If you have a question about TLC stickers or any of the Janice VanCleave Color Changing stickers, see Contact Janice in the side bar.
Following are some of the questions that I had when I started investigating thermochromic liquid crystals. (TLC). I am still researching and discovering new information all the time, thus along with questions from you, I will be adding my own questions.
Some questions that I add may not have an answer. This means that I am searching for the answer and would ever so like for you to help. Please send reference info with all answers you offer.
I love science and find it fun. Surfing the web for science information is entertaining for me. Yesterday while reading about liquid crystals, I read that the slime in the bottom of soap dishes has liquid crystal properties. WOW! That is cool, but is it true? One thing about researching, you need to make sure you check the information by comparing it with several sources. While I do compare and double-check the facts that I write in my books as well as on this website, there is always the possibility that an author can leave out a word that changes the meaning, such as: The chemical is not water-soluble. vs. The chemical is water-soluble. OOPS! This typo is not just a type error–it makes the science statement incorrect.
With this said,
PLEASE!!!!! Contact me if you find errors in my work.
PLEASE!!! Contact me if you have questions about topic and/or think my answers are not correct.
I have had lots of fun experimenting with the Hallcrest Chromic Chemicals. I know that you will enjoy using the products that I have designed to make teaching science fun. Check out the free lessons for the different types of color changing stickers in the side bar.
CAT Stickers are coated with a mixture of CAT dyes and Acrylic paint.
What is a Matter Classification Flowchart?
A flowchart is a type of tree diagram or organizing chart with the most general term at the top.
A Basic Matter Flowchart
1. Start with matter at the top.
2. Show the Matter Category divided into two groups: Pure Substance and Mixture
3. Divide each of the two groups into two other groups.
Pure Substance: elements and compounds
Mixtures: heterogeneous and homogeneous
The Matter Flowchart continues with examples for each groups.
Check here for more information:
Growing Crystals by the Evaporation Method
Crystals grown by the evaporation method are generally larger and better formed. This method is slower but the results are worth the wait. Basically a solution is made with water and a crystalline solid, such as copper sulfate or potassium aluminum sulfate (alum). Copper sulfate and alum crystals grow best with the evaporation method.
A saturated solution is made by adding a solute until no more will dissolve. At room temperature, about 32 g of copper sulfate will dissolve in 100 ml of water. Dissolve copper sulfate in distilled water until this solute stops dissolving. At this point there will be crystals of copper sulfate that settle to the bottom of the container and no amount of stirring will cause these crystals to dissolve.
A saturated solution of alum at room temperature has about 10 grams of alum in 100 ml of water.
Use the Wikapedia Solubility Table to decide how much solute you need to make a saturated solution. Note that it depends on the temperature of the water which should be at room temperature. http://en.wikipedia.org/wiki/Solubility_table
Caution: Adult supervision is required when handling copper sulfate because it is toxic. Copper sulfate is used to kill roots in drains. Do use special precautions when working with this chemical. Wear gloves, an apron, and goggles. Alum is not toxic. But you do not want to get it in your eyes. So wear goggles and an apron is always suggested.
2. Pour the saturated solution through filter paper to remove any undissolved solute. The diagram shows a funnel lined with filter paper. You can use a coffee filter or even a paper towel instead of filter paper.
The undissolved particles of solute will stay in the filter and the filtrate collected will be the saturated solution you’ll use for growing crystals. This step is done so that the crystal have a more perfect shape. The undissolved particle may have been broken and will form malformed crystals.
Collect Seed Crystals
3. Cover the container with paper and insert a craft stem through the center of the paper and down into the solution.
You want the solvent (water) to evaporate slowly at ambient temperature (room temperature). As the water evaporates the solution becomes more concentrated, meaning the ratio of solute to solvent increases. You now have a supersaturated solution. This means that there is more solute dissolved in the water than normal at ambient temperature (room temperature). Supersaturated solution are unstable, which means the excess solute tends to precipitate (fall out of solution).
As the water slowly evaporates tiny microscopic crystals start forming within the solution. The craft stem provides surfaces for these tiny invisible crystals to collect on. These are called seed crystals and will grow as solute particle attach to them.
Isomerization is the process by which one molecule is transformed into another molecule which has exactly the same elements and exactly the same number of atoms of each element. The difference between the two molecules is how the elements are arranged e.g. ABC changes to BCA. Each molecular form is called an isomer.
Thermochromic and photochromic materials, such as leuco dyes exhibit isomerization in which one molecular form is colorless and the other is colored.
Thermochromic molecules undergo isomerization in response to temperature fluctuations.
Photochromic molecules undergo isomerization in response to UV light.
Rust is a common name for Iron Oxide, the chemical formed when iron oxidizes (combines with oxygen).
The equation for rusting or the oxidation of iron is:
2Fe + 3O 2 ——–> Fe2O3
Corrosion is the deterioration of physical properties of a material due to a reaction with its environment.
Corrosion is caused by different things, such as the cracking of a polymer due to exposure to sunlight and the oxidation of metals.
Lately there have been several water leaks in my house. One was due to an old water heater that has not been used for years. I must admit that the beautifully colored crusted areas on the pipes caught my eye. If you look close you can see dark blue crystals. While I was intrigued with the colors and the chemistry behind their production, most of the excitement was dampened by the plumber’s bill. But I did learn something new from him. Seems that the corrosion started because two metals were in contact with each other. I later discovered that this is called two metal corrosion or galvanic corrosion.
The term corrosion seems to be a catch all for stuff deteriorating because of chemical reactions. My water pipes were made of different kinds of metals–copper, zinc, and iron–thus all the different colors of galvanic corrosion.
Oxidation is the chemical combination with oxygen.
Iron generally corrodes by combining with oxygen forming a reddish-brown chemical commonly called rust.
The equation for the rusting (oxidation) of iron is:
4Fe + 3O 2 ——-> 2Fe2O3
Zinc also corrodes by oxidation. The white corrosion in the photo is zinc oxide, commonly called zinc white.
The equation for the oxidation of zinc is:
2Zn + O2 —–> 2ZnO
Copper corrosion, except for water pipes, is desired by artists. Copper sculptures placed outdoors as well as copper used for roofs corrode and form what is called patina.
The corrosion of copper involves oxidation plus a reaction of other environmental chemicals, such as water and carbon dioxide. The summation of copper corrosion producing copper carbonate (CuCO3) the blue-green color on the pipe in the picture is:
2 Cu (s) + O2+ H2O (g) + CO2 ? Cu(OH)2 + CuCO3 (s)
This is not a one step process. The copper first oxidizes forming cupric oxide (copper I oxide) . The water and carbon dioxide form carbonic acid (H2CO3), then the copper I oxide and carbonic acid react forming copper hydroxide and copper carbonate.
Read more: http://www.physicsforums.com
A solution is a homogeneous mixture made of two parts:
1. A solvent, which is generally the greater amount and is what does the dissolving.
2. A solute, which is the part that is being dissolved and is the part that breaks into its smallest particles and is equally distributed throughout the solvent.
- amount of solute increases and the amount of solute remains the same.
- amount of solute remains the same and the amount of solute decreases.
2. Use two glasses marked A and B. Pour 1/4 cup of water in glass A . Pour 1/2 cup of water in glass B. Add 6 teaspoons of sugar to each glass and stir well. Taste each solution. Which ratio of sugar to water has the greater concentration of sugar? Glass A ratio= 6 teaspoon sugar: 1/4 cup water Glass B ratio = 6 teaspoons sugar: 1/2cup water Answer: Both glasses contain the same amount of sugar (solute). Glass A has less water (solute) for the sugar to spread out in. As a result, the molecules of sugar are closer together than they are in glass B with more water. Glass A has the greater concentration of sugar.
CAT Dyes are cold activated thermochromic dyes. Don’t panic, the terms are really not difficult if you take them in bite-size pieces.
1. Cold Activated— Cold refers to temperature. But cold is not a measurement. Instead it is a comparison between two temperatures. For all the Janice VanCleave color changing products, room temperature is the standard. This means that something would be cold if it is less than room temperature, which on the average is around 25 0 C (77 0F). Most of the JVC cold activated color changing products are activated around 15 0 C (59 0F), which can be achieved by cooling the CAT dyes with ice.
2. Thermochromic –Thermo refers to temperature and Chromic means a color change. Thus, thermochromic is a material that changes color in response to changes in temperature.
3. Dyes — A dye a colored substance that has an affinity to the materials that it is applied to. The dye is generally applied in an aqueous (water) solution. This means the dye is mixed with water.
Dyes usually require a mordant, which is a chemical that helps the dye to stick to surfaces. The Janice VanCleave Chromic dyes are ready to apply. The mordant or binder is already added.
Room temperature varies. Measure the room temperature using a thermometer.
If your investigation doesn’t need a precise room temperature measurements, the average
standard temperature used for room temperature is 25 °C ( 77 °F).
The Janice VanCleave CAT Stickers are not toxic.
But eating or licking CAT Stickers is not suggested.
A substance is a sample of matter that has a constant composition. In other words, a substance has the same physical and chemical properties throughout. Elements and compounds are examples of a substance.
Substances can change from one state of matter to another.
Ink is a mixture of a dye and a solvent. When ink is used on a surface, when the solvent evaporates the dye is left on the surface.
Inks used for the Janice VanCleave Color Changing Products are water soluble, meaning the solvent is water. The colorants in the color changing inks include a special type of dye called Leuco Dye. Leuco dye molecules have two structural forms, one form is colorless and transparent to visible light. The other leuco dye molecule structure is colored, meaning the molecules absorb part of the visible light spectrum and reflect the remaining part of the light spectrum. The changes in the structural forms of Leuco dye molecules are in response to a specific stimulus.
For more information see Leuco dyes.
Mixtures are the combination or blending of two or more things. There are two different types of mixtures:
Heterogeneous mixtures have an uneven distribution of the different things that have been mixed.
1. Fruit salad made up of apples, oranges, bananas, etc….
2. Combination of iron filings (prepared by grinding up a piece of iron, such as an iron nail) and sand.
3. Rocks are heterogeneous mixtures of different minerals.
Note: While the materials mix to form a heterogeneous mixture keep their own individual physical properties, not all materials in a heterogeneous mixtures are easily separated. The different minerals in rocks for example need special processes to separate.
Homogeneous mixtures have an even distribution of the different things added together.
Solutions are homogeneous mixtures. Solutions are a combination of two parts, a solute and a solvent. When two parts are mixed, the part with the larger volume is generally called the solute. Thus, the solute is the substance that breaks up and is spread throughout the solute.
A sugar solution is a mixture of sugar and water. Sugar is a molecular compound, meaning the smallest part of the sugar compound is a molecule. When sugar crystals dissolve in water, the molecules making up the sugar crystals pull apart and separate molecules of sugar are spread evenly throughout the water. Every drop of the sugar solution has the same number of sugar and water molecules.
Note: Table salt (sodium chloride) is an ionic compound made up of positively charged sodium ions and negatively charged chloride ions. A table salt solution is a mixture of table salt and water. When table salt is dissolved in water, unlike table sugar (sucrose) which is a molecular compound, table salt breaks up into ions. The sodium ions and chloride ions spread evenly throughout the water.
Adding Rigor–This section is not complete.
1. Properties of solutions are called colligative properties.
Colligative properties depend on the concentration of the solution.
Colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering and osmotic pressure.
2. Solvation (Dissolution) is the process of surrounding the solute in a solution with the solvent. Water molecules are polar, meaning they have a positive and negative side. The oxygen atom in each molecule of water has a slightly negative charge while the hydrogen atoms have a slightly positive charge. When sodium chloride dissolves in water, each positive sodium ion is surrounded by water molecules with the oxygen atom pointing toward the sodium ion. The negative chloride ions are surrounded by water molecules with the positive hydrogen ions pointing toward the negative chloride ion.
3. Solution Concentration
Saturated–No more solute can be dissolved at a specific temperature (room temperature).
A Way to Produce a saturated Solution at Room Temperature
I experimented with Cotton Candy, which is a fine webbing of sugar molecules.
I placed the cotton candy on a plate and using a straw let one drop of water fall on the puffy pink candy. Voom! A big hole appeared in the fluffy candy and red drops of liquid started running through the candy as if moving through tiny tubes. Each drop of water did the same thing and finally the candy had disappeared leaving only a small puddle of thick red liquid on the plate as well as on the paper cone the candy had been spun around. There were some sugar crystals on the paper cone as well as in the thick saturated solution in the plate.
Supersaturated Solution is when a solvent is heated above room temperature and more solute than will dissolve at room temperature is dissolved.
Types of Solutions
Solid + Solid
Alloy a homogeneous solution of two or more metals that have been melted, blended evenly together, then allow to cool; also a metal with any nonmetal where the two are evenly blended, such as iron and carbon forming steel.
Solid + Gas–
Gas + Gas– Air
Gas + liquid –Carbon dioxide dissolved in a soda.
Most mixtures can be separated by Physical means.
Methods for separating Homogeneous Mixtures
Methods for separating Heterogeneous Mixtures
Buoyancy is the force that pushes upward on an object placed in a fluid (gas or liquid).
An object placed in a fluid (gas or liquid):
How to Measure the Buoyancy Force on an Object placed in Water.
1. Weigh the objects to be placed in the water.
2. Fill the large container with water so that the water’s surface is just below the straw as shown.
3. Place the object in the water and allow the displaced water to drain into the collecting container.
4. Weigh the collecting container and displaced water. Subtract the weight of the container and the answer is equal to the water’s buoyancy force pushing up on the object.
Another Method to Measure the Buoyancy Force on an Object Placed in Water.
A solution is a homogeneous mixture. This mean the mixture is the same throughout.
There are different kinds of solution as shown in the chart.
A solution is made by dissolving a solute in a solvent. The solute is the part that breaks into the tiniest particles making up the solute, then these particles are dispersed (evenly spread) throughout the solvent.
Generally, the solvent is the part with the greater volume.
Solid Compound + Water
There are two types of compounds, ionic compounds, such as table salt and covalent compounds, such as table sugar.
The video very nicely describes what happens when table salt (sodium chloride) and table sugar (sucrose) dissolve in water.
Styrofoam is a polymer. Polymers are large molecules made of units of chemicals, called monomers, linked together, much like links in a chain.
Styrofoam is a brand name of Dow Chemical. Styrofoam is used to make products, such as disposable coffee cups, packing peanuts, and coolers.
Styrofoam was originally discovered by Geog Munters, a Swedish inventor. But in 1941, led by Ray McIntire and a team of researchers in Dow’s Chemical Physics Lab, McIntire, using Munters’ method, made foamed polystyrene. With exclusive rights to use Munters’ patents, Dow mass produced styrofoam.
Foamed polystyrene crunches when cut, breaks into what looks like tiny sponge balls, and is moderately soluble in some organic solvents, such as the solvents in spray paint. Actually, saying that polystyrene dissolves in acetone, much like sugar dissolves in water is incorrect. Instead, acetone causes the bonds (links) between the styrene monomers to break. When the bonds break, it opens air filled cells in the foam and the previously trapped air escapes. Styrofoam is about 90% air that is trapped in closed cells with polystyrene walls. The pressure of the air inside the cells, like air in a balloon, keep the walls rigid. Without the air, the walls collapse and thus the polystyrene has lost its foamed volume.
Discover for yourself
1. How to Produce Foam
- In a glass half- filled with tap water, add about 4 drops of liquid dish detergent.
- Using a drinking straw, blow air into the water until foamy soap bubbles fill the glass.
Results: The bubbles that form have a thin soap and water membrane filled with air. Styrofoam is made by bubbling gases into liquid polystyrene. Cells of polystyrene are filled with these gases. The foaming gases are replaced with air and voila’! Styrofoam is produced.
2. Model the effect of breaking Styrofoam cells
- Using a toothpick, pop the soap bubbles in the soapy foam from step 1.
Results: As the bubbles are broken air escapes and the soap-water membrane surrounding each soapy bubble collapses. Without the air, the unfilled soapy film takes up less space and in time dissolves in the water. Styrofoam is not soluble in many solvents. But it affected by acetone, an organic solvent used in some fingernail polish remover. See the following to discover how acetone breaks the air filled cells of Styrofoam.
3. How do break Styrofoam cells
- In a well ventilated area, outside is best, pour about 1/4 cup (63 mL) of acetone in a glass or ceramic bowl.
- Stand a Styrofoam cup in the bowl of acetone. Watch —the Styrofoam cup seems to melt into the acetone. Continue to add Styrofoam materials, peanuts, packing, etc…
Results: The Styrofoam is not dissolving into the acetone. Instead, the bond holding the monomers together are breaking allowing the trapped air to escape. Since about 90% of Styrofoam is air, there is an extreme reduction in the volume of the material when the air is released leaving only polystyrene.
It is not safe to be in a closed room with acetone because it evaporates so quickly. Thus, outdoors, allows the acetone to safely evaporate. Left in the bowl will be solid polystyrene.
The following video demonstrates what happens to Styrofoam in Acetone. Note, the Styrofoam is not dissolving in the Acetone.
The second video is an idea for a science fair project.
Science Project Idea.
What is the difference between elements and atoms?
Elements are pure substances that cannot be broken down to different substances.
Atoms are the smallest part of an element that retains the properties of the element.
A chemical symbol is an abbreviated way of representing the name of an element. Symbols are made of one or two letters. If one letter is use, such as H for hydrogen, the letter is capitalized.
If the symbol has two letters, such as Ca for Calcium, the first letter is capitalized and the second letter is lower case.
The periodic table shows all the know elements in order of their atomic number, which is the number of protons in the atoms of each element. In other words, all the atoms of a specific element have the same atomic number. Thus, the atomic number is how elements are identified.
Click the Periodic table for a copy of the table with information about each element.
Know that elements are the smallest
Why students need books
To provide interrelating information.
For example, if students use the internet they might come up with the following definitions.
abundance-having plenty of what you need
trend-a tendency or inclination
With a textbook, they would find how these terms are used in connection to the subject being studied, which is the periodic table. The textbook would provide the following information.
Abundance is the percent of each part of the whole. The two most abundant elements in the Earth’s crust is Silicon and Oxygen. The abundance of Silicon is about 47% and Oxygen is 28%, thus together, Silicon and Oxygen make up about 75% of the materials in Earth’s crust.
Matter is anything that has mass and takes up space.
Mass is how much stuff something is made of or contains.
Space is the volume that something occupies.
The states of matter, include gas, liquid, and solid.
Air is a gas, thus is an example of matter. While most gases are invisible, never the less, they have mass and take up space.
Use an empty plastic sack, such as an empty plastic bread sack.
1. Fill the sack with air by holding the top open and moving the sack through the air.
2. Close the sack by twisting the opening and holding it with your hand.
3. With your free hand, squeeze the sack as shown.
4. Look into the sack.
The sack is blown up, but appears to be empty. The sack resists being squeezed. This means the content of the sack takes up space.
Demonstrate that Air Has Mass
Mass is a measure of the amount of “stuff” something has. For example, if you fill two bread sacks with different amounts of air, the one with the small amount of air has less mass. The bag with the largest amount air has more mass.
I. Design an investigation that compares the mass of different amounts of air (exhaled breath).
1. Use two balloons of equal size to hold different amounts of air–inflate with your breath.
2. Design a balance, such as suspending a yardstick or meter stick so that the stick rotates about its center. This is the fulcrum of the balance (the point the stick rotates about).
3. Hang the largest balloon near one end of the stick. Secure with tape. The mass of this balloon will be m1.
3. Hang the smaller balloon on the opposite side and position so that the stick hangs as parallel to the floor as possible. The mass of this balloon will be m2.
4. Calculate the ratio of the masses of each balloon to the distance they are from the center, the fulcrum of the stick.
In other words, measure how far is m1 and m2 are from the fulcrum. Use this equation to show the ratio of the the masses of the balloons. m1/m2 = d1/d2
II. Mass vs. Inertia
Design and calibrate an inertia balance. Use the balance to measure the mass of objects.
Clues can be found here:
What is the difference between atoms and molecules?
Atoms are the smallest part of an element that retains the properties of the element.
Molecules are made of two or more atoms bonded together to form one particle with specific characteristics.
All matter is made up of atoms which may combine to form molecules.
Some elements, such as neon, Ne, are made up of single atoms. Other elements, such as oxygen, is made up of molecules. The molecules of oxygen have two atoms of oxygen, O2. The symbol for the oxygen element is O. The symbol or one can say, the formula for the molecules of oxygen is, O2. There is another type of oxygen molecule called ozone, O3. The subscript 3 describes the ozone molecule as having three bonded atoms of oxygen.
These diatomic elements aka diatomic molecules should be memorized. H2, N2, O2, F2, Cl2, Br2, I2,
What is the difference between molecules and compounds?
Compound are substances with two or more different kinds of elements bonded together. Water is a compound made of elements hydrogen and oxygen. Table sugar is a compound made of elements carbon, hydrogen, and oxygen.
Some compounds are made up of molecules, such as table sugar, known as sucrose. The molecular formula for sucrose is C12H22O11. This is read as 12 atoms of carbon, 22 atoms of hydrogen, 11 atoms of oxygen.
The molecular formula for sucrose tell how many atoms of each kind of element makes up one molecule, but does not tell how the atoms are bonded together.
Structural and skeletal formulas do give information about how the elements are bonded together. Compounds made up of molecules have covalent bonds (bonds in which the atoms share electrons). Each single covalent bond is indicated by a single line in structural and skeletal formulas. Each covalent bond represents the sharing of two electrons between the connected atoms.
Count the number of carbon, hydrogen, and oxygen atoms in each of the formulas. Do they add up to the molecular formula for sucrose, which is C12H22,O11?
FYI: Each carbon atom should have 4 connecting bonds. You will find this true in the structural formula, but the skeletal formula only shows what is connected to the rings. You are suppose to know that carbons have 4 connecting covalent bonds. Compare the two formulas to see where carbon and hydrogen atoms are left off in the skeletal formula.
The structural formula for sucrose is:
At the particle level, the difference between the states of matter of a substance is in the behavior of the substance’s particles.
When a substance changes state due to a gain or loss of energy, it is because of the spacing between the particles. Water is a common substance that can exist in three different states of matter. Water’s solid state is called ice. As ice, water particles are close together. As ice is heated and the water particles making up ice gain energy, the water particles move farther apart, eventually the ice changes into the liquid state, called water. The process of changing from ice (solid) to water (liquid) is called melting.
As the water (liquid) is heated and the water particles making up the liquid state of matter gain energy, the water particles move even farther apart, eventually the liquid changes into the gas state, called vapor. The process of changing from water (liquid) to a gas is called vaporization.
When water vaporizes at its boiling point, the process is called boiling.
When water vaporizes at a temperature lower than it boiling point, the process is called evaporation.
Leuco dyes are chemicals that can have two structural forms, one is colorless and transparent and the other is colored. Things that are colorless and transparent because visible light energy passes through, much like light passes through window glass. In fact, transparent means to allow something to pass through. Earth’s atmosphere is transparent to some of the solar energy from the Sun but not all of it.
The dye’s colored molecular structure absorbs and reflects different parts of the visible light energy spectrum. The reflected visible light energy enters your eyes where it is absorbed by special cells that sends an electric massage via nerves to your brain. Your brain decodes the message and voila’ the dye appears to be a specific color.
The Leuco dyes in the Janice VanCleave color changing products are microencapsulated with special materials needed to activate a structural change in the leuco dye molecules. The color change is due to the leuco dye molecules mixing with the chemicals inside the microcapsules.
Believe me, I am amazed that all the necessary stuff to cause the leuco dyes to change from one structure to another and then reverse back again is within such a tiny–microscopic –capsule.
Technically, the HAT (Heat Activated Thermochromic) Stickers and the CAT (Cold Activated Thermochromic) Stickers do not directly change colors because of the fluctuation of temperature. These stickers are coated with special mixtures containing leuco dye microcapsules. One more bit of information that ties this all together is that the solvent content of the microcapsules changes state, from solid to liquid in response to temperature changes. Thus, the color changes for the CAT Stickers and HAT stickers are thermochromic only in the sense that a change in temperature is necessary for the content of the microcapsules to mix and separate. The color change within the microcapsules is called halochromism (color change due to changes in the pH inside the microcapsules.
HAT Stickers : Heat Activated Thermochromic stickers.
CAT Stickers: Cold Activated Thermochromic stickers.
A chromophore is a group of atoms in a chemical compound that are responsible for the color of the compound.
Pigments in leaves are compounds with chromophores. Chlorophyll is a green pigment with a chromophore that reflects green light.
Chromophores absorbs, reflect and/or transmit visual light energy.
The color of reflected visual light enters your eyes and is absorbed by special cells on the back surface inside your eyes. These cells send a message to your brain. Your brain decodes the message and voila’ the object has color. For leaves with chlorophyll, the reflected light is perceived as a shade of green.
Even though the same light energy is reflected from the leaf, the color of the leaf may not seem to be the same color to every person. We assume that most people see the same colors, but I always wonder about this. After all, what you see depends on a lot of information being transferred. Vision is a very interesting topic to read about.
More to Know and Find Out About
The chromophore is the part of an organic molecule that affects visible light, thus it can be said that the chromophore section is a functional group or moiety of the molecule that is responsible for color.
The chromophore of some organic chemicals, such as Leuco dyes, change color in response to different stimuli. Some Leuco dyes change color in response to fluctuations in temperature and other in response to being exposed to UV light. These changes, which can be permanent but often are reversible, depend on changes in the chromophore.
Leuco Dyes are chemicals whose molecules can have two form, one form is colorless and the other form is colored. The colorless form is transparent, meaning that visible light is not absorbed by the molecules. Window glass is transparent, thus visible light passes through without being absorbed or reflected.
One example of a leuco dye is phenolphthalein, a chemical whose molecules respond to changes in pH (a scale measuring the acidic or basic nature of a substance). At a pH of 7 or lower, phenolphthalein molecules are colorless and transparent, thus a solution of phenolphthalein would have no color and visible light would pass through the solution. At a pH greater than 7, phenolphthalein molecules absorb most of visible light, reflecting parts that you perceive as pink to fuchsia in color.
Leuco dyes change color due to halochromism. In other words, color changes in response to changing in pH.
So What is pH?
pH is a scale that describes how acidic or basic a substance is. A pH of 7 is neutral, meaning it is neither an acid nor a base. A pH less than 7 is an acid. A pH greater than 7 is a base.
Since color is due to chromophores, changes in pH affects the chromophores of the molecules of halochromic chemicals.
This sounds so complex and at the molecular level there is a lot going on.
In the diagrams below, the structure of a chromatophore of a phenolthalein molecules changes. To understand this change, one needs to understand organic chemistry. But, even elementary kids can observe and determine that the two structures are different. Because of the differences, the structure on the left is transparent. This means that visible light passes through, much like visible light passes through a glass of water. The structure on the right reflects parts of visible light that make the phenolthalein molecules have a fuchsia color.
The change in the structural shape of the chromatophore section of phenolthalein molecules is due to changes in pH.
Indicators are chemicals that exhibit halochromism. Common indicators are red litmus and blue litmus. Red litmus turns blue when it comes in contact with an acid. Blue litmus turns red when it comes in contact with a base.
In the same way as phenolphthalene, Leuco dyes in thermochromic inks have chromatophores that respond to changes in pH. Find out how dyes that are actually halochromic are the color changing part of thermochromic inks.
|Changes in Chromatophore Structure|
|Conditions||acidic or near-neutral||basic|
|Color name||pink to fuchsia|
HAT Stickers are stickers coated with a mixture of HAT dye and acrylic paint. In other words, Heat Activated Thermochromic Leuco dye, which changes color when warmed above room temperature, is mixed with acrylic paint.
HAT Dyes are Chromic Dyes.
Chromic Dyes are color changing substances, and like all dyes has an affinity to materials to which it is applied.
The Janice VanCleave chromic dyes are prepared with the necessary binder that helps the dyes to stick to surfaces. These dyes are all water-soluble, which means they dissolve in water.
HAT is a acronym that stands for Heat Activated Thermochromic
Heat Activated —-To activate the dyes heat must be applied. Each dye has an activation temperature, which for the Janice VanCleave HAT dyes is around 290C (850F).
Thermochromic –Thermo refers to temperature and Chromic means a color change. Thus, thermochromic is a material that changes color in response to changes in temperature.
Variables that Affect the Rate of Heat Transfer by Conduction
1. Temperature Difference
In conduction, heat is transferred from a hot temperature region to a cold temperature region. As long as there is a difference between the temperature, heat will move from the hotter region to the colder region.
Thermal equilibrium means the temperature of the two regions in contact are the same, thus any heat transfer between the two regions would be the same.
In the graphs above, the slope of the line represents the rate at which the temperature of each individual sample of water is changing. The temperature is changing because of the heat transfer from the hot to the cold water. The hot water is losing energy, so its slope is negative. The cold water is gaining energy, so its slope is positive. The rate at which temperature changes is proportional to the rate at which heat is transferred. The temperature of a sample changes more rapidly if heat is transferred at a high rate and less rapidly if heat is transferred at a low rate. When the two samples reach thermal equilibrium, there is no more heat transfer and the slope is zero. So we can think of the slopes as being a measure of the rate of heat transfer. Over the course of time, the rate of heat transfer is decreasing. Initially heat is being transferred at a high rate as reflected by the steeper slopes. And as time progresses, the slopes of the lines are becoming less steep and more gently sloped.
What variable contributes to this decrease in the heat transfer rate over the course of time? Answer: the difference in temperature between the two containers of water. Initially, when the rate of heat transfer is high, the hot water has a temperature of 70°C and the cold water has a temperature of 5°C. The two containers have a 65°C difference in temperature. As the hot water begins to cool and the cold water begins to warm, the difference in their temperatures decrease and the rate of heat transfer decreases. As thermal equilibrium is approached, their temperatures are approaching the same value. With the temperature difference approaching zero, the rate of heat transfer approaches zero. In conclusion, the rate of conductive heat transfer between two locations is affected by the temperature difference between the two locations.
The first variable that we have identified as affecting the rate of conductive heat transfer is the temperature difference between the two locations. The second variable of importance is the materials involved in the transfer. In the previous discussed scenario, a metal can containing high temperature water was placed within a Styrofoam cup containing low temperature water. The heat was transferred from water through the metal to water. The materials of importance were water, metal and water. What would happen if the heat were transferred from hot water through glass to cold water? What would happen if the heat were transferred from hot water through Styrofoam to cold water? Answer: the rate of heat transfer would be different. Replacing the inner metal can with a glass jar or a Styrofoam cup would change the rate of heat transfer. The rate of heat transfer depends on the material through which heat is transferred.
The effect of a material upon heat transfer rates is often expressed in terms of a number known as the heat transfer coefficient. Heat transfer coefficients are numerical values that are determined by experiment. The higher that the coefficient is for a particular material, the more rapidly that heat will be transferred through that material. Materials with relatively high heat transfer coefficients are referred to as thermal conductors. Materials with relatively low heat transfer coefficients are referred to as thermal insulators. The table below lists heat transfer coefficients (k) for a variety of materials, in units of W/m/°C.
Polyvinyl chloride (PVC)
Concrete (Low Density)
Concrete (High Density)
As is apparent from the table, heat is generally transferred by conduction at considerably higher rates through solids (colored red) in comparison to liquids (blue) and gases (green). Heat transfer occurs at the highest rates for metals (first eight items in left-hand column) because the mechanism of conduction includes mobile electrons (as discussed on a previous page). Several of the solids in the right-hand column have very low heat transfer coefficients and are considered insulators. The structure of these solids is characterized by pockets of trapped air interspersed between fibers of the solid. Since air is a great insulator, the pockets of air interspersed between these solid fibers gives these solids low heat transfer coefficients. One of these solid insulators is expanded polystyrene, the material used in Styrofoam products. Such Styrofoam products are made by blowing an inert gas at high pressure into the polystyrene before being injected into the mold. The gas causes the polystyrene to expand, leaving air filled pockets that contribute to the insulating ability of the finished product. Styrofoam is used in coolers, pop can insulators, thermos jugs, and even foam boards for household insulation. Another solid insulator is cellulose. Cellulose insulation is used to insulate attics and walls in homes. It insulates homes from heat loss as well as sound penetration. It is often blown into attics as loose fill cellulose insulation. It is also applied as fiberglass batts (long sheets of paper backed insulation) to fill the spacing between 2×4 studs of the exterior (and sometimes interior) walls of homes.
Another variable that affects the rate of conductive heat transfer is the area through which heat is being transferred. For instance, heat transfer through windows of homes is dependent upon the size of the window. More heat will be lost from a home through a larger window than through a smaller window of the same composition and thickness. More heat will be lost from a home through a larger roof than through a smaller roof with the same insulation characteristics. Each individual particle on the surface of an object is involved in the heat conduction process. An object with a wider area has more surface particles working to conduct heat. As such, the rate of heat transfer is directly proportional to the surface area through which the heat is being conducted.
Thickness or Distance
A final variable that affects the rate of conductive heat transfer is the distance that the heat must be conducted. Heat escaping through a Styrofoam cup will escape more rapidly through a thin-walled cup than through a thick-walled cup. The rate of heat transfer is inversely proportional to the thickness of the cup. A similar statement can be made for heat being conducted through a layer of cellulose insulation in the wall of a home. The thicker that the insulation is, the lower the rate of heat transfer. Those of us who live in colder winter climates know this principle quite well. We are told to dress in layers before going outside. This increases the thickness of the materials through which heat is transferred, as well as trapping pockets of air (with high insulation ability) between the individual layers.
A Mathematical Equation
So far we have learned of four variables that affect the rate of heat transfer between two locations. The variables are the temperature difference between the two locations, the material present between the two locations, the area through which the heat will be transferred, and the distance it must be transferred. As is often the case in physics, the mathematical relationship between these variables and the rate of heat transfer can be expressed in the form of an equation. Let’s consider the transfer of heat through a glass window from the inside of a home with a temperature of T1 to the outside of a home with a temperature of T2. The window has a surface area A and a thickness d. The coefficient of heat transfer of the window glass is k. The equation relating the heat transfer rate to these variables is
Rate = k•A•(T1 – T2)/d
The units on the rate of heat transfer are Joule/second, also known as a Watt. This equation is applicable to any situation in which heat is transferred in the same direction across a flat rectangular wall. It applies to conduction through windows, flat walls, slopes roofs (without any curvature), etc. A slightly different equation applies to conduction through curved walls such as the walls of cans, cups, glasses and pipes. We will not discuss that equation here.
To illustrate the use of the above equation, let’s calculate the rate of heat transfer on a cold day through a rectangular window that is 1.2 m wide and 1.8 m high, has a thickness of 6.2 mm, a coefficient of heat transfer of 0.27 W/m/°C. The temperature inside the home is 21°C and the temperature outside the home is -4°C.
To solve this problem, we will need to know the surface area of the window. Being a rectangle, we can calculate the area as width • height.
Area = (1.2 m)•(1.8 m) = 2.16 m2.
We will also need to give attention to the unit on thickness (d). It is given in units of cm; we will need to convert to units of meters in order for the units to be consistent with that of k and A.
d = 6.2 mm = 0.0062 m
Now we are ready to calculate the rate of heat transfer by substitution of known values into the above equation.
Rate = (0.27 W/m/°C)•(2.16 m2)•(21°C – -4°C)/(0.0062 m)
Rate = 2400 W (rounded from 2352 W)
It is useful to note that the thermal conductivity of a house window is much lower than the thermal conductivity of glass itself. The thermal conductivity of glass is about 0.96 W/m/°C. Glass windows are constructed as double and triple pane windows with a low pressure inert gas layer between the panes. Furthermore, coatings are placed on the windows to improve efficiency. The result is that there are a series of substances through which heat must consecutively pass in order to be transferred out of (or into) the house. Like electrical resistors placed in series, a series of thermal insulators has an additive effect on the overall resistance offered to the flow of heat. The accumulative effect of the various layers of materials in a window leads to an overall conductivity that is much less than a single pane of uncoated glass.
Lesson 1 of this Thermal Physics chapter has focused on the meaning of temperature and heat. Emphasis has been given to the development of a particle model of materials that is capable of explaining the macroscopic observations. Efforts have been made to develop solid conceptual understandings of the topic in the absence of mathematical formulas. This solid conceptual understanding will serve you well as you approach Lesson 2. The chapter will turn slightly more mathematical as we investigate the question: how can the amount of heat released from or gained by a system be measured? Lesson 2 will pertain to the science of calorimetry.
Check Your Understanding
1. Predict the effect of the following variations upon the rate at which heat is transferred through a rectangular object by filling in the blanks.
a. If the area through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ (increased, decreased) by a factor of _________ (number).
b. If the thickness of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ by a factor of _________.
c. If the thickness of the material through which heat is transferred is decreased by a factor of 3, then the rate of heat transfer is ________________ by a factor of _________.
d. If the thermal conductivity of the material through which heat is transferred is increased by a factor of 5, then the rate of heat transfer is ________________ by a factor of _________.
e. If the thermal conductivity of the material through which heat is transferred is decreased by a factor of 10, then the rate of heat transfer is ________________ by a factor of _________.
f. If the temperature difference on opposite sides of the material through which heat is transferred is increased by a factor of 2, then the rate of heat transfer is ________________ by a factor of _________.
2. Use the information on this page to explain why the 2-4 inch thick layer of blubber on a polar bear helps to keep polar bears warm during frigid artic weather.
3. Consider the example problem above. Suppose that the area where the window is located is replaced by a wall with thick insulation. The thermal conductivity of the same area will be decreased to 0.0039 W/m/°C and the thickness will be increased to 16 cm. Determine the rate of heat transfer through this area of 2.16 m2.
SA dyes are chromic, which means they change colors in response to some stimulus.
- SA dyes respond to ultraviolet light.
- Ultraviolet light is part of sunlight, thus SA dyes are activated by sunlight.
SA is the acronym for Sun Activated.
Thermochromic Liquid Crystals (TLC) have reversible color changes when heated then cooled. This means they go through the light spectrum of colors as they are heated and as cooled the colors disappear in reverse order.
Shelf life is the length of time that the TLC Stickers will continue to change color with fluctuations of temperature.
All TLC products are sensitive to UV light as well as high temperatures. Exposure to UV light and high temperatures will cause the TLC chemicals to degrade. The first signs that the TLC Stickers are degrading is a loss of brightness in color. Eventually the stickers will not change color at all.
If stored properly, in a cool (room temperature), dark place, the TLC Stickers have a shelf life of at least 6 months. I have TLC stickers that are much older.
Thermochromic Liquid Crystals (LCs) can be highly temperature sensitive, change to many colors, and are more expensive than leuco dyes.
Leuco dyes are the thermochromic dyes used on CAT Stickers and HAT Stickers.
Liquid Crystals are used on the TLC Stickers.
LC’s start black below their temperature range; go through the colors of a rainbow, and then black again above the temperature range. LC’s are reversible in that they can be used over and over again. The picture shows an example of a liquid crystal sheet in response to warming.
Popular liquid crystal applications include medical devices, forehead, aquarium and room thermometers, promotional pieces and advertising applications. Additionally functional devices such as a propane tank gas level indicator are achieving much notoriety. Liquid crystal thermometers are being used for thermal mapping and other industrial applications where custom inexpensive temperature monitoring is warranted. We offer a wide range of LC thermometers as stock products, but also offer literally thousands of products for your label.
LC’s are very similar to the liquid crystal displays used in watches and laptop computers, but our thermochromic LC’s change color with temperature instead of voltage. LC’s can be formulated to change temperature from -22 to +248ºF (-30 to 120ºC), and can be sensitive enough to detect changes as small as 0.2ºF. If kept out of UV light and away from high temperatures and strong solvents, LC products will last for years.
The most prevalent use of liquid crystal is as a self adhesive reversible temperature indicator label that continually monitors temperature offering a visual readout that ranges from traditional numeric displays to custom graphics that can serve as an alert or warning.
What They Are:
These self adhesive labels consist of a series of temperature-sensitive elements containing microencapsulated Thermochromic Liquid Crystal TLC coated on a black backing. Each element changes color distinctly as its rated temperature is reached, passing through the colors of the spectrum in sequence (Tan, Green, and Blue before turning black at a higher temperature. The TLC strips are calibrated so that the indicator that shows green indicates the actual temperature. The color changes are reversible and the reflected colors will be observed in the reverse order upon cooling.
TLCs react to changes in temperature by changing color. They have chiral (twisted) molecular structures and are optically active mixtures of organic chemicals.
The correct scientific name for the materials is CHOLESTERIC or CHIRAL NEMATIC liquid crystals.
The term cholesteric is an historical one, and derives from the first materials to show the characteristic properties and structure of this particular type of liquid crystal were esters of cholesterol. This can be misleading, as many non-sterol derived optically active chemicals (and mixtures containing them) also show the cholesteric liquid crystal structure. It is important to differentiate this sterol and non-sterol derived materials because, although they change color in the same way, they have different properties and can be used in different ways to achieve different effects.
TLC mixtures can be divided into 3 types based on their compositions:
(a) CHOLESTERIC – consist entirely of sterol-derived chemicals;
(b) CHIRAL NEMATIC – composed entirely of non-sterol based chemicals.
(c) COMBINATION – containing both cholesteric and chiral nematic components. Combination mixtures extend the application possibilities and working ranges of TLC formulations by combining the respective advantages of both groups of component chemicals.
All TLCs are CHOLESTERIC LIQUID CRYSTALS, whether sterol-derived, non-sterol-derived or a mixture of the two.
Cholesterics are one of the three major classes of THERMOTROPIC liquid
crystals (produced by the action of heat).
The two others are called smectics and nematics.
LYOTROPIC liquid crystals result from the action of a solvent.