Notes from Janice’s Desk

**Density** is a physical property describing the **mass **of one unit **volume** of a substance. If the mass is measured in grams, generally the volume is measured in milliliters or cubic centimeters. (**NOTE:** 1 ml = 1 cm^{3})

A **pure substance **is an element, such as copper or gold **or** a compound, such as water or sodium chloride (table salt).

The density of pure substances is constant. This means that they do not change as long as other materials are not mixed with them.

Memorize the density of water, which is: **density of water = 1g/1ml.**

This means that 1 gram of water occupies a space of 1 ml.

Since density is a constant, it can be used as a **conversion factor**, which is a **ratio** used in math calculations to change units.

**Dimensional analysis **is the math process in which one or more conversion factors are used to solve a problem.

DON’T PANIC! IT SOUNDS HARDER THAN IT IS.

**Problem:**

**1. **The density of a liquid is 4 g/ml. What is the mass of 3 ml of the liquid?

ARE YOU THINKING, “I CAN DO THIS PROBLEM IN MY HEAD. THE MASS IS 12 g. SO WHY BOTHER WITH DIMENSIONAL ANALYSIS?”

**TRUST ME.** **IF YOU PLAN TO DO WELL IN YOUR CHEMISTRY CLASS, YOU NEED TO BE A WHIZ AT DIMENSIONAL ANALYSIS**.

For step-by-step instructions for using dimensional analysis to solve the problem, see **MATH: DIMENSIONAL ANALYSIS**

More Later,

Janice