Chemistry Essentials for Dummies

Chapter 1. Matter and Energy: Exploring the Stuff of Chemistry

Nice Properties You've Got There

When chemists study chemical substances, they examine two types of properties:

Chemical properties: These properties enable a substance to change into a brand-new substance, and they describe how a substance reacts with other substances. Does a substance change into something completely new when water is added — like sodium metal changes to sodium hydroxide? Does the substance burn in air?

Physical properties: These properties describe the physical characteristics of a substance. The mass, volume, and color of a substance are physical properties, and so is its ability to conduct electricity. Physical properties can be extensive or intensive:

· Extensive properties, such as mass and volume, depend on the amount of matter present.

· Intensive properties, such as color and density, don’t depend on the amount of matter present. A large chunk of gold, for example, is the same color as a small chunk of gold.

Intensive properties are especially useful to chemists because intensive properties can be used to identify a substance. For example, knowing the differences between the density of quartz and diamond allows a jeweler to check out that engagement ring quickly and easily.

REMEMBER. Density (d) is the ratio of the mass (m) to volume (v) of a substance. Mathematically, it looks like this:

d = m/v

Usually, mass is described in grams (g) and volume is described in milliliters (mL), so density is g/mL. Because the volumes of liquids vary somewhat with temperature, chemists usually specify the temperature at which they made a density measurement. Most reference books report densities at 20°C, because it’s close to room temperature and easy to measure without a lot of heating or cooling. The density of water at 20°C, for example, is 1 g/mL.

TIP. You may sometimes see density reported as g/cm3 or g/cc, both of which mean grams per cubic centimeter. These units are the same as g/mL.

Calculating density is pretty straightforward. You measure the mass of an object by using a balance or scale, determine the object’s volume, and then divide the mass by the volume.

TIP. With an irregular solid, like a rock, you can measure the volume by using the Archimedes principle. The Archimedes principle states that the volume of a solid is equal to the volume of water it displaces. Simply read the volume of water in a container, submerge the solid object, and read the volume level again. The difference is the volume of the object.