Students know differences in chemical and physical properties of substances are used to separate mixtures and identify compounds.

This standard raises two potentially confusing issues: 1) the difference between mixtures and compounds, and 2) the use of physical and chemical properties to identify specific compounds.

1) Two major characteristics distinguish between mixtures and compounds. In mixtures, the parts keep their distinct physical properties. Mixtures also do not have a fixed ratio of components. In contrast, the parts of a compound do not retain their physical properties. Instead, the compound generally has properties that are very different from those of the parts. The parts of a compound also combine in a very specific ratio.

COMPARING MIXTURES AND COMPOUNDS
	Physical Properties	Ratio of Components
Mixtures	In a mixture, the parts keep their physical properties.	A mixture does not have a fixed ratio of components.
Compounds	Compounds have properties that are different than those of the parts.	The parts of a compound combine in a very specific ratio.

In a mixture of iron, salt, and sand, each of the parts still has the properties it had when it exists by itself. For example, you can separate the iron filings from the other parts by using a magnet. Since it is a mixture, you can also change the amounts of each of the components as much as you want.

When hydrogen and oxygen chemically combine to form the compound water, the physical properties have changed dramatically. Both hydrogen and oxygen are gases at room temperature. Water is a liquid at room temperature. Hydrogen is explosive, and oxygen is required for all fires. Water puts out fires. Note also that water has a very specific ratio of components: two hydrogens for each oxygen.

2) The term “identify compounds” in this standard has two meanings. One is that you can identify what a compound is. The other is that you can tell which compound a substance is (e.g., distinguishing sugar, saccharine, and aspartame). As explained in the California Science Framework (page 69), “every compound has a unique set of chemical and physical properties that can be used to identify it.”

Each compound melts at a specific temperature which is the same for any pure sample of that compound. Chemists can use this temperature (called the “melting point”) to help identify compounds. For example, the following solid white compounds could all be distinguished from each other simply by heating a sample and observing the temperature at which it melts (all melting points are in degrees Celsius).

Aspirin: 139 degrees
Sugar (sucrose): 186 degrees
Vitamin C (ascorbic acid): 192 degrees
Saccharine: 229 degrees
Aspartame (artificial sweetener): 246 degrees
Salt (sodium chloride): 801 degrees