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You may have noticed it...maybe you didn't...we pulled another verbal "fast one". Here's what we said: "All pure substances are either elements or compounds. This adds another entry to our flow chart (see below). Just as you can make a mixture by mixing together two or more pure substances, you can make a compound by combining elements together." We said you MIX pure substances together to get a mixture, but that you COMBINE elements to form a compound. As always, we choose our words carefully to mean something very precise. When we say elements are combined to form a compound we mean that they are CHEMICALLY COMBINED. On the other hand the process of mixing is called a PHYSICAL process. Compounds are pure substances which can be altered only by means of a chemical change. To break up a compound into its component elements is an example of such an alteration, and so is also considered to be a chemical change. Mixtures, on the other hand, can be separated into their component pure substances by means of physical changes. There is one and only one kind of chemical change: it is called a chemical reaction, and it is central to the study of chemistry. There are many different kinds of physical changes: evaporation, dissolution, and freezing to name a few. Some examples: A mixture of sand and iron filings could be separated using a magnet. The sand/iron mixture would have undergone a physical change. A solution of salt water could be separated into salt and water by allowing the water to evaporate, another physical change. When you make ice cubes, the dissolved gases are expelled (forming those tendril-like veins in the center), again a physical change. On the other hand, there is no way that you can make a compound simpler by a physical change. Sucrose (sugar) is something that we eat. Our cells first use a chemical change to alter it to glucose, and then metabolize it giving us energy and two entirely new compounds: carbon dioxide and water. This contrast illustrates the major difference between a physical and chemical change. In a physical change, the substances maintain their identity--if you dissolved sugar in water, and then evaporate the water, you get back the sugar, unchanged. Sugar water is a mixture and not a compound. But sugar itself is a compound, because there is no way to "separate out" the things that make it up without destroying the sugar. Put another way, a chemical change involves the breaking and forming of chemical bonds, whereas a physical change does not. Of course, we have not yet officially covered the idea of a chemical bond, and so this last sentence might not make sense. If not, don't worry...it should make sense by the end of the day.

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