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Reading Selection, Lesson 6

Heat and Changing Matter

You have discovered that heat affects different substances in different ways. When you heated one of the solids, it melted. Then when the substance cooled, it solidified again. Another substance turned into a gas and then turned back to a solid farther up the tube. When a substance changes from one phase to another it is called a phase change. Did any of the substances you heated exist in all three phases of matter (solid, liquid, and gas) within the test tube?

Many of the other substances changed their appearance when you heated them and did not return to their original form when they cooled. This is usually a sign that a chemical reaction has taken place. In a chemical reaction, one or more substances (called reactants) are changed into new substances (called products).

In Inquiry 6.1, the substances that underwent chemical reactions decomposed when they were heated and then formed products that did not look like the original substances. This type of chemical reaction is called thermal decomposition. Some of the substances you heated decomposed and gave off invisible gas as one of the products. Can you identify one of the substances that you heated that did this? What evidence do you have that an invisible gas may have been produced? There are many other types of chemical reactions. You will investigate some later in the module.

Some substances change in other ways when they are heated. Can you identify any of these changes? How could these types of changes be useful to people? The way a substance behaves when it is heated is a characteristic property of that substance.

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