http://www.ck12.org Chapter 2. Matter and Change
and settles out of a liquid mixture. The formation of a precipitate may also indicate the occurrence of a chemical
reaction.
FIGURE 2.15
A yellow precipitate of solid lead(II) io-
dide forms immediately when solutions of
lead(II) nitrate and potassium iodide are
mixed.All chemical changes involve a transfer of energy. When zinc reacts with hydrochloric acid, the test tube becomes
very warm as energy is released during the reaction. Some other reactions absorb energy. While energy changes are
a potential sign of a chemical reaction, care must be taken to ensure that a chemical reaction is indeed taking place.
Physical changes may also involve a transfer of energy. A solid absorbs energy when it melts, and the condensation
of a gas releases energy. The only way to be certain that a chemical reaction has occurred is to test the composition
of the substances after the change has taken place to see if they are different from the starting substances.
Combustion is a chemical change. Watch as magnesium reacts with oxygen to form magnesium oxide. View this
lab-scale fireworks display at http://www.youtube.com/watch?v=EZ3JT2nWfMA (1:31).
MEDIA
Click image to the left for more content.Lesson Summary
- The composition of matter changes during a chemical reaction. The ability of a substance to react with other
substances is reflected in its chemical properties. - In a chemical reaction, the reacting substances are called reactants, and the substances that are produced by
the reaction are called products. A chemical equation shows reactants becoming products. - Four clues to a possible chemical reaction include a color change, the production of a gas, the formation of a
precipitate, and an observable transfer of energy.