http://www.ck12.org Chapter 2. Matter and Change
- A gas is produced.
- A solid product called a precipitate is produced.
- A change of energy is apparent, due to a change in temperature or the appearance of light such as a flame.
Mercury(II) oxide is a red solid. When it is heated to a temperature above 500°C, it easily decomposes into mercury
and oxygen gas. The red color of the reactant, mercury oxide, is gradually replaced by the silver color of the product,
mercury. The color change is one sign that this reaction is occurring. Watch this decomposition take place at http
://www.youtube.com/watch?v=_Y1alDuXm6A (1:12).
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Click image to the left for more content.When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced (Figure
2.12). The production of a gas is also an indication that a chemical reaction may be occurring.
FIGURE 2.12
Zinc reacts with hydrochloric acid to pro-
duce bubbles of hydrogen gas.When a colorless solution of lead(II) nitrate is added to a colorless solution of potassium iodide, a yellow solid
called a precipitate is instantly produced (Figure2.13). A precipitate is a solid product that forms from a reaction
and settles out of a liquid mixture. The formation of a precipitate may also indicate the occurrence of a chemical
reaction.
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