2.3. Changes in Matter http://www.ck12.org
Recognizing Chemical Reactions
How can you tell if a chemical reaction is taking place? Certain visual clues indicate that a chemical reaction is
likely (but not necessarily) occurring, including the following examples:
- A change of color occurs during the reaction.
- A gas is produced during the reaction.
- A solid product, called a precipitate, is produced in the reaction.
- A visible transfer of energy occurs in the form of light as a result of the reaction.
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).
MEDIA
Click image to the left for more content.When zinc reacts with hydrochloric acid, the reaction bubbles vigorously as hydrogen gas is produced (Figure
2.14). The production of a gas is also an indication that a chemical reaction may be occurring.
FIGURE 2.14
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 (Figure2.15). Aprecipitateis a solid product that forms from a reaction