Chapter 8
Objective 8-1-1 List three observations that suggest that a chemical reaction has taken
place.
A chemical equation represents, using symbols and
formulas, the identities and relative amounts of the
reactants and products in a chemical reaction.
2H2O (l) 2H2 (g) + O2 (g) (Balanced)
To know for certain that a chemical reaction has taken
place requires evidence that one or more substances have
undergone a change in identity.
Absolute proof of such a change can be provided only by chemical analysis of the
products. However, certain easily observed changes usually indicate that a chemical
reaction has occurred.
Possibly a reaction has Occurred when:
1. Evolution of heat and light. A change in matter that
releases energy as both heat and light is strong evidence
that a chemical reaction has taken place.
2. Production of a gas. The evolution of gas bubbles when
two substances are mixed is often evidence of a chemical
reaction.
3. Formation of a precipitate. Many chemical reactions take
place between substances that are dissolved in liquids. If a
solid appears after two solutions are mixed, a reaction has
likely occurred.
Precipitate: a solid that is produced as a result of a chemical
reaction in solution and that separates from the solution.
4. Color change. A change in color is often an indication of
a chemical reaction.
NOTE: None of these are absolute proof, only strong evidence.
