Chemical Symbols. In writing chemical equations, we use a number of
symbols. The most common symbols are shown below with their meanings.
Heat (a form of energy)
"yields," indicates direction of reaction
given off as a gas
given off as a precipitate
As we illustrate several types of reactions, the uses of these symbols will become
d. Types of Reactions. There are four types of chemical reactions, which are
possible: Combination reactions, decomposition reactions, single replacement
reactions, and double replacement reactions.
(1) Combination reactions. A combination reaction can be represented by
the chemical equation A + B --> AB (one atom of A plus one atom of B yield one
molecule of AB). A specific example of this type of reaction is the combination of a
metal with oxygen to yield a metallic oxide.
2 Mg + O2 --> 2 MgO
This equation tells us that two atoms of magnesium and one molecule of oxygen react
to form two molecules of magnesium oxide.
(2) Decomposition reactions. The general equation representing
decomposition reactions is AB → A + B. Here is a good example:
CaCO3 → CaO + CO2
This equation tells us that calcium carbonate will yield calcium oxide and carbon
dioxide. The ∆ also tells us that this reaction occurs when heat is applied to calcium
carbonate. The ↑ indicates that the carbon dioxide is given off as a gas.