Now that you are familiar with atoms and molecules and how they make up the solids, liquids, and gases we see around us, let’s discuss what happens when two molecules meet: namely, chemical reactions. A chemical reaction is defined as a process by which one or more substances are changed into one or more new substances. Chemical reactions are usually portrayed in this way:
reactant
product
The
is read as “yields” or “produces.” You will often see the states of matter in parentheses as subscripts after the chemical formulas of the reactants and products. The symbols for the states of matter and some other chemical reaction symbols you should be familiar with are given in the table below.
| Symbol |
Meaning |
 |
“Yields” or “produces” |
| + |
“Reacts with” or “and” |
| (g) |
Gaseous state |
| (l ) |
Liquid state |
| (s) |
Solid state |
| (aq) |
Aqueous state (dissolved in water) |
| number subscript |
Represents the number of atoms of the element it’s to the right of |
| number coefficient |
How many molecules or moles of the substance are reacting |
 |
A substance named above the arrow represents a catalyst in the reaction |
 |
A gas is produced |
 |
A precipitate is formed |
| kJ or J |
Energy term (kilojoules or joules) |
 |
Reversible equation; equilibrium |
 |
A delta above the reaction arrow indicates that heat is added to the reaction |
