A chemical change indicates that a chemical reaction has occurred. A chemical equation is shorthand for expressing these reactions using symbols and formulas.

The law of conservation of mass requires that the number of atoms or moles of atoms of each element in the reaction be the same on both sides of the equation.

The substances that interact with one another (reactants), are written on the left side of the equation. The substances that are formed (products), are written on the right side of the equation. A single arrow  or a double arrow separates the reactants from the products. A plus sign ( + ) separates the formulas of reactants and the products from each other.

A catalyst is a substance that increases the rate (speed) of a chemical reaction but is recovered unchanged at the end of the reaction.

Guidelines for Balancing Chemical Equations

1. Write the correct formulas for the reactants and the products, with the products on the right and the reactants on the left separated by an arrow. Separate each formula using a + sign.

Note: Once you have written a correct formula do not change it during balancing. You may only place, place numbers (coefficients) in front of each formula to balance an equation.

2. Select an element from the formula containing the most atoms. The selected element should not be an element in a polyatomic ion, a H or an O.

Balance the number of atoms of this element by placing a coefficient in front of the appropriate formula on the other side of the equation containing the selected element.

3. Balance the polyatomic ions that exist on both sides of the equation.

4. Balance the H atoms and then the O atoms. Any O or H atoms previously balanced as polyatomics do not need to be balanced in this step.

(BALANCE HYDROGENS)

2 molecules of H₃PO₄ has 6 atoms of hydrogen and 3 molecules of Ca(OH)₂ has 6 atoms of hydrogen.  To balance the product side, 6 molecules of H₂O gives 12 atoms of hydrogen.

(BALANCE OXYGENS)

3 molecules of Ca(OH)₂ has 6 atoms of oxygen.  The oxygen molecules of in H₃PO₄ are not counted since they had previously been balanced as a polyatomic.   To balance the product side, 6 molecules of H₂O gives 6 atoms of oxygen (already balanced).  The oxygen molecules of in H₃PO₄ are not included since they had previously been balanced as a polyatomic.

5. Check for fractions.   If you have any fractions, multiply the fraction by a number to make the fraction a whole number.   Then multiple all the other coefficients by this number also.

6. Check your work.  Count the number of atoms of each element on each side of the equation to make sure the numbers match.

Combination Reactions

A combination reaction occurs when two or more substances (either elements or compounds) react to produce one substance (a compound).

Decomposition Reactions

In decomposition reactions, one substance undergoes a reaction to form two or more substances.

Single-Replacement Reactions

In single-replacement reactions, an element and a compound react in such a way that the element replaces one of the elements in the compound.   Single-replacement reactions are also called replacement, substitution, or displacement reactions.

Double-Replacement Reactions

In double-replacement reactions, two compounds are involved in a reaction, with the positive ion (cation) of one compound exchanging with the positive ion (cation) of the other compound.

Neutralization Reactions

A neutralization reaction is one in which an acid or an acid oxide reacts with a base or basic oxide. In most of these reactions, a salt and water is produced.