Wolfram Alpha:

```Balancing Chemical Equations
----------------------------

A chemical equation describes what happens in a chemical reaction.
The equation identifies the reactants (starting materials) and products
(resulting substance), the formulas of the participants, the phases of
the participants (solid, liquid, gas), and the amount of each substance.
Balancing a chemical equation refers to establishing the mathematical
relationship between the quantity of reactants and products.  The
quantities are expressed as grams or moles.

It takes practice to be able to write balanced equations. There are
essentially three steps to the process:

Write the unbalanced equation:

- Chemical formulas of reactants are listed on the lefthand side of the
equation.
- Products are listed on the righthand side of the equation.
- Reactants and products are separated by putting an arrow between
them to show the direction of the reaction. Reactions at equilibrium
will have arrows facing both directions.

Balance the equation:

- Apply the Law of Conservation of Mass to get the same number of
atoms of every element on each side of the equation. Tip: Start by
balancing an element that appears in only one reactant and product.
- Once one element is balanced, proceed to balance another, and
another, until all elements are balanced.
- Balance chemical formulas by placing coefficients in front of them.
Do not add subscripts, because this will change the formulas.

Indicate the states of matter of the reactants and products:

- Use (g) for gaseous substances.
- Use (s) for solids.
- Use (l) for liquids.
- Use (aq) for species in solution in water.

Write the state of matter immediately following the formula of the
substance it describes.

Example:

Tin oxide is heated with hydrogen gas to form tin metal and water
vapor. Write the balanced equation that describes this reaction.

- Write the unbalanced equation.

SnO2 + H2 -> Sn + H2O

- Balance the equation.

Look at the equation and see which elements are not balanced.
In this case, there are two oxygen atoms on the lefthand side
of the equation and only one on the righthand side. Correct this
by putting a coefficient of 2 in front of water:

SnO2 + H2 -> Sn + 2H2O

This puts the hydrogen atoms out of balance. Now there are two
hydrogen atoms on the left and four hydrogen atoms on the right.
To get four hydrogen atoms on the right, add a coefficient of 2
for the hydrogen gas. Remember, coefficients are multipliers, so
if we write 2 H2O it denotes 2x2 = 4 hydrogen atoms and 2x1 = 2
oxygen atoms.

SnO2 + 2H2 -> Sn + 2H2O

The equation is now balanced. Be sure to double-check your math!
Each side of the equation has 1 atom of Sn, 2 atoms of O, and 4
atoms of H.

- Indicate the physical states of the reactants and products.

To do this, you need to be familiar with the properties of various
compounds or you need to be told what the phases are for the
chemicals in the reaction. Oxides are solids, hydrogen forms a
diatomic gas, tin is a solid, and the term 'water vapor' indicates
that water is in the gas phase:

SnO2(s) + 2H2(g) -> Sn(s) + 2H2O(g)

This is the balanced equation for the reaction. ```