Chemical equations are ways that we represent chemical reactions. If you think back to Math, you could represent a problem either with words or with numbers. For example we could say, "Two plus three equals five" or we could say, "2 + 3 = 5." In chemistry, we can represent a chemical reaction either with words or with symbols. Most chemists prefer symbols when writing because the symbols are easier to read and write.

´╗┐Parts of an Equation

Chemical equations are like mathematical equations in that they involve things being added together on one side to create one or more things on the other side. Our example is "A + B --> C." This is not a real equation, but we use it to represent real equations because it works the same, we're just using A, B, and C to represent some unknown chemicals. You may notice that instead of an equals sign there is an arrow. Because chemical equations represent reactions (which are chemical changes) there is a definite 'before' and 'after' and therefore we use an arrow. In math, an equals sign works both ways but in chemistry, the reaction only happens in the direction that the arrow says. In the case of our example, A and B can make C but C cannot make A and B. Unlike math, where the two sides do not have names, we call everything before the arrow "reactants" and everything after the arrow "products." Reactants are the things that go into a reaction (notice the words look similar) and products are what are produced from the reaction (so they come after). In our case, A and B are the reactants and C is the product.

Below are some examples of chemical equations. Can you identify the reactants and products?
  • Ca + O --> CaO
  • 2H2 + O2 --> 2H2O
  • 2Li + Cl2--> 2LiCl
  • 2PCl5 --> 2P + 5Cl2

Balancing Chemical Equations

Chemical equations need to be balanced. That means that you need the same number of things on one side of the arrow as the other side of the arrow. As you know from chemical compounds, the subscript, or small number, tells you how many atoms of something you have in a molecule. The number in front of a substance is called a coefficient and it tells you how much of that substance you have. Below is a list showing how these all work together.
  • X by itself means one atom. (1 x 1 = 1)
  • 2X means that you have two units of one atom each, or two atoms. (2 x 1 = 2)
  • X2 means that you have one units of two atoms, for two atoms total. (1 x 2 = 2)
  • 2X2 means that you have two units of two atoms each, for a total of four atoms. (2 x 2 = 4)
The rules for balancing an equation, or figuring out how many things are on each side are that you cannot change the subscript, or small number, you can only change the coefficient (the number in front). This is like having a box of macaroni and cheese at the store: you cannot change what's inside the box, you can only change how many boxes you buy.
The reason that you have to balance equations is because of the Law of Conservation of Mass. Remember that chemical equations are symbols we use to tell us about chemical reactions. The Law of Conservation of Mass says that matter cannot be created or destroyed, so if we have a 10 grams of stuff to start with, we must have 10 grams of stuff to end with. Balancing equations tells us how many things are involved in the reaction at once. In the equation "A + B --> C" one atom of A and one atom of B react to make one molecule of C (where C is really AB). So if we had 10 atoms of A and 5 atoms of B, we could only make 5 molecules of C before we ran out. On the other hand, the equation "2A + B --> 2C" tells us that we need two A's to react with every B to make two C's. If we had 10 A's and 5 B's, they would react to make 10 C's. We can use this skill of balancing equations to figure out how much stuff we need if we wanted to make a certain amount of something.