Excess (too much reactant)

This reaction represents a hydrogen molecule and a chlorine molecule reacting to form a hydrogen chloride molecule. Using nuts and bolts students can animate their own reactions. Click to see an 80kb video.
The reaction on the left represents the reaction between hydrogen and chlorine gases to form the strong acid, hydrochloric acid. All the reactants are in the right proportion, they will all be used up.
The equation on the right represents a more complex chemical reaction. Two nuts, two washers and two bolts form two units of product.
Looking at the reactants on the left it is obvious that some reactants are in excess. The reactant that will be used up is the bolt. Both bolts will be used up and therefore the bolts are known as the limiting reactant.
Two units of product (bolts+nuts+washers) are formed. The excess reactants (4 nuts and 2 washers) remain unreacted.