The rate of a chemical reaction is a measure of how fast the reactants are being used up and how fast the products are being made.
Reactions in which a gas is produced can be used to monitor the rate.
For example, hydrogen gas is one of the products released when dilute hydrochloric acid reacts with zinc metal.
By collecting the hydrogen gas that is produced over water or in a syringe, rate graphs can be produced. The volume of gas produced and the time taken need to be recorded.
The rate of the same reaction could be monitored by measuring the change in the mass of reactants as they react to form products.
If the reaction was set up on a balance as shown, the mass of the apparatus can be monitored and recorded at time intervals throughout the reaction. As hydrogen bubbles escape, the apparatus will lose mass.
In chemistry, graphs can be used to follow the course of a reaction. A graph can tell us many things about a reaction.
The graph below shows two similar reactions.
The magenta line has a steeper gradient and represents conditions favouring a faster reaction than the green line. When the reaction is finished (the end-point) the graph goes flat as no more products are being produced.