Four changes affect how fast a reaction goes. Increasing the temperature increases the rate. Increasing the concentration of a solution increases the rate. Increasing the pressure of reacting gases increases the rate. Using a solid in smaller pieces, which gives it a larger surface area, increases the rate.
The opposite changes slow the reaction down. A lower temperature, a more dilute solution, a lower pressure or larger lumps of solid all make a reaction slower.
You can investigate surface area with marble chips and hydrochloric acid. Marble reacts with the acid to give off carbon dioxide gas. If you use the same mass of marble in each experiment, lumps react more slowly than small chips, and powdered marble reacts fastest of all. You can follow each reaction by measuring the volume of gas given off or the loss in mass over time.
You can investigate temperature by reacting magnesium with acid at different temperatures and timing how long the magnesium takes to disappear. The warmer acid gives a shorter time, which means a higher rate. In each investigation only one factor is changed.