Different chemical reactions occur at different rates or speeds. Indicators of reactions are colour change, precipitate formation, release of gas, and/or a detectable energy change. Some are very slow, like a car rusting, while others are very fast like a sudden explosion.
There are four factors that affect the rate of a chemical reaction:
Increasing the temperature a reaction takes place at increases the rate of reaction. At higher temperatures, particles can collide more often and with more energy, which makes the reaction take place more quickly.
The graph below shows how changing the temperature affects the rate of reaction between an acid and chalk. The magenta line represents a faster reaction because it is steeper. Both reactions release a gas and both finish at the same volume.
If you increase the concentration of a reactant, there will be more of the chemical present. More reactant particles moving together allow more collisions to happen and so the reaction rate is increased. The higher the concentration of reactants, the faster the rate of a reaction will be.
By decreasing the particle size of a reactant, there are more surfaces that collisions can take place on. The smaller the particle sizes the faster the reaction.
This video clip has demonstrations with charcoal, gunpowder and steel wool that show the effect of particle size and concentration on reaction rate.