The two containers below show particles that can react with each other. These particles are dissolved in water, but the water particles are not shown.
Both containers have the same volume, but the second container has a higher concentration of particles than the first one. This means that the particles in the second container are more crowded, which leads to more frequent collisions and a higher rate of reaction.
In reactions that involve gases, increasing the pressure will also make the particles more crowded. This is because increasing pressure causes the gas particles to be closer together. As a result, the particles collide more frequently, which increases the rate of reaction.
We can represent the effect of concentration or pressure on reaction rate using a rate of reaction graph.
The reactant at a higher concentration (solution) or higher pressure (gas) shows a higher rate of reaction than the reactant at a lower concentration or lower pressure. We know this because the line for the reaction at a higher concentration or higher pressure has a steeper gradient and it becomes horizontal faster.