Breaking up a solid into smaller pieces will increase its surface area. In any mass of a solid, smaller pieces will have a greater surface area-to-volume ratio than larger pieces. This means that there are more particles on the surface that can react with particles in a solution.
Let’s look at an example:
Imagine we have a solid and we put it in a solution. The particles in the solution can only react with particles on the surface of the solid.
Before – The solid is in one large piece, so many of its particles are not on the surface. Therefore, they cannot react with the solution.
After – We have divided the solid into smaller pieces, so there is now a greater surface area exposed to the solution, and more particles can react. This leads to more frequent collisions and an increase in the rate of reaction.
We can represent the effect of surface area on reaction rate using a rate of reaction graph.
The graph line for the greater surface area (powdered reactant) shows a higher rate of reaction than the line for the smaller surface area (lumps). We know this because the line for the larger surface area has a steeper gradient and it becomes horizontal faster.