Reading the graph
There are three things the graph tells us, and all three fit what we saw in the earlier sections.
1. The steeper curve is the faster one
The powder curve is steeper at the start — in the first minute alone it climbs to 3.1 g, while the lumps curve reaches only 2.1 g. So the rate of reaction is greater in the case of the powder.
2. Both stop at the same height
Both curves level off at 4.8 g. The reason is simple: the two masses of CaCO3 used at the start were equal. So the amount that can be used up in the end is equal too.
A common mistake
Neither a catalyst nor grinding to a powder increases the amount of product. All it changes is the time taken to get that product. Both curves stopping at the same height is the evidence for this.
3. Both curves flatten out
Neither curve is a straight line. The slope is steep at the start and grows gentler afterwards. As the reaction proceeds the reactant left over dwindles — both its concentration and its surface area fall — so the rate falls away with it.
What accounts for the difference
The difference lies in surface area. The powder has the greater surface area, so the number of collisions with acid particles in unit time is greater. Hence the greater rate of reaction.