Concept:A solubility curve shows how the solubility of a solute changes with temperature.
Explanation:The curve helps us read the solubility of a substance at different temperatures.
When a hot saturated solution is cooled, its solubility decreases.
As a result, the amount of solute that was dissolved at the higher temperature becomes more than the solution can hold at the lower temperature.
This excess solute separates out from the solution as solid crystals.
By comparing the solubility values at the two temperatures, we can calculate the mass of solute that has crystallised.
Hence, a solubility curve is mainly used to determine the amount of crystals formed during cooling.
Answer:A. amount of crystals formed