F.3Chemistry

Generally you are correct, but to put it more clearly we can interpret the answer in this way:

First of all we have to understand that a substance can either be (i) completely soluble; (ii) slightly soluble; or (iii) insoluble in water.

In case (i), the solid completely dissolve in water until the solution become saturated. In case (ii), only very small amount of solid dissolves in water, the solution become saturated at very low solute concentration. In case (iii), the solid does not dissolve in water at all.

When we add a white solid to water, and it does not dissolve, we can be sure that it is NOT completely soluble in water. Case (i) is not true.

But then how can we determine whether the solid is slightly soluble or completely insoluble?

That's the reason for having the evaporation step.

If white solid is left after solution is evaporated to dryness, that means some solid has dissolved in water. Therefore the solid is slightly soluble in water.

Only if there's nothing left behind after evaporation, we can say the solid is completely insoluble in water.

you are correct! be confident in yourself

you are correct! be confident in yourself!