Determine which salt crystallizes at 20 °C and calculate the mass crystallized. Define hard water and state one use of it.

For Salt K:
Amount present = 28 g
Solubility at 20 °C = 31 g
Since $28 \text{ g} < 31 \text{ g}$, Salt K will remain dissolved.

For Salt L:
Amount present = 17.5 g
Solubility at 20 °C = 8 g
Since $17.5 \text{ g} > 8 \text{ g}$, Salt L will crystallize.

• Salt that crystallizes: $\boxed{\text{L}}$

II)

Mass of Salt L initially present = 17.5 g
Maximum mass of Salt L that can remain dissolved at 20 °C = 8 g
Mass of Salt L that crystallizes = (Mass initially present) - (Mass that remains dissolved)

$$\text{Mass crystallized} = 17.5 \text{ g} - 8 \text{ g}$$ $$\text{Mass crystallized} = 9.5 \text{ g}$$

• Mass of the salt that crystallizes: $\boxed{9.5 \text{ g}}$

Hard water is water that contains a high concentration of dissolved mineral ions, primarily calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) ions. It does not readily form lather with soap.

• What is hard water: $\boxed{\text{Water that contains high concentrations of dissolved mineral ions, mainly calcium and magnesium ions, and does not easily form lather with soap.}}$

Hard water can be used in certain industrial processes or for drinking as it provides essential minerals.

• One use of hard water: $\boxed{\text{Used for brewing beer.}}$