Solution, solubility and saturation

Solubility is the maximum solute that dissolves in a fixed amount (100 mL) at a given temperature. For solids, solubility usually increases with temperature, but for gases it decreases as the liquid warms.

In real life: More sugar dissolves in hot tea than in cold water, while a warm fizzy drink loses its gas faster because gas solubility falls.

Worked example 1

Question. In salt water, name the solute and the solvent.

Answer: Salt is the solute and water is the solvent.

Density

Mass is measured with a weighing balance and volume with a measuring cylinder. Density usually decreases as a substance is warmed, because it expands. Density decides whether something floats or sinks.

In real life: A small stone sinks while a large log floats, because the log’s density is less than water’s, not because of size.

Worked example 2

Question. A block has mass 200 g and volume 100 cm³. Find its density.

Answer: The density is 2 g/cm³.

Worked example 3

Question. Why does more sugar dissolve in hot water than in cold?

Answer: Because the solubility of a solid usually rises with temperature, so hot water holds more sugar.