Solubility, defined
To compare how well two solutes dissolve, the comparison has to be fair: the same amount of the same solvent, at the same temperature. Fix those and the amount that dissolves becomes a number worth quoting.
Solubility
the maximum mass of a solute that dissolves in 100 g of a solvent at a given temperature
measured in grams, and only meaningful with the temperature stated
Three things the definition pins down
Leave any one of them out and the figure means nothing.
- A fixed mass of solvent — 100 g, so two solutes can be compared
- A stated temperature, because the answer changes with it
- The maximum mass, which is to say the solution is saturated
Two solutes at 25 °C
Magnesium chloride
The solubility of magnesium chloride in water at 25 °C is 53.0 g.
Potassium sulphate
At that same temperature the solubility of potassium sulphate in water is 12.0 g.
Both figures are for water, and both are for 25 °C. That is what makes them comparable, and comparing them says magnesium chloride is more than four times as soluble as potassium sulphate.
Always quote the temperature
A solubility without a temperature attached is an incomplete answer. Change the temperature and the number changes, which is exactly what the next skill is about.