Solubility and Solubility Curves (Paper 2)

Explained

Solubility, and reading a solubility curve

Solubility has a precise definition and a loose one, and only the precise one earns marks. It is the mass of solute that dissolves in 100 g of solvent at a stated temperature. Three parts: a mass, a fixed 100 g of solvent, and a temperature. Leaving out the temperature is the usual way to lose the mark, because solubility changes with it.

Saturated solutions

A saturated solution holds the maximum amount of dissolved solute possible at that temperature. Add more and it will not dissolve. It sits at the bottom as undissolved solid, which is how you can tell by eye that saturation has been reached.

Warm that same solution and more will dissolve, because solubility of most solids increases with temperature. Cool it and the excess comes back out of solution as crystals.

What the curve is telling you

A solubility curve plots solubility, in grams per 100 g of water, against temperature. Any point on the line is a saturated solution. Any point below the line is unsaturated, because less solute is present than could dissolve. A point above the line cannot be a stable solution at all, so if a question puts one there, something must come out as solid.

The most common calculation asks how much crystallises when a saturated solution cools. Read the solubility at the higher temperature, read it at the lower temperature, and subtract. The difference is the mass that can no longer stay dissolved.

Scaling to a different amount of solvent

Curves are always given per 100 g of solvent, and questions rarely use exactly 100 g. If the solubility is 40 g per 100 g of water at a given temperature and you have 250 g of water, multiply by 2.5 to get 100 g of solute.

Do the scaling as a separate step and write it down. Most errors in this topic are arithmetic rather than chemistry, and an examiner can award method marks for working they can follow.

Measuring solubility in the laboratory

A workable method is to weigh an evaporating basin, add a known volume of saturated solution at a known temperature, evaporate the water carefully, then reweigh to find the mass of solid left behind. Scaling that to 100 g of solvent gives the solubility.

The control that matters most is temperature, because the whole quantity is defined at a stated temperature. If a question asks how to improve the method, keeping the solution at a constant, measured temperature is usually the strongest answer.