1.6 Interpreting Solubility Curves

Prior knowledge: You should understand what a solution is, what it means for a solution to be saturated, and the definition of solubility.

Interpreting a solubility curve

This is simply a matter of being able to read the graph correctly and being able to relate what the graph is telling you to the question you are being asked.

This is best learnt by example:

1)  What is the lowest temperature at which we can dissolve 20g of the solute in 100g of water?

SolCurveQ1

We start by finding 20g/100g of water on the y-axis and read across the graph to find the solubility curve. We then read down to the x-axis, to find that the temperature at which the solution would be saturated. This is the lowest temperature at which 20g of solute could be dissolved.

Answer: 60°C

2)  How much solute will dissolve in 100g of water at 40°C?

SolCurveQ2

We start by finding 40°C on the x-axis, and reading up until we get to the solubility curve. We then read across to the y-axis to find the mass of solute dissolved in 100g of water when the solution is saturated. This is the maximum amount that will dissolve in 100g of water at this temperature.

Answer: 15g

3)  How much solute will dissolve in 50g of water at 60°C?

SolCurveQ3

The key to answering this question is to realise that the amount of solute that dissolves in 50g of solvent will always be exactly half of the amount that dissolves in 100g of solvent.  From the graph we can see that at 60°C we can dissolve 20g of solute in 100g of water. Therefore, we can dissolve half this amount in 50g of water.

Ans: 10g

4)  What is the lowest temperature required to dissolve 50g of solute in 200g of water?

As mentioned in Q3, whatever amount of solute will dissolve in 100g of water, twice that amount will dissolve in 200g of water.

SolCurveQ4

So 50g will dissolve in 200g of water at exactly the same temperature as 25g will dissolve in 100g of water. We can read that off the graph, finding 25g on the y-axis, and reading across then down to find the temperature.

Answer: 73°C

5)  25g of solute is dissolved in 100g of water at  90°C. The solution is then cooled to 20°C (room temperature). What would you observe? Would the solution still be saturated?

SolCurveQ5

Reading up from 90°C on the x-axis and across from 25g per 100g of water on the y-axis, we can find the starting point on the graph (marked X). The solution contains 25g of solute, all dissolved, and isn’t saturated.  As the temperature decreases and the solution cools, we move along horizontally on the graph until we reach the curve. At this temperature, 73°C, the solution is saturated. If we continue to cool the solution, crystals will form as the mass of solute that can stay dissolved will get less.  At 20°C we can see that 9g of solute will remain dissolved in the solution. This means all the rest of the solute (25g  – 9g = 16g) will have appeared as crystals. The solution will still be saturated, though, and any more cooling will produce more crystals.