Solubility and Solubility Curves

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 1.5 — Solubility in g per 100 g of solvent (bold C statement — Paper 2 only)
• 1.6 — Plotting and interpreting solubility curves (bold C statement — Paper 2 only)
• 1.7 — Practical: investigate the solubility of a solid in water at a specific temperature (bold C statement — Paper 2 only) (required practical)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Stir a spoon of sugar into tea and it vanishes. Stir in another, and another, and at some point it stops vanishing — a layer sits at the bottom however long you stir. Nothing has gone wrong with that sugar. What has happened is that the solution is full, and a full solution is called saturated. Dissolving is not a yes-or-no property of a substance; it is a quantity, with a limit, and everything in this lesson is about knowing where that limit is.

What 'full' actually means, at particle level

1. For a solid to dissolve, water molecules have to surround its particles and separate them from the rest of the solid.
2. There is only a fixed number of water molecules in a given volume, and each solute particle needs a share of them.
3. As more solute dissolves, fewer water molecules are free to do the surrounding.
4. Eventually particles are returning to the solid as fast as they are leaving it. Dissolving has not stopped — it has reached a balance, and no further NET dissolving happens.
5. The solution is now saturated: it holds as much of that solute as it can at that temperature.

That last phrase is doing a lot of work, and it forces a decision about how solubility is quoted. Saying '36 g of salt dissolved' tells you nothing — in a bathtub that is barely anything and in a teaspoon it is impossible. The mass of solute only means something alongside the mass of solvent it dissolved in. So solubility is always given for a FIXED amount of solvent: grams of solute per 100 g of water. It is a rate, not a total.

Why heating helps, and why cooling gives you crystals back

1. Warmer water molecules have more kinetic energy and move faster.
2. They separate solute particles more effectively, so more solute can be held before the balance point is reached. For most solids, solubility rises with temperature.
3. Now saturate a solution at a high temperature and cool it down. The solubility falls — but the solute is already in there.
4. The solution now holds more solute than it can, so the excess has no choice but to come out of solution as solid.
5. It comes out slowly and in an orderly way, which is why it forms crystals. This is exactly how crystallisation is used to purify a solid.

A solubility curve is simply a map of that limit: temperature along the bottom, grams per 100 g of water up the side. Reading it is three ideas. A point ON the curve is a saturated solution. A point BELOW it is unsaturated — there is room for more. A point above the curve cannot exist for long, because the solution is holding more than its limit and the surplus will crystallise out. And that gives you the most useful trick in the topic: to find how much solid appears when a solution is cooled, read the solubility at the top temperature, read it at the bottom temperature, and take the difference — then scale it to however much water you actually have.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Solubility

The mass of a solute that dissolves in 100 g of a solvent to form a saturated solution at a stated temperature. Quoted in g per 100 g of solvent, and the temperature must always be stated.

Learn this definition · Saturated solution

A solution that contains as much dissolved solute as it can at that temperature, in the presence of undissolved solute, with no further net dissolving taking place.

solubility = (mass of solute ÷ mass of water) × 100

mass of solute and mass of water both in grams, at a stated temperature

Units: g per 100 g of water. Rearranged: mass of solute = (solubility × mass of water) ÷ 100.

Reading a solubility curve — the three positions and what each means

• ON the curve: the solution is saturated at that temperature
• BELOW the curve: the solution is unsaturated — more solute could still dissolve
• ABOVE the curve: the solution holds more than its limit, so the surplus crystallises out
• Mass crystallising on cooling = (solubility at the higher temperature − solubility at the lower temperature), then scaled to the mass of water actually present
• A curve that rises steeply means a large yield of crystals for a small drop in temperature

Calculating the mass of crystals formed on cooling

1. Read the solubility at the starting temperature, in g per 100 g of water.
2. Read the solubility at the final temperature.
3. Subtract to get the mass that can no longer stay dissolved, per 100 g of water.
4. Multiply by (mass of water present ÷ 100) to scale it to the actual solution.

Required practical 1.7 — measuring the solubility of a solid at a given temperature

1. Add solute to water in a boiling tube and stir at the chosen temperature until no more dissolves and some solid remains undissolved, so the solution is certainly saturated.
2. Hold the tube at that temperature in a water bath and allow it to stand, so the solution reaches equilibrium with the excess solid.
3. Weigh a clean dry evaporating basin.
4. Decant or filter a sample of the saturated solution into the basin, taking none of the undissolved solid, and weigh the basin and solution.
5. Evaporate the water carefully, then dry the residue, cool and reweigh. Reheat and reweigh until the mass is constant.
6. Mass of solute = final mass − empty basin. Mass of water = solution mass − solute mass. Solubility = (solute ÷ water) × 100.

Variables

Independent (changed) — temperature of the water
Dependent (measured) — mass of solute dissolved per 100 g of water

Sources of error

Model answer [3 marks]

A saturated solution of potassium nitrate is prepared at 60 °C and then cooled to 20 °C. Explain why crystals form. [3]

At 60 °C the solution holds as much potassium nitrate as it can at that temperature. Solubility decreases as temperature falls, so at 20 °C the solution can hold less than it already contains. The excess solute can no longer stay in solution, so it comes out as solid and forms crystals.

Model answer [2 marks]

Explain why solubility must always be quoted per 100 g of water and at a stated temperature. [2]

A mass of dissolved solute means nothing on its own, because the same mass could be dissolved in any amount of water — so it is fixed to 100 g of solvent to make different substances comparable. The temperature must be stated because solubility changes with temperature, so one substance has a different value at every temperature.

Mark-losing trap. Solubility is g per 100 g of WATER, not per 100 g of solution. The water is weighed separately from the solute.

Mark-losing trap. A saturated solution is not one where nothing dissolves. Dissolving continues — it is exactly balanced by solute coming back out.

Mark-losing trap. Always state the temperature with a solubility. A value without one is not an answer, because it changes at every temperature.

Mark-losing trap. Cooling does not stop solute dissolving; it lowers the limit. The surplus already in solution is what crystallises out.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — What is meant by a saturated solution?
2. Grade 7 · Calculate [2 marks] — At 25 °C, 18 g of a salt dissolves in 50 g of water to make a saturated solution. Calculate the solubility of the salt at 25 °C, in g per 100 g of water.
3. Grade 8 · Explain [4 marks] — A saturated solution of a salt is made in hot water and then left to cool. Crystals appear at the bottom of the beaker. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — The solubility of potassium nitrate is 110 g per 100 g of water at 60 °C and 32 g per 100 g of water at 20 °C. A saturated solution is made using 250 g of water at 60 °C and then cooled to 20 °C. Calculate the mass of crystals formed.
5. 9+ · Evaluate [5 marks] — Two students measure the solubility of a salt at 40 °C. Student A samples the solution as soon as the last visible crystal disappears. Student B keeps adding salt until some remains undissolved, leaves the tube in the water bath for twenty minutes, then filters and samples. Student A's value is lower than Student B's, and neither student's tube was allowed to cool. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who used tiny differences in solubility to find a new element, and the one who spent years on what happens when something dissolves. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Marie Curie — the one who used tiny differences in solubility to find a new element

Curie isolated radium from tonnes of pitchblende by fractional crystallisation — dissolving, cooling, collecting crystals and repeating the cycle thousands of times, because radium and barium salts differ only very slightly in solubility. This lesson's curve is the entire basis of that method, and almost nobody has ever relied on it harder.

• “How can a very small difference in solubility be used to separate two substances?”
• “Why did the same crystallisation have to be repeated thousands of times?”
• “How did you know a fraction was becoming purer rather than just smaller?”
• “What made you certain there was a new element in the residue at all?”

Dmitri Mendeleev — the one who spent years on what happens when something dissolves

Long before the periodic table, Mendeleev's doctoral research was on solutions, and he argued that dissolving is not simple mixing but involves real interaction between solvent and solute. That is the argument underneath this lesson's particle explanation of saturation, and he pursued it for years against people who thought solutions were merely stirred-together substances.

• “Is dissolving just mixing, or is something more happening between the particles?”
• “Why does a solution reach a limit instead of taking solute for ever?”
• “What did measuring solutions so carefully tell you that guessing could not?”
• “Why does warming water let it hold more of a dissolved solid?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Joseph Louis Gay-Lussac in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Electronic Configurations and the Periodic Table
• Next lesson: Formulae and Balanced Equations
• The Three States of Matter — What the particles are doing in a solid, a liquid and a gas — and what really changes when ice melts.
• Pure Substances, Mixtures and How to Separate Them — Why a pure substance melts at one temperature, and which separation technique to reach for when.
• Inside the Atom — Protons, neutrons and electrons — where they are, what they weigh, what they carry, and how isotopes give an element a decimal mass.
• Electronic Configurations and the Periodic Table — Why the table has the shape it has: the group number IS the outer-shell electron count, and everything follows from that.
• Formulae and Balanced Equations — Why the ash weighs more than the metal, what balancing an equation is really doing, and how to add up a formula mass without losing a bracket.
• All of Chemistry · Incandio Science