How to read a solubility curve
A solubility curve shows how many grams of a solute dissolve in 100 g of water at each temperature. To read it, go up from the temperature to the curve, then across to the solubility axis. A point on the curve is a saturated solution, below it unsaturated, above it supersaturated. Readings are marked right within half a small square and close within one; worked answers are right to two or more significant figures.
Pick a course first. Regents style writes numbers as the New York exam does (100. g, 30.°C) but draws the handbook curves, not the ones on NYSED's Table G. GCSE and IGCSE sets use names with formulas. US chemistry and CBSE Class 9 also ask the mass percent of a saturated solution, as in the new NCERT Grade 9 chapter on mixtures. Choose invented curves for compounds nobody can look up.
Example: crystals of potassium nitrate
In set KN5222 (GCSE), a saturated solution of potassium nitrate in 100 g of water at 50 °C is cooled to 10 °C. The curve is at 84.2 g at 50 °C and 21.4 g at 10 °C, so 84.2 − 21.4 = 62.8 g of crystals form. In 50 g of water it would be half that, 31.4 g. Open set KN5222
Solubility of the salts used
| Substance | 0 °C | 20 °C | 40 °C | 60 °C | 80 °C | 100 °C |
|---|---|---|---|---|---|---|
| Potassium nitrate KNO₃ | 13.6 | 31.9 | 62.9 | 109.2 | 170.3 | 242.5 |
| Sodium nitrate NaNO₃ | 73.0 | 87.3 | 104.1 | 123.7 | 147.5 | 176.2 |
| Potassium chloride KCl | 27.8 | 34.0 | 40.0 | 45.8 | 51.2 | 56.4 |
| Sodium chloride NaCl | 35.6 | 35.9 | 36.4 | 37.0 | 37.9 | 39.0 |
| Ammonium chloride NH₄Cl | 29.7 | 37.5 | 45.9 | 55.0 | 65.0 | 76.2 |
| Potassium chlorate KClO₃ | 3.1 | 7.2 | 13.7 | 23.1 | 36.8 | 57.9 |
| Potassium dichromate K₂Cr₂O₇ | 4.5 | 12.2 | 26.3 | 46.4 | 70.9 | 95.7 |
| Lead(II) nitrate Pb(NO₃)₂ | 39.8 | 55.4 | 73.1 | 91.9 | 111.5 | 131.2 |
| Potassium sulfate K₂SO₄ | 7.7 | 11.0 | 14.8 | 18.3 | 21.5 | 23.9 |
| Potassium iodide KI | 127.3 | 143.9 | 160.4 | 176.2 | 191.5 | 206.7 |
| Lithium sulfate Li₂SO₄ | 35.7 | 34.4 | 33.3 | 32.5 | 31.6 | 30.9 |
| Sulfur dioxide SO₂ | no data | 10.6 | 5.6 | no data | no data | no data |
From the CRC Handbook (mass percent w, converted as 100w ÷ (100 − w)). Sulfur dioxide is at 1 atm of the gas, 5 to 55 °C. Lithium sulfate is a solid whose solubility falls a little.
Questions
How do you read a solubility curve?
Find the temperature on the bottom axis, go straight up to the salt's curve, then straight across to the left axis. That number is the solubility: the grams that dissolve in 100 g of water at that temperature. To go the other way, start at the solubility, go across to the curve, then down.
How do you tell if a solution is saturated, unsaturated or supersaturated?
Scale the mass of solute to 100 g of water, then mark that point at its temperature. On the curve it is saturated. Below the curve it is unsaturated, so more can dissolve. Above the curve it is supersaturated, which only lasts until a crystal or a knock starts crystallization.
How do you work out the mass of crystals formed on cooling?
Read the solubility at the starting temperature and at the cooler one. The difference is the mass that crystallizes from 100 g of water. For 50 g of water, halve it; for 250 g, multiply by 2.5. Salts that crystallize with water of crystallization, such as copper sulfate, need a different calculation, so they are not asked here.
How do you find the solubility in 50 g of water?
Curves give grams per 100 g of water, so scale by the mass of water: multiply by 50 and divide by 100. If 64 g of a salt dissolves in 100 g of water, 32 g dissolves in 50 g. To find the water needed for a mass of salt, use water = 100 × mass ÷ solubility.
Why does a gas get less soluble as water warms?
Dissolving a gas gives out heat, and warmer water gives the gas molecules more energy to escape from the liquid. That is why the sulfur dioxide curve falls while most salt curves rise, and why warm fizzy drinks go flat faster.
Why are these numbers a little different from my textbook's graph?
Textbooks and exam boards, including the New York Regents reference table, draw their own curves and round them, so a reading can differ a little. These curves pass through the CRC Handbook values, which use the evaluated IUPAC Solubility Data Series where it exists. In an exam, read the graph you are given.
Can I give my class the same questions?
Yes. Share with class copies a link with the set code, and everyone who opens it gets the same curves and numbers. To stop copying, print a class set: each copy asks the same kinds of question in the same order with different salts and numbers, and each has its own key.
Sources, read October 27, 2026: CRC Handbook of Chemistry and Physics, 91st edition, section 8, tables “Aqueous Solubility of Inorganic Compounds at Various Temperatures” and “Solubility of Selected Gases in Water”; spot-checked against the IUPAC Solubility Data Series (volumes 12, 30 and 47, NIST online copies); Pearson Edexcel International GCSE Chemistry specification 1.5C to 1.7C; NCERT Exploration, Grade 9, chapter 5; NYSED Regents chemistry exams.