General Chemistry · Solutions · SOLUBILITY CURVES OF IONIC SOLIDS
Solubility Curves and Crystallization on Cooling
Use the solubility curves of nine ionic compounds. Change the mass dissolved in 100 g of water and the temperature,
decide whether the solution is unsaturated, saturated or supersaturated, and read from the curve how many grams of solid crystallize when it is cooled.
Learning objectives After this activity you can …
read the solubility at a given temperature from a solubility curve and decide the state of the solution (unsaturated / saturated / supersaturated).
calculate the mass of crystals formed on cooling from the difference between the mass dissolved and the solubility.
compare the shapes of the solubility curves of several substances and choose a substance suitable for recrystallization.choose a substance suitable for recrystallization, using the fact that the temperature dependence of solubility differs from substance to substance.
Estimated time 20 min · Prerequisites: definition of solubility, saturated solution, mass calculations · Next up: supersaturated solutions and seed crystals, Gas solubility and Henry's law
Solubility is the maximum mass of solute that can dissolve in 100 g of water at a given temperature.
Comparing the mass dissolved with the solubility at that temperature tells you whether the solution is unsaturated, saturated or supersaturated.
The mass of crystals formed on cooling is set by the difference between the mass first dissolved and the solubility at the lower temperature.
Solubility curves vs temperature
Click or drag on the canvas, use the arrow keys (left/right: temperature, up/down: mass dissolved), or the sliders belowT=— · dissolved —
Readout
The solubility, state and crystal mass readouts open after you lock in a prediction.
Substance—
Solubility S(T) · reference curve—
Literature check—
State—
Crystals at equilibrium—
Saturation temperature—
Limits Data sources and limits of the model
The curve values are digitized from a reference solubility-curve graph; the reading uncertainty is about ±1 g. The literature value at the same temperature (CRC Handbook 91st ed., 2010, pp. 8-112 to 8-117, mass percent converted to g per 100 g of water) is shown in the readout, and differences above 5% are marked. Ranges above 5%: K₂Cr₂O₇ at 20 ℃ and above (reference curve 12–19% lower; 0.9 g higher at 0 ℃), KNO₃ at 10–20 ℃ (8–9% lower), Pb(NO₃)₂ at 0 ℃ (8% lower), NaCl at 0 ℃ and KCl at 0–30 ℃ (5–6% lower), KClO₃ at 10–20 ℃ (6–8% lower, under 1 g). Ce₂(SO₄)₃ is not in the CRC 91st ed. table (pp. 8-112 to 8-117; the only cerium compound listed is Ce(NO₃)₃, rechecked 2026-10-02), so only the reference curve is used and it is not used for quantitative grading. The crystal mass assumes that crystals form until equilibrium; in reality a supersaturated solution can persist for a while. Crystal masses are for the anhydrous salt. When crystals take water of crystallization with them, as for CaCl₂ (CaCl₂·6H₂O below 30 ℃) or Ce₂(SO₄)₃ (hydrates), the real crystal mass is larger and less water remains.
Entry check Before you start — prerequisites and key terms
At most 36 g of a solid dissolves in 100 g of water at 20 ℃. What is this "maximum 36 g" called?
SolubilityMolarityBoiling-point elevation
Key terms
Solubility: the maximum mass (g) of solute that can dissolve in 100 g of solvent at a given temperature. ·
Saturated solution: a solution that holds as much solute as the solubility allows; no more dissolves. ·
Unsaturated solution: a solution that holds less solute than the solubility. ·
Supersaturated solution: an unstable solution that holds more solute than the solubility. ·
Crystallization · recrystallization: dissolved solute coming out as a solid, and the purification method based on it.
Predict Think first, before you observe
A saturated solution of Ce₂(SO₄)₃ (cerium(III) sulfate) in 100 g of water, with no solid at the bottom, is heated from 20 ℃ to 80 ℃. What happens?
Even more solid can dissolve — the solubility increasesPart of the dissolved salt comes out as a solid — the solubility decreasesNothing changes — the solubility stays the same
ConfidenceMedium
—
—
The explanation opens after you lock in a prediction
Observation task: after predicting, choose Ce₂(SO₄)₃ and change the temperature
0 ℃100 ℃
0 g100 g / 100 g water
Temperature and the solubility of ionic solids
For most compounds in the figure the solubility increases with temperature, but for Ce₂(SO₄)₃ it decreases. In a saturated solution, the temperature dependence of solubility is linked to the heat taken in or given out on dissolving (Le Chatelier). If dissolving is endothermic, more dissolves on heating; when ion hydration releases a lot of heat, as for Ce₂(SO₄)₃, less dissolves on heating. The NaCl curve is almost flat, so little crystallizes on cooling.
Fertilizer salts such as KNO₃ show why a concentrated fertilizer solution can form crystals in a cold tank.
crystals formed = mass dissolved − S(lower temperature)
Meaning of points above and below the curve
A point below the curve is unsaturated (more can dissolve); a point on the curve (within 0.5 g) is saturated; a point above the curve holds more than can dissolve and is supersaturated. A seed crystal or a shock makes the extra amount crystallize, and the solution returns to saturation. This simulation calculates that equilibrium crystal mass. A substance with a large solubility difference (KNO₃) suits recrystallization by cooling; NaCl, with a small difference, is usually obtained by evaporation.
saturation temperature: the temperature at which S(T) = mass dissolved
Check Formative check — pass all items to complete
Q1—
Hint
—
Q2 · Hands-on task—
Current state: —
Q3 · Calculation—
Hint
First check that everything dissolved at the starting temperature. Then read the solubility at the lower temperature from the curve and subtract it from the mass dissolved. Reading tolerance ±1.5 g.
Q4 · Fill in the blank—
Hint
—
Q5 · Apply (higher-order, not required for completion)KNO₃ and NaCl solutions, each saturated at 50 ℃ in 100 g of water, are cooled to 10 ℃. Compare the crystal masses using the curves.
KNO₃ gives tens of grams, far more than the few grams of NaClBoth have a large solubility at 50 ℃, so the crystal masses are similarNaCl gives more
Hint
The crystal mass is the difference in solubility between the two temperatures. Read the 50 ℃ and 10 ℃ values on each curve and subtract.
Reflect Reflect — metacognition · completion
My prediction: Not predicted yet
In one sentence, answer: "Does a solid always dissolve more when it is hotter?"
ApplyThe solubility of lithium sulfate, Li₂SO₄, is about 35.7 g per 100 g of water at 0 ℃ and about 30.9 g at 100 ℃ (CRC 91st ed.). What happens when a Li₂SO₄ solution saturated at 20 ℃ is heated to 90 ℃?
Part of it comes out as a solid — the same type as Ce₂(SO₄)₃It becomes unsaturated, so more can dissolve — the same type as KNO₃Temperature has no effect
Hint
Compare the solubilities at the two temperatures. After heating, is the maximum that can dissolve larger or smaller than the amount now dissolved?
COMPLETE · Completed — all checks passed.
Worksheet — Solubility curves and crystallization on cooling
Name ________ Class ________ Date ________
1. Predict — On the graph above, sketch the curve you expect for Ce₂(SO₄)₃ (0–100 ℃), and write what happens when a solution saturated at 20 ℃ is heated to 80 ℃.
2. Observe — — crystals = ______ g
3. Conclusion — Use the shape of the curve to explain why NaCl is hard to obtain by recrystallization on cooling.
4. Fill in the blank — —
Answer key (teacher only · not on the student print) — 1. a curve that falls from about 17 g at 0 ℃ to about 2.6 g above 30 ℃; on heating, dissolved Ce₂(SO₄)₃ comes out as crystals · 2. — · 3. the solubility changes by only about 7 g over 0–100 ℃, so almost nothing comes out on cooling (it is obtained by evaporation) · 4. —
Teacher notes — open the lesson plan
Lesson plan · 5E phase and time
Explore · Elaborate stage, 20 min. Right after the topic "temperature effects on solubility". Whole-class demo (?mode=teacher) or individual work.
Demo in 3 steps
(1) Dissolve 80 g KNO₃ at 50 ℃ and cool to 20 ℃ — match the crystal mass in the readout with the vertical gap to the curve.
(2) Do the same with NaCl — explain the small crystal mass with the slope.
(3) Heat a saturated Ce₂(SO₄)₃ solution — crystals form the moment the point crosses the falling curve.
3 key questions
"What kind of solution is a point above the curve, in reality?"
"What does the curve look like for a substance that is good for recrystallization?"
"If a substance forms crystals when heated, what can you say about the heat of its dissolving process?"
Common misconceptions
MC-sol-01 "The solubility of every solid increases with temperature, without exception" → counterexamples Ce₂(SO₄)₃ and Li₂SO₄.
MC-sol-02 "Crystal mass = solubility at the lower temperature" → the crystal mass is the difference from the mass dissolved.
Prerequisites · data
Definition of solubility, mass relationships in solutions, enthalpy of solution.
Digitized reference curve compared with CRC 91st ed. (20 ℃ / 60 ℃, g per 100 g water):
Second check for KClO₃: the IUPAC-NIST Solubility Data Series vol. 30 (evaluators H. Miyamoto, M. Salomon) recommended values agree with CRC within 3% (0–80 ℃); at 100 ℃ the two sources differ by 9% (52.9 g vs 57.9 g). K₂Cr₂O₇ shows a figure–literature mismatch (12–19%), so it is not used in quantitative questions. The absence of Ce₂(SO₄)₃ from CRC 91st ed. was rechecked on 2026-10-02 by searching all 6 pages of the table (cerium: only Ce(NO₃)₃). The other compounds are checked against CRC only; the CRC values for NaCl, KCl and Pb(NO₃)₂ are taken from the IUPAC vol. 47 and Clever–Johnston (1980) evaluations.
University general chemistry · SolutionsGeneral ChemistryEngine E3 data-drivenVerified: digitized reference curve + CRC 91st ed. and IUPAC-NIST vol. 30 check, independent Python recalculation (2026-10-02)