Stir salt into water one spoonful at a time and at first it dissolves easily, but from some point on the salt stays on the bottom however much you stir.
A solution that has dissolved as much as it can at that temperature is a saturated solution, and the largest mass of solute (in g) that dissolves in 100 g of water is that substance's solubility.
Solubility differs from substance to substance and also changes with temperature: potassium nitrate dissolves far more as it warms, while table salt (sodium chloride) barely changes.
Change the amount of salt and the temperature to watch the moment the beaker becomes saturated, and read the same temperature off the solubility curves to compare.
Dissolving and Saturation
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Adding salt to 100 g of water: beyond the solubility, the rest stays on the bottom
20 g
40°C
💡 Dissolving
①Salt mixing evenly into water is called dissolving.
②The substance that dissolves (salt, sugar) is the solute, the one doing the dissolving (water) is the solvent, and the mixture is a solution.
③A solution that has dissolved as much solute as it can at that temperature is saturated; if more can dissolve, it is unsaturated.
④At 20 ℃ about 36 g of salt dissolves in 100 g of water; in the picture, any amount above this sinks to the bottom.
Solubility Curves
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Solubility against temperature: a different curve for each substance
40°C
🔍 Reading Solubility Curves
①Solubility is the largest mass of solute (in g) that dissolves in 100 g of water at a given temperature.
②For most solids, solubility increases with temperature.
③Potassium nitrate has a steep curve, so its solubility changes a lot: about 32 g at 20 ℃ and about 110 g at 60 ℃.
④Sodium chloride's curve is nearly flat, changing only from about 36 to 40 g between 0 and 100 ℃.
⑤Where two curves cross, the two substances have the same solubility; potassium nitrate and sodium nitrate cross at about 70 ℃.
Getting Crystals from Solubility Differences
Solubility
S = solute (g)100 g water
A value fixed by the temperature and the kind of solute.
Recrystallization
saturate at high temperature → cool → the excess comes out as crystals
As the temperature drops, less can stay dissolved, and the overflow becomes crystals.
Calculating the amount crystallized
amount crystallized = solubility at high temperature − solubility at low temperature
e.g. cooling a saturated potassium nitrate solution (100 g water) from 60 °C to 20 °C gives 110 − 32 = 78 g of crystals.
🧪 How Recrystallization Works
①Dissolve plenty of the substance at a high temperature to make a saturated solution.
②Cool the solution and its solubility drops.
③The amount that can no longer stay dissolved comes out as crystals.
④Impurities are present in small amounts and stay dissolved, so the crystals are pure.
⑤It suits potassium nitrate, whose solubility changes a lot with temperature; salt, whose solubility hardly changes, is obtained by evaporating the water instead.
Exam Key Points
Summary
solids: solubility mostly rises with temperature · gases: solubility falls with temperature
Gases dissolve better at low temperature and high pressure; opening a fizzy drink lowers the pressure, so bubbles form.
🎯 Exam Points
①Solubility is the largest mass of solute that dissolves in 100 g of water at a given temperature.
②A saturated solution can dissolve no more at that temperature.
③Recrystallization uses differences in solubility with temperature to obtain pure crystals.
④Sodium chloride's solubility changes little with temperature, so it is hard to obtain by recrystallization.
⑤The solubility of gases decreases as temperature rises and increases as pressure rises.
Examples and Unit Test
Example 1
At a certain temperature, 36 g of salt dissolved in 100 g of water and no more would dissolve. What is the solubility of salt at this temperature, and what state is the solution in?
1
Solubility is the largest mass of solute (in g) that can dissolve in 100 g of water, and a solution that can dissolve no more is saturated.
2
36 g dissolved in 100 g of water and no more would, so the solubility of salt at this temperature is 36 and the solution is saturated.
▸ Solubility = 36 (g / 100 g water) / saturated
With 200 g of water, twice as much, 72 g, would dissolve; solubility is always stated per 100 g of water.
Example 2
Potassium nitrate has a solubility of 110 at 60 °C and 32 at 20 °C. How much potassium nitrate crystallizes when a saturated solution at 60 °C (100 g water) is cooled to 20 °C?
1
Cooling lowers the solubility, and the difference comes out as crystals: amount crystallized = solubility at the high temperature − solubility at the low temperature.
2
110 − 32 = 78, so 78 g crystallizes.
▸ 78 g
Under the same conditions sodium chloride gives only 37.3 − 36.0 = 1.3 g; the steeper the curve, the more comes out on cooling.
Unit-test style
Which statement about solubility is correct?
①Solubility is the maximum solute that dissolves in 100 g of solvent
②For most solids, higher temperature gives lower solubility
③A saturated solution can still dissolve more solute
④For a gas, higher temperature gives higher solubility
⑤All substances have the same solubility regardless of temperature
▸ ① Solubility is the maximum solute that dissolves in 100 g of solvent
1
Solubility is the largest mass of solute (in g) that dissolves in 100 g of solvent (water) at a given temperature, so ① is correct.
2
Most solids become more soluble as temperature rises (②), a saturated solution dissolves no more (③), gases become less soluble as temperature rises (④), and solubility depends on the substance and the temperature (⑤).