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1.1.4States of Matter

Solubility — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Section 1 of 5What Solubility Means

Solubility is the maximum mass of solute that dissolves in a stated mass of solvent at a set temperature.

Units
  • Units Solubility is usually given as grams of solute per 100 g of water, which allows the values for different substances to be compared directly.
Different substances
  • Different substances Each substance has its own solubility,
    • so some dissolve far more readily than others in the same solvent under the same conditions.
Temperature and solids
  • Temperature and solids For most solids, solubility increases as the temperature of the water rises,
    • because the extra energy helps the particles separate.
Gases
  • Gases Gas solubility falls as temperature rises and rises as pressure increases, which is why a warm fizzy drink loses its fizz faster.

Section 2 of 5Reading a Solubility Curve

A solubility curve plots solubility against temperature for a given solute.

1
Each point on a solubility curve gives the maximum mass of solute that dissolves in 100 g of water at one particular temperature.
2
To find the solubility at another temperature, read across from the temperature axis to the curve and then down to the mass axis.
3
A steep curve, as for potassium nitrate, shows that solubility changes greatly with temperature, so cooling causes large amounts of crystals to form.
4
Sodium chloride has an almost flat curve, so its solubility hardly changes between cold and hot water.
5
A mass of solute above the curve at a given temperature leaves undissolved solid, because the solution is already saturated at that temperature.

Section 3 of 5Calculating with Solubility

Calculating with Solubility
Scaling
  • Scaling If the solubility is known for 100 g of water, multiply it by the mass of water divided by 100 to find the solute for another mass of water.
Crystals on cooling
  • Crystals on cooling The mass of crystals formed equals the solubility at the high temperature minus the solubility at the low temperature, scaled to the water used.
Water mass
  • Water mass Mass of water equals mass of solution minus mass of solute, so the water mass should be found before using solubility values.
Units check
  • Units check Check whether the question gives solubility per 100 g of water or per 100 g of solution before applying any scaling step.

Section 4 of 5Worked Example

Question

  • Sodium chloride has a solubility of 36 g per 100 g of water at 20 °C.
  • Calculate the maximum mass of sodium chloride that dissolves in 25 g of water at 20 °C.

  1. Write the solubility for 100 g of water: 36 g.
  2. Scale the value to 25 g of water: 36 × (25 ÷ 100).
  3. Calculate36 × 0.25 = 9.0 g.

Answer 9.0 g of sodium chloride dissolves in 25 g of water at 20 °C.

Section 5 of 5Exam Checklist & Tips

1.1.4 – Key Points for the Exam

  • Solubility The maximum mass of solute that dissolves in a stated mass of solvent at a given temperature.
  • Solubility curve A graph of solubility in grams per 100 g of water against temperature.
  • Saturated solution A solution that holds the maximum mass of solute at that temperature.
  • Crystallisation The formation of solid crystals when a saturated solution is cooled or solvent is evaporated.

Common Mistakes

  • Using a solubility value per 100 g of water as though it were per 100 g of solution, which gives the wrong mass.
  • Saying that gases become more soluble when heated; gas solubility falls as the temperature increases.
  • Reading a value off the curve at the wrong temperature because the axes were not checked first.
  • Assuming all solids dissolve much better in hot water; for sodium chloride the change is very small.

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