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IGCSE · Practice questions · Hydrated and anhydrous salts

Count the water.

Six original Cambridge-style questions on hydrated salts: definitions, colour changes, tests for water and finding the formula of a hydrate.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Mass lost is water.

01
Recall
[3 marks]

Define the terms hydrated, anhydrous and water of crystallisation.

  • Hydrated: a substance that is chemically combined with water. ✓
  • Anhydrous: a substance that contains no water. ✓
  • Water of crystallisation: water molecules present in the crystals of a hydrated salt. ✓
02
Recall
[2 marks]

Describe what is seen when blue hydrated copper(II) sulfate is heated strongly, and write the equation.

  • The blue crystals turn to a white powder, and steam (water vapour) is given off. ✓
  • CuSO4·5H2O → CuSO4 + 5H2O ✓
03
Calculation
[3 marks]

(Extended) 5.00 g of hydrated copper(II) sulfate, CuSO4·xH2O, is heated to constant mass. 3.20 g of anhydrous copper(II) sulfate (Mr 160) remains. Find x.

  • Water lost = 5.00 − 3.20 = 1.80 g = 1.80 ÷ 18 = 0.100 mol. ✓
  • CuSO4 = 3.20 ÷ 160 = 0.0200 mol. ✓
  • x = 0.100 ÷ 0.0200 = 5. ✓
04
Calculation
[3 marks]

(Extended) 2.86 g of washing soda, Na2CO3·xH2O, is heated to constant mass, leaving 1.06 g of anhydrous sodium carbonate (Mr 106). Find x.

  • Water lost = 2.86 − 1.06 = 1.80 g = 0.100 mol. ✓
  • Na2CO3 = 1.06 ÷ 106 = 0.0100 mol. ✓
  • x = 0.100 ÷ 0.0100 = 10. ✓
05
Explain
[2 marks]

(Extended) A student heats a hydrated salt only once and not to constant mass. Explain the effect on the calculated value of x.

  • Some water of crystallisation may remain in the solid, so the mass of water lost is too small. ✓
  • The calculated value of x is therefore too low. ✓
06
Recall
[2 marks]

Describe a chemical test for water, and explain why it does not show that the water is pure.

  • Add to anhydrous copper(II) sulfate: it turns from white to blue (or blue cobalt(II) chloride paper turns pink). ✓
  • Any liquid containing water gives this result; purity is shown by a boiling point of exactly 100 °C. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.