Water Across IGCSE Chemistry

Water (H2O) appears in almost every topic of the 0620 syllabus. This page maps its connections so you can see how a single question about water might draw on knowledge from several chapters.

1. States of matter

Water is the standard example for all three states of matter:

State	Name	Particle arrangement
Solid	Ice	Regular lattice, particles vibrate in fixed positions
Liquid	Water	Particles close together, move randomly
Gas	Steam	Particles far apart, move rapidly in all directions

Key terms: melting (0 C), boiling (100 C), evaporation, condensation, freezing.

Heating and cooling curves for water illustrate energy changes during changes of state.

2. Testing for water

Test for presence of water

Add the liquid to anhydrous copper(II) sulfate (white powder):

• Turns blue if water is present (forms CuSO4.5H2O)
• This is a reversible reaction

Alternatively, add to anhydrous cobalt(II) chloride paper (blue):

• Turns pink if water is present

Test for pure water

Measure the boiling point: pure water boils at exactly 100 C (at standard pressure). Impure water has a higher boiling point than 100 C or boils over a range.

Measure the freezing point: pure water freezes at exactly 0 C.

3. Water in acid-base chemistry

As a product of neutralisation

acid + base -> salt + water

The ionic equation: H+(aq) + OH-(aq) -> H2O(l)

See neutralisation reactions.

As a solvent for acids and alkalis

• HCl(g) dissolved in water gives H+(aq) — only then does it act as an acid
• NaOH dissolved in water gives OH-(aq) — an alkali is a soluble base
• Pure water has pH 7 (neutral)

Water of crystallisation

Some salts crystallise with water molecules bound into the crystal structure:

CuSO4.5H2O (blue) ⇌ CuSO4 (white) + 5H2O

See water of crystallisation.

4. Water in electrochemistry

Electrolysis of water

Dilute sulfuric acid is electrolysed (water acts as the compound being decomposed):

At cathode: 2H+(aq) + 2e- -> H2(g)

At anode: 4OH-(aq) -> 2H2O(l) + O2(g) + 4e-

Volume ratio: H2 : O2 = 2 : 1. See electrochemistry equations.

Aqueous electrolysis

H+ and OH- ions from water compete with solute ions at the electrodes, determining what is discharged.

Hydrogen-oxygen fuel cell

2H2 + O2 -> 2H2O

Water is the only product, making this a clean energy source. See hydrogen-oxygen fuel cells.

5. Water in environmental chemistry

Water treatment

Steps to make potable water:

1. Sedimentation (heavy particles settle)
2. Filtration (through sand and gravel)
3. Chlorination (kills bacteria)

See water and water treatment.

Dissolved oxygen

Aquatic organisms need dissolved O2 in water. Pollution (eutrophication) can deplete oxygen levels.

Acid rain

Acid rain (containing H2SO3/H2SO4/HNO3) acidifies lakes and rivers, harming aquatic life.

6. Water in metal reactions

Metal	Reaction with water
K, Na, Li	React vigorously with cold water
Ca	Reacts steadily with cold water
Mg	Very slow with cold water, reacts with steam
Zn, Fe	React with steam only
Cu, Ag, Au	No reaction

7. Water as a reactant

• CaO + H2O -> Ca(OH)2 (exothermic, limestone cycle)
• Hydration of ethene: C2H4 + H2O -> C2H5OH
• Dissolving: NaCl(s) + water -> Na+(aq) + Cl-(aq)

Describe two different tests that could confirm a liquid is pure water. [4 marks]

• Test 1: Add the liquid to anhydrous copper(II) sulfate; it turns from white to blue, confirming water is present [1]
• Test 1 only shows water is present, not that it is pure [1]
• Test 2: Measure the boiling point; pure water boils at exactly 100 C at standard atmospheric pressure [1]
• If the boiling point is exactly 100 C (not a range and not higher), the water is pure [1]

Water appears as a product in neutralisation, combustion, and the reaction of Group I metals with water. Write one equation for each. [3 marks]

• Neutralisation: NaOH + HCl -> NaCl + H2O [1]
• Combustion: CH4 + 2O2 -> CO2 + 2H2O [1]
• Group I + water: 2Na + 2H2O -> 2NaOH + H2 (water is a reactant here) [1]