Playbook
Hydrogen and Water
11 q · 9% HARD. Permanent vs temporary hardness (CaSO₄/MgSO₄ vs Ca(HCO₃)₂), softening methods (boiling/ion-exchange/lime-soda), pure-water source ranking, and dihydrogen properties + storage. The 'permanent hardness can't be removed by boiling' trap recurs.
- questions in the bank
- 11
- tagged HARD
- 9%
- subtopic(s)
- 3
- worked examples
- 2
When you’ll see it
A permanent vs temporary hardness question, a softening-method recall (boiling/ion-exchange/lime-soda/permutit), a dihydrogen property statement, or a 'purest source of water' / 'water's anomalous behaviour' question.
How this chapter is tested
11 q in 10 years · 9% HARD. Three subtopics. Hardness of Water (5 q) is the dominant subtopic. Temporary hardness = Ca(HCO₃)₂ and Mg(HCO₃)₂ (bicarbonates) — removed by BOILING (decomposes to insoluble carbonate that precipitates out). Permanent hardness = CaSO₄, MgSO₄, CaCl₂, MgCl₂ — NOT removed by boiling. Removed by ion-exchange (Na⁺ zeolite/permutit) or lime-soda (Ca(OH)₂ + Na₂CO₃) or distillation.
Properties of Hydrogen (3 q · 33% HARD): H₂ is lightest gas, colourless, odourless, burns with pale-blue flame, can be liquefied at very low T (one HARD q tests how to store/transport large volumes — answer: solid metal hydrides or compression to liquid). H₂ in water = 'dihydrogen monoxide.' Heavy water D₂O = isotopic variant, used as neutron moderator in nuclear reactors.
Water's anomalous behaviour (3 q): max density at 4 °C (not 0 °C), ice less dense than water (floats — that's why aquatic life survives winter), high specific heat (climate moderation), high boiling point for its molar mass (due to H-bonding). 'Purest source of water' = rainwater (in unpolluted air); ocean water is most saline.
The sub-skills
The rules and habits that decide whether you get a question right.
Temporary vs permanent hardness
Temporary = bicarbonates (Ca(HCO₃)₂, Mg(HCO₃)₂). Boil → CaCO₃ ↓ + CO₂ ↑ + H₂O. Permanent = sulphates + chlorides (CaSO₄, MgSO₄, CaCl₂, MgCl₂). NOT removed by boiling.
Softening methods
Temporary: boiling, OR add Ca(OH)₂ (lime — Clark's method). Permanent: lime-soda (Ca(OH)₂ + Na₂CO₃); ion-exchange (zeolite / Permutit, Na+ replaces Ca²⁺/Mg²⁺); synthetic resin (Calgon, EDTA); distillation. Distillation removes BOTH but is energy-intensive.
Hydrogen properties + storage
H₂ = lightest gas, ~0.09 g/L at STP. Burns with PALE BLUE flame. Diatomic. Cannot be liquefied easily (very low critical T). Large-volume storage: as liquid at ~−253 °C, OR adsorbed in metal hydrides (LaNi₅H₆, etc.). H₂ → H₂O on combustion (no CO₂, hence 'clean fuel').
Water anomaly + purity
Max density at 4 °C (not 0 °C). Ice density 0.92 g/cm³ < water 1.00 → ice floats. Specific heat 4.18 kJ/(kg·K) — highest among common liquids. Purest source: RAINWATER (before pollution); distilled > river > lake > sea (most saline).
2 worked examples from the bank
Real past-year questions illustrating the playbook. Click to reveal options + solution.
[Q88 · Sep · 2019]
[Q78 · Sep · 2018]
Traps to expect
Distractor shapes specific to this chapter. The page-wide Traps section covers the bank-level patterns.
Boiling removes permanent hardness
Boiling removes ONLY temporary hardness (decomposes bicarbonate). Sulphates and chlorides don't decompose on boiling — permanent hardness persists. Wrong option lists boiling as a solution for all hardness types.
Water densest at 0 °C
Water is densest at 4 °C, NOT 0 °C. As temperature drops from 4 °C to 0 °C, water EXPANDS (anomalous). At 0 °C, ice forms and density drops further to 0.92. Wrong option says '0 °C' as the densest point.
Drill every hydrogen and water question
11 questions from the bank, scoped to 3 bundled subtopics.
Related playbooks
Often paired with this one — drill these next if you found the worked examples above tractable.