CDS Chemistry · Teaching notes
Hydrogen and Water — CDS Chemistry
Hydrogen and Water has 5 past-year questions in CDS General Knowledge, from 2018 (I) to 2026 (I), and all of them are about water rather than hydrogen. Three ask about hard water: what makes it temporary or permanent and what removes each kind. The others ask which drying agent fills a guard tube and how water's density changes as it warms from 0 °C to 4 °C. One page covers all five.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Formula & revision sheet
0 formulas · 3 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
0 formulas · 3 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (3)
Drying agents4 rows
| Drying agent | Dries | Not used for |
|---|---|---|
| Anhydrous calcium chloride | HCl and most neutral gases | AmmoniaQ |
| Concentrated H₂SO₄ | Acidic and neutral gases | Ammonia |
| Quicklime (CaO) | Ammonia | Acidic gases such as HCl |
| Phosphorus pentoxide | Neutral and acidic gases | Ammonia |
Water's maximum density at 4 °C3 rows
| Temperature change | Volume | Density |
|---|---|---|
| 0 °C → 4 °C (heating) | Decreases | IncreasesQ |
| Above 4 °C (heating) | Increases | Decreases |
| Water → ice at 0 °C | Increases | Decreases (ice floats) |
Watch out for (12)
- Bicarbonates are temporary, chlorides and sulphates permanent→ Temporary and permanent hardness
- Washing soda, not baking soda, softens water→ Temporary and permanent hardness
- Hardness is calcium and magnesium, not sodium→ Temporary and permanent hardness
- Calcium is in both kinds→ Temporary and permanent hardness
- A drying agent must not react with the gas→ Drying agents
- Calcium chloride, not calcium fluoride→ Drying agents
- Ammonia needs a basic drying agent→ Drying agents
- Anhydrous means without water→ Drying agents
- Water expands below 4 °C→ Water's maximum density at 4 °C
- Density does not return to its old value at 4 °C→ Water's maximum density at 4 °C
- Ice is lighter than water→ Water's maximum density at 4 °C
- 4 °C, not 0 °C→ Water's maximum density at 4 °C
PYQ weightage by concept
3 concepts · 5 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
3 concepts · 5 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Temporary and permanent hardness | 3 | 60% |
| Drying agents | 1 | 20% |
| Water's maximum density at 4 °C | 1 | 20% |
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.