CDS Chemistry · Hydrogen and Water
Water: Hardness, Drying Agents and Density
Temporary and permanent hardness and how each is removed, the drying agents that take water out of gases, and why water is densest at 4 °C.
Why this matters
Five CDS questions. Three are on hard water, one on the drying agent in a guard tube (HARD) and one on how water's density changes between 0 and 4 °C.
Concept 1 of 3: Temporary and permanent hardness
Hard water carries dissolved calcium and magnesium salts, which stop soap from lathering. If the salts are hydrogen carbonates, boiling breaks them down and the hardness goes: that is temporary hardness. Chlorides and sulphates do not break down on boiling, so that hardness is permanent and needs a chemical such as washing soda.
Definition
The two kinds:
- Temporary hardness: hydrogen carbonates (bicarbonates) of calcium and magnesium, Ca(HCO₃)₂ and Mg(HCO₃)₂. Removed by boiling (they become insoluble carbonates) or by adding lime (Clark's method).
- Permanent hardness: chlorides and sulphates of calcium and magnesium (CaCl₂, MgSO₄). Boiling does not remove it.
- Permanent hardness is removed by washing soda, Na₂CO₃, which precipitates calcium and magnesium as carbonates, or by ion exchange (permutit) or Calgon (sodium hexametaphosphate).
- Hard water forms scum with soap because calcium and magnesium make insoluble salts with it.
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q27Moderate
Example 1 · Hydrogen and Water · Water: Hardness and Properties
Consider the following statements :
1. The temporary hardness in water is due to the presence of bicarbonates of calcium.
2. The permanent hardness in water is due to the presence of soluble chlorides and sulphates of calcium and magnesium.
3. The permanent hardness in water is removed by adding to water.
Which of the statements given above are correct ?
Bicarbonates are temporary, chlorides and sulphates permanent
Boiling removes only the hydrogen carbonate hardness. CaCl₂ and MgSO₄ cause permanent hardness, which boiling leaves behind.
Washing soda, not baking soda, softens water
Permanent hardness is removed with washing soda, Na₂CO₃. Baking soda (NaHCO₃) and caustic soda are distractors.
Hardness is calcium and magnesium, not sodium
Sodium salts do not make water hard. Hardness comes from dissolved calcium and magnesium compounds.
Calcium is in both kinds
Temporary hardness is calcium (and magnesium) hydrogen carbonate; permanent hardness is calcium (and magnesium) chloride and sulphate. The anion, not the metal, tells them apart.
Concept 2 of 3: Drying agents
A drying agent takes up water vapour from a gas. The rule is that it must not react with the gas being dried. An acidic drying agent cannot dry a basic gas such as ammonia, and a basic one cannot dry an acidic gas.
Definition
The common drying agents:
- Anhydrous calcium chloride: packed in a guard tube; dries most neutral and acidic gases, including HCl. It is not used for ammonia, with which it combines.
- Concentrated sulphuric acid: dries acidic and neutral gases, but not ammonia (it reacts with it).
- Quicklime (CaO): dries ammonia.
- Phosphorus pentoxide (P₄O₁₀): a very strong drying agent for neutral and acidic gases.
| 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 |
Practice this concept2 quick reps
The same idea in a real exam question:
CDS · 2021 · CDS (II) 2021 — General Knowledge · Q26Hard
Example 2 · Hydrogen and Water · Water: Hardness and Properties
Which one of the following materials is present in a guard tube (drying tube) that is used for preparation of HCl gas ?
A drying agent must not react with the gas
Concentrated sulphuric acid absorbs water well but reacts with ammonia, so it cannot dry it. Choose a drying agent of the same acid-base nature as the gas.
Calcium chloride, not calcium fluoride
The guard-tube drying agent is anhydrous calcium chloride, which absorbs moisture strongly. Calcium bromide, iodide and fluoride are distractors.
Ammonia needs a basic drying agent
Ammonia is a base, so it is dried over quicklime. Acidic agents (H₂SO₄, P₄O₁₀) and calcium chloride combine with it.
Anhydrous means without water
Only the anhydrous salt can dry a gas. Calcium chloride that has already taken up water cannot absorb more.
Concept 3 of 3: Water's maximum density at 4 °C
Most liquids shrink steadily as they cool. Water shrinks only down to 4 °C. Below that its molecules start lining up into the open, hydrogen-bonded pattern of ice, so it expands again. That is why ice floats and why a pond freezes from the top.
Definition
The facts:
- Water is densest at 4 °C.
- Heating water from 0 °C to 4 °C, it contracts, so its density increases. Above 4 °C it expands like other liquids.
- Ice is less dense than water (an open, hydrogen-bonded structure), so it floats.
- In winter the water at 4 °C sinks to the bottom of a lake, the surface freezes first, and life survives below.
| 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) |
Practice this concept2 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (II) 2022 — General Knowledge · Q86Moderate
Example 3 · Hydrogen and Water · Water: Hardness and Properties
When water is heated from to , its density
Water expands below 4 °C
Between 0 and 4 °C water behaves oddly: cooling makes it expand, and heating makes it contract. Above 4 °C it behaves like other liquids.
Density does not return to its old value at 4 °C
From 0 to 4 °C the density simply rises, to a maximum at 4 °C. It does not rise and then fall back to where it started within that range.
Ice is lighter than water
Ice is less dense than liquid water, which is why icebergs float. For most substances the solid is denser than the liquid; water is the exception.
4 °C, not 0 °C
Water is densest at 4 °C, not at its freezing point. At 0 °C, both as water and as ice, it is less dense.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
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
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