NDA Chemistry · Hydrogen and Water

Hardness and Purity of Water

Hard water contains dissolved calcium and magnesium salts; temporary hardness (from bicarbonates) is removed by boiling, while permanent hardness (from sulphates and chlorides) needs chemical softening — and pure drinking water sits in a narrow pH band.

All Hydrogen and Water notes NDA Chemistry strategy

Why this matters

Five PYQs — the densest subtopic in the chapter — split between the temporary-versus-permanent hardness distinction (which ions, which salts, which removal method) and water-quality recall (pH range of drinking water, purest natural source). The single highest-yield fact: boiling removes ONLY temporary hardness. Get the two columns of the hardness table straight and the marks follow.

Concept 1 of 3

Temporary versus permanent hardness

Intuition

Water is called hard when it carries dissolved calcium and magnesium salts — the salts that stop soap lathering and leave scale in kettles. Hardness comes in two flavours depending on which salt is dissolved: bicarbonates give temporary hardness (it leaves when you boil the water), while sulphates and chlorides give permanent hardness (boiling does nothing).

Definition

The two types of hardness, by the dissolved salt:

Temporary hardness — caused by the bicarbonates (hydrogencarbonates) of calcium and magnesium, i.e. Ca(HCO₃)₂ and Mg(HCO₃)₂. Removed by boiling, which precipitates the insoluble carbonate.
Permanent hardness — caused by the chlorides and sulphates of calcium and magnesium (e.g. CaCl₂, CaSO₄, MgSO₄). NOT removed by boiling.
The hardness-causing ions are always the metal cations Ca²⁺ and Mg²⁺.

Type	Caused by	Removed by boiling?
Temporary	Bicarbonates (hydrogencarbonates) of Ca and Mg	YesQ Temporary hardness = hydrogencarbonates (bicarbonates) of Ca²⁺/Mg²⁺ — e.g. Mg(HCO₃)₂.
Permanent	Chlorides and sulphates of Ca and Mg	NoQ

Bicarbonates → temporary (boil it off); chlorides/sulphates → permanent (needs chemical softening).

Practice this conceptself-check · 5 quick reps

Try it yourself

A sample of water leaves no scale after boiling and lathers freely with soap. Was its hardness temporary or permanent, and which salts were responsible?

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which salts cause temporary hardness?
2.
Which salts cause permanent hardness?
3.
Which two ions are responsible for hardness of water?
4.
Is hardness from Mg(HCO₃)₂ temporary or permanent?
5.
Is hardness from CaSO₄ temporary or permanent?

From the bank · past-year question

Example 1Hydrogen and WaterEASY

Temporary hardness in water is due to which one of the following of Calcium and Magnesium ?

[Q57 · Apr · 2017]

Hardness ions are Ca²⁺ and Mg²⁺, not Na⁺

Hardness is caused by calcium and magnesium salts. Sodium salts make water alkaline but do not cause hardness — a Na⁺ option is a distractor.

Drill 1 more on temporary versus permanent hardness

Concept 2 of 3

Removing hardness of water

Intuition

Temporary hardness is easy — just boil the water and the bicarbonates fall out as carbonate scale. Permanent hardness needs a chemical: washing soda, the ion-exchange resin, or Calgon all swap out or lock up the Ca²⁺ and Mg²⁺. The catch the bank loves: boiling does NOTHING to permanent hardness.

Definition

How each type is removed:

Boiling — removes temporary hardness only (precipitates Ca/Mg carbonates). Does NOT remove permanent hardness.
Treatment with washing soda (Na₂CO₃) — removes both temporary and permanent hardness by precipitating Ca²⁺/Mg²⁺ as carbonates.
Ion-exchange method — a resin swaps the hardness ions for harmless ones; removes both types.
Calgon's method — sodium hexametaphosphate locks Ca²⁺/Mg²⁺ into a soluble complex; removes both types.

So permanent hardness is removed by washing soda, ion exchange and Calgon — but never by boiling alone.

Method	Removes temporary?	Removes permanent?
Boiling	Yes	NoQ Boiling cannot remove permanent hardness — this is the most-tested single fact in the chapter.
Washing soda (Na₂CO₃)	Yes	Yes
Ion-exchange method	Yes	Yes
Calgon's method	Yes	Yes

Only boiling is selective (temporary-only); washing soda, ion exchange and Calgon clear both.

Practice this conceptself-check · 5 quick reps

Try it yourself

Boiling a water sample reduces its hardness but does not remove it completely. Explain what kind of hardness remains and name one method to remove it.

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

1.
Which method removes ONLY temporary hardness?
2.
Does boiling remove permanent hardness?
3.
Name one method that removes permanent hardness.
4.
How does washing soda soften water?
5.
Which method uses a resin to swap out hardness ions?

From the bank · past-year question

Example 2Hydrogen and WaterMODERATE

Permanent hardness of water

\textbf{\text{cannot}}

be removed by which one of the following methods?

[Q78 · Sep · 2018]

Boiling cannot remove permanent hardness

A 'which method CANNOT remove permanent hardness' question is answered by boiling. Washing soda, Calgon and ion exchange all work on permanent hardness; boiling does not.

Concept 3 of 3

Purity and quality of drinking water

Intuition

Beyond hardness, the bank tests two water-quality facts: the pH band that drinking water should sit in, and which natural source is the purest. Rain water is distilled by nature — evaporation leaves the dissolved salts behind — so it is the purest natural source, while safe drinking water keeps to a gently-near-neutral pH.

Definition

The drinking-water quality facts:

Desirable pH range of drinking water — about 6.5 to 8.5 (close to neutral, slightly either side).
Purest natural source of water — rain water; it is naturally distilled by evaporation, leaving dissolved salts behind.
River/ground/sea water all carry more dissolved salts, so none is as pure as rain water before it touches the ground.

Quality marker	Value / fact
Desirable pH of drinking water	About 6.5 to 8.5Q Drinking-water pH band = 6.5 to 8.5 — near neutral, not strongly acidic or alkaline.
Purest natural source	Rain water (naturally distilled)Q

Rain water is the purest natural source; safe drinking pH is about 6.5 to 8.5.

Practice this conceptself-check · 4 quick reps

Try it yourself

Why is rain water regarded as the purest natural source of water, even though it can still pick up some impurities?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Desirable pH range for drinking water?
2.
Which is the purest natural source of water?
3.
Why is rain water pure?
4.
Is a drinking-water pH of 4 acceptable?

From the bank · past-year question

Example 3Hydrogen and WaterEASY

The desirable range of pH for drinking water is

[Q139 · Sep · 2017]

Drinking-water pH is near neutral

The desirable pH band is 6.5 to 8.5 — close to neutral. Options far from neutral (like 4 to 5 or 9 to 11) are wrong.

Rain water, not river or sea water, is purest

Rain water is the purest natural source because it is naturally distilled. River and sea water carry more dissolved salts.

Drill 1 more on purity and quality of drinking water

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)

Temporary versus permanent hardness2 rows

Type	Caused by	Removed by boiling?
Temporary	Bicarbonates (hydrogencarbonates) of Ca and Mg	YesQ Temporary hardness = hydrogencarbonates (bicarbonates) of Ca²⁺/Mg²⁺ — e.g. Mg(HCO₃)₂.
Permanent	Chlorides and sulphates of Ca and Mg	NoQ

Bicarbonates → temporary (boil it off); chlorides/sulphates → permanent (needs chemical softening).

Removing hardness of water4 rows

Method	Removes temporary?	Removes permanent?
Boiling	Yes	NoQ Boiling cannot remove permanent hardness — this is the most-tested single fact in the chapter.
Washing soda (Na₂CO₃)	Yes	Yes
Ion-exchange method	Yes	Yes
Calgon's method	Yes	Yes

Only boiling is selective (temporary-only); washing soda, ion exchange and Calgon clear both.

Purity and quality of drinking water2 rows

Quality marker	Value / fact
Desirable pH of drinking water	About 6.5 to 8.5Q Drinking-water pH band = 6.5 to 8.5 — near neutral, not strongly acidic or alkaline.
Purest natural source	Rain water (naturally distilled)Q

Rain water is the purest natural source; safe drinking pH is about 6.5 to 8.5.

Watch out for (4)

Hardness ions are Ca²⁺ and Mg²⁺, not Na⁺
→ Temporary versus permanent hardness
Boiling cannot remove permanent hardness
→ Removing hardness of water
Drinking-water pH is near neutral
→ Purity and quality of drinking water
Rain water, not river or sea water, is purest
→ Purity and quality of drinking water

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Hydrogen and WaterEASY

Which one of the following salts is responsible for temporary hardness of water ?

[Q106 · Sep · 2025]

Example 2Hydrogen and WaterEASY

Which one of the following is considered to be the purest source of water ?

[Q105 · Sep · 2025]

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