Water, Hardness and Water Treatment – NDA Chemistry Notes

Water is the most common liquid on Earth. It covers about 71% of the Earth’s surface. Yet clean, usable water is limited. Chemistry helps us understand what makes water hard, how to soften it, and how to purify it for drinking.

Hardness types and their removal methods are tested every two to three years without fail. Properties of water – especially what is true and what is NOT true – come second. Know both topic clusters thoroughly.

1.  Properties of Water

Water (H₂O) is a polar molecule. It has a bent shape. The oxygen end carries a partial negative charge and the hydrogen end carries a partial positive charge. Water is NOT non-polar.  [NDA 2019-II, NDA 2013-I]

Water molecules are held together by hydrogen bonds. The O−H bond in one water molecule attracts the oxygen of a neighbouring molecule. This hydrogen bonding is responsible for most of water’s unusual properties.  [NDA 2013-I]

Properties of water that are HIGH because of hydrogen bonding:

Water is a good coolant in car engines because of its high specific heat. It absorbs a large amount of heat without its temperature rising much.

The latent heat of fusion of water is HIGH (334 J/g) – not low. A statement saying it is low is INCORRECT.  [NDA 2024-I]

Pure water does not conduct electricity. It is a non-conductor because it has almost no free ions. Dissolved salts make water conduct.  [NDA 2024-I]

Water is a good solvent because its dielectric constant is very high (~80 at 25°C). A high dielectric constant means water greatly reduces the attraction between oppositely charged ions. This allows ionic compounds to dissolve. Water dissolves ionic and polar covalent compounds well. It does NOT dissolve non-polar covalent compounds (like oils).  [NDA 2013-I, NDA 2010-II]

2.  Anomalous Properties of Water

Most liquids become denser as they cool. Water behaves differently below 4°C.

Water has maximum density at 4°C = 277 K. As water cools from 100°C to 4°C, it contracts normally and becomes denser. But below 4°C, water starts to expand. Its density decreases. This is anomalous expansion.  [NDA 2024-II, NDA 2010-II]

Because of this, ice (formed below 0°C) is less dense than liquid water at 4°C. Ice floats on water. This protects aquatic life in winter. The ice layer insulates the water below.

3.  Hard Water and Soft Water

Soft water lathers freely with soap. Hard water does not give lather with soap.

Hard water contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts. These ions react with soap (sodium stearate) to form insoluble calcium and magnesium stearates. This insoluble precipitate is called scum. The scum consumes the soap before any lather can form.  [NDA 2013-I]

Temporary Hardness

Temporary hardness is caused by dissolved bicarbonates (hydrogencarbonates) of calcium and magnesium:

Ca(HCO₃)₂ and Mg(HCO₃)₂

It is called temporary because it can be removed by simply boiling the water.  [NDA 2017-I, NDA 2025-II, NDA 2013-I]

Permanent Hardness

Permanent hardness is caused by dissolved sulphates and chlorides of calcium and magnesium:

CaSO₄, MgSO₄, CaCl₂, MgCl₂

It is called permanent because boiling does NOT remove it.  [NDA 2015-I, NDA 2011-I]

4.  Methods of Removing Hardness

Boiling – removes TEMPORARY hardness only. On boiling, bicarbonates decompose to insoluble carbonates that precipitate out:

Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂

Permanent hardness is NOT removed by boiling.  [NDA 2018-II]

Clark’s Process (adding slaked lime / lime water) – removes TEMPORARY hardness. Lime water (Ca(OH)₂) reacts with bicarbonates to form insoluble carbonates. This is also called the lime-soda process when used with soda. Clark’s method removes TEMPORARY hardness, not permanent.  [NDA 2013-I]

Washing Soda (Na₂CO₃) – removes PERMANENT hardness. Sodium carbonate reacts with dissolved Ca²⁺ and Mg²⁺ salts to form insoluble CaCO₃ and MgCO₃ precipitates, which settle out.

Calgon’s Method – removes PERMANENT hardness. Calgon (sodium hexametaphosphate, Na₆P₆O₁₈) forms soluble complexes with Ca²⁺ and Mg²⁺ ions, preventing them from reacting with soap. Calgon removes PERMANENT hardness, NOT temporary.  [NDA 2013-I, NDA 2018-II]

Ion Exchange / Permutit Process – removes BOTH temporary and permanent hardness. Water is passed through a resin or zeolite (sodium aluminium silicate) that exchanges Na⁺ ions for Ca²⁺ and Mg²⁺ ions. The resin can be regenerated with brine.

When both cation and anion exchange resins are used together, the result is deionised water – completely free of all dissolved ions. Deionised water is produced by the ion-exchange resin process, not by Calgon, Clark’s, or permutit alone.  [NDA 2012-II]

5.  Water Purification

KMnO₄ purifies water because it is an oxidising agent. It oxidises and destroys bacteria and organic impurities. It does not purify water by a reducing action.  [NDA 2017-II]

The desirable pH range for drinking water is 6.5 to 8.5 (WHO and BIS standard). Below 6.5, water is too acidic and may be corrosive. Above 8.5, it is too alkaline and unpalatable.  [NDA 2017-II]

6.  Heavy Water (D₂O)

Heavy water is deuterium oxide. Its chemical formula is D₂O. It is made of deuterium (²H) and oxygen.

Heavy water has a molecular weight of about 20 u (compared to 18 u for ordinary water, H₂O). This is because each hydrogen atom (mass 1) is replaced by a deuterium atom (mass 2).

Heavy water is used as a moderator in nuclear reactors. A moderator slows down fast neutrons so that they can sustain a chain reaction.  [NDA 2009-I, NDA 2015-II]

The dielectric constant of heavy water is smaller than that of ordinary water.  [NDA 2015-II]