Hardness of Water – Types, Causes, Effects & Detailed Explanation | Engineering Chemistry Notes

Hardness of Water – Types, Causes, Effects & Detailed Explanation

Hardness of water is defined as the property by virtue of which water does not readily produce lather with soap. Instead, it forms a white precipitate known as scum. This phenomenon is caused mainly due to the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) salts. Hardness is an important parameter in engineering chemistry since it affects domestic use, industrial processes, and boiler operations.

1. Chemistry of Hardness

When soap (sodium stearate, C₁₇H₃₅COONa) is added to hard water, instead of forming lather, it reacts with Ca²⁺/Mg²⁺ ions to form insoluble precipitates:

$$2C_{17}H_{35}COONa + Ca^{2+} \; \rightarrow \; (C_{17}H_{35}COO)_2Ca \downarrow + 2Na^+$$

Thus, a large amount of soap is wasted before any lather is formed.

2. Types of Hardness

Based on the nature of dissolved salts, hardness is of two types:

(a) Temporary Hardness

Caused by the presence of bicarbonates of calcium and magnesium:

Ca(HCO₃)₂, Mg(HCO₃)₂

Removal: By boiling. On heating, bicarbonates decompose to insoluble carbonates:

$$Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3 \downarrow + CO_2 \uparrow + H_2O$$ $$Mg(HCO_3)_2 \xrightarrow{\Delta} MgCO_3 \downarrow + CO_2 \uparrow + H_2O$$

(b) Permanent Hardness

Caused by the presence of chlorides, sulfates, and nitrates of calcium and magnesium:

CaCl₂, MgSO₄, Ca(NO₃)₂

Removal: Cannot be removed by boiling; requires chemical treatment such as lime-soda process, zeolite process, or ion-exchange method.

Example reaction using washing soda (Na₂CO₃): $$CaSO_4 + Na_2CO_3 \; \rightarrow \; CaCO_3 \downarrow + Na_2SO_4$$

Figure: Classification of Hardness of Water

3. Causes of Hardness

Percolation of rainwater through soil and rocks containing limestone (CaCO₃), dolomite (CaCO₃·MgCO₃), and gypsum (CaSO₄·2H₂O).
Dissolution of these minerals releases Ca²⁺ and Mg²⁺ ions into water.
Industrial effluents, fertilizers, and sewage discharge further add chlorides, sulfates, and nitrates.

4. Measurement of Hardness

Hardness is usually expressed in terms of parts per million (ppm) of CaCO₃ equivalent, or in mg/L of CaCO₃. One degree of hardness corresponds to 10 mg of CaO per liter of water.

$$\text{Hardness (mg/L as CaCO}_3) = \frac{M_{\text{solute}}}{Eq.wt.} \times 50,000 / V_{\text{water (mL)}}$$

5. Effects of Hard Water

Domestic use: Wastage of soap, difficulty in washing clothes, scaling of utensils and geysers.
Industrial use: Scale formation in boilers, condensers, and heat exchangers → loss of efficiency and risk of boiler accidents.
Textile industry: Poor dyeing quality and uneven coloring.
Paper and sugar industry: Affects final product quality.
Food processing: Alters taste, increases cooking time.
Health: Hard water is not harmful; moderate hardness provides essential Ca and Mg, but very high hardness may cause digestive issues.

Comparison of Temporary & Permanent Hardness

Aspect	Temporary Hardness	Permanent Hardness
Causing salts	Bicarbonates of Ca and Mg	Chlorides, sulfates, nitrates of Ca and Mg
Removal method	Boiling	Chemical treatment (lime-soda, zeolite, ion exchange)
Examples	Ca(HCO₃)₂, Mg(HCO₃)₂	CaSO₄, MgCl₂, Ca(NO₃)₂
Reversibility	Temporary (reversible by boiling)	Permanent (non-reversible by boiling)

Quick Revision Points

Hardness = Ca²⁺ + Mg²⁺ salts.
Temporary hardness: Due to bicarbonates; removable by boiling.
Permanent hardness: Due to chlorides/sulfates/nitrates; chemical treatment needed.
Expressed as mg/L (ppm) CaCO₃.
Effects: soap wastage, scaling, industrial losses, but no serious health hazards.

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