Water ChemistryPrepared byMuhammad Shakaib QureshiMsc (Analytical Chemistry)

Introduction

• Water is a chemical compound. Each molecule of water, H2O or HOH, consists of two atoms of hydrogen bonded to one atom of oxygen.

• Or

A transparent, odorless, tasteless liquid, a compound of hydrogen and oxygen.

Freeze at 32F and Boil at 212F

• For the existence of all living beings (human, animals and plants), water is very crucial. Almost all human activities—domestic, agricultural and industrial, demand use of water although water is nature’s most wonderful and abundant compound but only less than 1% of the world’s water resources are available for ready use. Hence it is required to use it carefully and economically.

Sources of Water

• Surface water: It includes flowing water (streams and rivers) and still water (lakes, ponds and reservoirs).

• Underground water: It includes water from wells and springs.

• Rain water

• Sea water

Dissolved Impurities

• (a) Dissolved gases: O2, CO2, H2S etc.

• (b) Inorganic salts:

• (i) Cations: Ca++, Mg++, Na+, K+, Fe++, Al+++ etc.

• (ii)Anions: CO3–, Cl–, SO4–, NO3– etc.

Impurities Continue

• Organic Salts

• Suspended Impurities

• (a) Inorganic: Clay and sand.

• (b) Organic: Oil globules, vegetables, and animal material.

• Colloidal Impurities

• Finally divided clay and silica Al(OH)3, Fe(OH)3, organic waste products, colouring

• matter, amino acids etc.

• Microscopic Matters

• Bacteria, algae, fungi etc.

Sources of Impurities in water

• Gases (O2, CO2 etc.) are picked up from the atmosphere by rainwater.

• Decomposition of plants and animals remains introduce organic impurities in water.

• Water dissolves impurities when it comes in contact with ground, soil or rocks.

• Impurities are also introduced in water when it comes in contact with sewage or industrial waste.

Impurities in Various Sources of Water• River water contains dissolved minerals like chlorides,

sulphates, bicarbonates of sodium, magnesium, calcium and iron. It also contains suspended impurities of sand, rocks and organic matter. The composition of river water is not constant. The amount of dissolved impurities in it depends on its contacts of the soil. Greater the duration of contact, more soluble is the minerals of soil in it.

• Lake water has high quantity of organic matter present in it but lesser amount of dissolved minerals. Its chemical composition is also constant.

Impurities in Various Sources of Water• Rain water is obtaining as a result of evaporation from the surface

water. Probably it is the purest form of natural water. But during its downward journey through the atmosphere it dissolves organic and inorganic suspended particles and considerable amount of industrial gases like (CO2, NO2, SO2 etc.). Rain water is expensive to collect and is irregular in supply.

• Underground water is free from organic impurities and is clearer in appearance due to the filtering action of the soil. But it contains large amount of dissolved salts.

• Sea water is very impure due to two reasons:

• 1. Continuous evaporation increases the dissolved impurities content, which is

• further increased by the impurities thrown by rivers as they join sea.

• 2. It is too saline for most industrial uses except cooling.

Hardness of Water

• “Hardness in water is that characteristics, which prevents the lathering of soap”.

• In other way we may define it as “soap consuming capacity of water”.

Cause of Hardness

• Hardness is due to presence of certain salts of Ca2+, Mg2+ and other heavy metal ions like Al3+, Fe3+ and Mn2+ in water.

Reaction of soap with Soft Water• When soft water is treated with soap, lather is produced

according to the following reaction:

• C17H35COONa + H2O C17H35COOH + NaOH

• Soap (Sodium stearate) Stearic acid

• C17H35COONa + C17H35COOH Lather

• (Sodium stearate) Stearic acid

Reaction of Soap with Hard water• A sample of hard water, when treated with soap (sodium or

potassium salt of higher fatty acid like oleic, palmitic and stearic acid), does not produce lather, but on the other hand forms insoluble white scum or precipitate which do not possess any detergent action. This is due to the formation of insoluble soap of calcium and magnesium.

Types of Hardness

Effect of Hardness

Effect of Hardness

Types of water on basis of hardness

Temporary hardness can be removed by two methods Boiling and

• Clark's process (the addition of a controlled quantity of slaked lime).

Clark’s Process

Permanent Hardness

Ion Exchange

By Polyphosphate

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