Environmental Chemsitry II Water Punjab Class 10 Notes Unit 15 Pdf Download

Limestone (CaCO₃) is basically insoluble in water. But it is readily dissolved by water that has become slightly acidic, usually from carbon dioxide in the air.
CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂
The reaction produces hydrogen carbonate HCO₃^– ions, which are very soluble in water, and so the limestone dissolves.

The hardness of water is because of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. The rainwater while coming down absorbs carbon dioxide from the atmosphere. The water mixed with carbon dioxide when comes into contact with limestone and dolomite, converts insoluble carbonates of calcium and magnesium into soluble bicarbonates. It may also dissolve chlorides and sulphates of calcium and magnesium. These salts make the water hard.
CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂
MgCO₃ + CO₂ + H₂O → Mg(HCO₃)₂

The amount of minerals in water affects the amount of soap and detergent used for cleaning.  Soap combines with the minerals of hard water to form sticky soap curd. Some synthetic detergents are less effective in hard water because the active ingredient is partially inactivated by hardness. When doing laundry in hard water, soap curds lodge in fabric and make it stiff and rough. Minerals can be deposited in pipes, fixtures and appliances, clogging them prematurely.

Disadvantages of detergents
Many detergents are resistant to the action of microorganisms like bacteria and are not biodegradable.
i) Household water containing these detergents when discharged in lakes and rivers, cause water pollution.
ii) They produce stable foams in the rivers and pose a danger to aquatic life.
iii) The phosphate salts present in detergents cause the rapid growth of algae in the water. These plants die and decay by consuming oxygen gas present in the water. Thus, the depletion of oxygen results in the death of aquatic life.

As detergents are non-biodegradable, they remain in the water for a long time and make the water unfit for aquatic life. The phosphate salts present in detergents cause the rapid growth of algae in the water. These plants die and decay by consuming oxygen gas present in the water. Thus, the depletion of oxygen results in the death of aquatic life.

Pesticides are used either directly to kill or control the growth of various pests and disease carriers, such as mosquitoes, rats and mice. Pesticides are used in agriculture to control weeds, insect infestation and diseases, which damage crops and transmit diseases both to humans and animals.

Water-borne diseases can be due to drinking water contaminated by human or animal faeces, which contain pathogenic microorganisms. Large poorly functioning municipal water distribution systems and toxins also contribute to the transmission of waterborne bacterial diseases.

Waterborne diseases can be prevented by taking the following measures:
i) Ensuring that the drinking water is properly treated and purified.
ii) Keeping control over the use of pesticides and other chemicals.
iii) Not throwing or discharging the waste directly in waterways, streams or reservoirs.

Water has the ability to flow in narrow spaces without the assistance of external forces and in opposition to gravity. This property is called capillary action and is responsible for its rise in plants from roots to leaves. Plants and trees can’t thrive without capillary action.

Water molecule is polar in nature i.e. hydrogen end of the molecule is partially positive while the oxygen end is partially negative due to the electronegativity difference between both atoms. Polar substances dissolve in water by the ion-dipole force of attraction between ion and water molecules. In this way, the positive end of the substance is attracted by the negative end of the water and vice versa. For example, NaCl is soluble in water.

Non-polar compounds do not have positive and negative ends. They cannot interact favourably with water molecules. Therefore, they do not dissolve in water.