Chemistry

3.54. The elements of group 1 (alkali metals), group 2 (alkaline earth metals) and group 13 to 17 constitute the representative elements. They are elements of the s-block and the p-block.

1 ppm or 1 mg dm^-3.

Toxic substance that is used to kill insects are called insecticides. For example: DDT, BHC.

Carbon monoxide (CO), sulphur dioxide (SO₂) and oxides of nitrogen (NO₂).

Mixing of soil or rock particles in water is called siltation.

Green chemistry can be applied as mentioned in the following examples:

(i) In dry-cleaning, use of liquefied CO₂ in place of tetrachloroethene.

(ii) In bleaching of paper using hydrogen peroxide instead of chlorine.

(iii) In the manufacture of chemicals like ethanal using environment-friendly chemicals and conditions.

PCBs are polychlorinated biphenyls. They are contaminants of water. They are used as fluids in transformers and capacitors

Organic pollutants in water

3.53. They have filled s-orbitals and hence do not tend to accept an additional electron. That is why energy is needed if an extra electron is to be added. Therefore, the electron gain enthalpies of Be and Mg are positive.