Category Chemistry

It is a phenomenon caused by industrial pollution. Natural rain always contains a small amount of dissolved carbon dioxide which makes it slightly acidic. But large-scale burning of coal or oil in industries, power plants and vehicles produce large amounts of gases such as sulphur dioxide, nitrogen oxides, etc., which are released into the atmosphere. These gases rise up with air currents and may be carried by prevailing winds many hundred of kilometres away from their source. Under favourable conditions they react with water vapour and oxygen in the atmosphere to produce sulphuric and nitric acids which eventually come down with rain, snow or fog. Compared to a pH of around 6 for normal rain, acid rains cause considerable damage to vegetation, trees and marine life. As the gases retry and acid rains fall in another damaging its flora and marine life, it has become a cause of concern to all countries. The countries worst affected by acid rains are southern Sweden, Norway, parts of central Europe and eastern regions of North America. Most European countries are therefore making efforts to reduce sulphur emissions by their industries and power plants.

The shine of a surface depends on its smoothness. Smoother the surface, more it will reflect light and thus will appear shiny. Unpolished leather does not shine because its surface is full of tiny pores and is rough. When we apply polish on this rough surface, the wax in the polish fills up the very small microscopic pits of the leather surface. Vigorous brushing or rubbing with a soft cloth spreads the wax evenly by melting. This makes the surface shine. If the shoe is rubbed after a few drops of water is sprinkled on it, it shines even more because the cloth surface after absorbing little water does not stick to the wax on the leather surface but glides smoothly. Liquid shoe polishes do not need rubbing as these leave a quick drying film on the leather which itself is reflecting and gives a glossy look. 

Curd is formed by a chemical reaction which involves casein, the protein that occurs in milk. The coagulation of this protein results in the formation of curd. This occurs if bacteria called Lactobacillus acidophilus is present in milk. This bacterium produces lactic acid by the fermentation of the milk sugar lactose. The lactic acid so formed by the bacterial action contains positive hydrogen ions which are attracted to the negative particles of the protein casein. As the latter are neutralized, these protein molecules no longer repel one another but coagulate. The optimum temperature at which Lactobacillus acts is around 40°C. So milk is usually warmed to this temperature before setting it to curdle.