CORROSION


 When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.


 The black coating on silver and the green coating on copper are other examples of corrosion.


 Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron


Rancidity


 When fats and oils are oxidised, they become rancid and their smell and taste change.


 Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil.


 Keeping food in air tight containers helps to slow down oxidation.


 chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised


Importance of pH in Everyday Life


 Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change.


 When pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water. The survival of aquatic life in such rivers becomes difficult.


pH in our digestive system


 our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.


 During indigestion the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, people use bases called antacids.


 These antacids neutralise the excess acid. Magnesium hydroxide (Milk of magnesia), a mild base, is often used for this purpose.


pH change as the cause of tooth decay


 Tooth enamel, made up of calcium phosphate is the hardest substance in the body.


 It does not dissolve in water, but is corroded when the pH in the mouth is below 5.5.


 Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating.


 The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.


 Self defence by animals and plants through chemical warfare Bee-sting leaves an acid which causes pain and irritation.


 Use of a mild base like baking soda on the stung area gives relief.


 Stinging hair of nettle leaves inject methanoic acid causing burning pain


Bleaching powder is used –


 for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;


 as an oxidising agent in many chemical industries; and


 for disinfecting drinking water to make it free of germs 


Uses of sodium hydrogen carbonate


 Sodium hydrogencarbonate is also an ingredient in antacids.


 Being alkaline, it neutralises excess acid in the stomach and provides relief.


 It is also used in soda-acid fire extinguishers


Uses of washing soda


 Sodium carbonate (washing soda) is used in glass, soap and paper industries.


 It is used in the manufacture of sodium compounds such as borax.


 Sodium carbonate can be used as a cleaning agent for domestic purposes.


 It is used for removing permanent hardness of water


 A neutral solution has a pH of exactly 7,


 an acidic solution has a pH less than 7


 a basic solution a pH more than 7.