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Many metals are chemically active elements and get easily affected by substances like moisture, air, acids, etc. One must have observed iron articles that are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron. The problem with iron (as well as many other metals) is that oxidation takes place and the oxide formed does not firmly adhere to the surface of the metal causing it to flake off easily. This eventually causes structural weakness and disintegration of the metal.
When a metal is attacked by substances around it, it is said to corrode and this process is called corrosion. Corrosion causes deterioration of essential properties in a material.
What happens to copper vessels or artifacts when exposed to air and water? They slowly get tarnished by acquiring a thin green oxide layer. Similarly, silver quickly acquires a thin black oxide coating in moist air. The heaviest metal lead also tarnishes in moist weather. The black coating on silver and the green coating on copper are examples of corrosion in which the oxides formed strongly bond to the surface of the metal, preventing the surface from further exposure to oxygen and consequently slowing down corrosion.
Billions of rupees are lost each year because of corrosion and a huge amount of money is spent in prevention of corrosion and tarnishing of metals. Corrosion causes damage to car bodies, buildings, bridges, iron railings, underground water and sewage pipes, ships and all objects made of metals. Much of this loss is due to the corrosion of iron and steel, although many other metals may corrode as well. You will learn more about corrosion in Chapter 3.
Take a new iron nail, a gold item (ring or chain) and a silver item (ring or chain) during rainy weather. See that these are exposed to the effects of moist weather for a few days (3 to 4 days).
1) What changes are observed to all these items in the moist air?
2) How will you know which metal is more reactive?
Suggested answer: With 3 to 4 days of exposure to moist air (rainy days), the iron nail will form a coat of rust (a reddish brown coating of ferrous oxide); the silver will acquire a very thin coating of blackish silver oxide and become dull; gold does not undergo any change and remains the same. Thus, it is seen that iron and silver are more reactive to moist air than gold.
Have you ever tasted or smelt the fat/oil containing food materials left for a long time? This unpleasant change in the flavor and odour of a food is called rancidity. The most important cause of rancidity is the deterioration in fats and fatty foods because of oxidation process. When an oxygen atom replace hydrogen atom in the fatty acid molecule it destabilizes the molecule. Factors which accelerate fat oxidation include, salt, light, water, bacteria, moulds trace metals (iron, zinc, etc.).
Usually substances which retard fat oxidation or rancidity are called antioxidants (such as BHT, BHA, vitamin E, and vitamin C, and spices such as sage and rosemary). These are added to foods containing fats and oil to prevent such spoiling. Keeping food in air tight containers or air tight wrapping also helps to slow down oxidation.
| Some high fat foods such as potato chips are packaged in materials that protect them from light and oxygen and the containers are flooded with nitrogen to further exclude oxygen.
At times, to avoid the presence of oxygen altogether, vacuum packaging is used in some processed foodstuff.
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