Corrosion and Rancidity

Have you ever observed a green coating on coins or statues made of copper? Why does this happen? This happens because the metal undergoes a process called corrosion. Also, have you observed a bad smell emanating from food containing fats or oils? This process is called rancidity. Both these processes are everyday examples of oxidation reactions. Let’s understand why and how both these processes take place.

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Corrosion

It is a process where metals are attacked by moisture, acids etc. Some daily examples of corrosion that you may have observed are the black coating that develops on articles made of silver or the green coating on copper articles. Corrosion that occurs on materials made of iron is ‘rusting‘.

Corrosion is the wearing away of metals due to chemical reactions, mainly oxidation. In this process, metals lose their metallic properties. This causes a lot of damage to car bodies, ships, bridges, iron railings and to all objects made of iron. Iron is used in a lot of places like the construction of bridges, tools, pipes, heavy machinery etc.

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Therefore, corrosion of iron is a very big problem. A lot of money is spent each year to replace damaged iron. There are a few ways to keep metals from corroding. They are:

• Painting the metal surfaces.
• Powder coating the surfaces.
• Oiling the metal surfaces.
• Galvanization – It is the process of applying a protective zinc coating to iron or steel articles, to prevent them from rusting.

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Rancidity

Have you observed a foul smell in nuts that have been stored for months? Or potato chips that have been sitting in the kitchen cupboard for too long? When such foul odour emanates from food, it is said to be rancid. In simple terms, rancidity is the spoilage of food such that it becomes unsuitable and undesirable for consumption.

Food turns rancid when the fats and oils within them get oxidized and the taste and smell of the food changes. Oxidation of fats generates highly reactive molecules in the rancid food which is responsible for the unpleasant odour and flavour. These chemical reactions also reduce the nutrition of the food. Some studies show that rancid food can have harmful health effects if consumed.

Rancidity can be prevented using the following methods:

• Adding antioxidants (substances which prevent oxidation) to food.
• Storing food in airtight containers to slow the process of rancidification.
• Refrigerating food also helps to slow down rancidification.
• Replacing oxygen in the containers with another gas. For example, to prevent chips from turning rancid, chips manufacturers flush the bags with nitrogen gas.

Solved Examples for You

Question: Corrosion and rancidity are both due to which type of chemical reaction?

1. Decomposition
2. Reduction
3. Oxidation
4. Displacement

Solution: Option C. Corrosion and Rancidity are both a result of the oxidation reaction.

Question: Which of the following is not a method of preventing corrosion of metals?

1. Galvanization
2. Exposure to moisture
3. Painting the metal surface
4. Oiling the metal surface

Solution: Option B. Exposure to moisture will increase corrosion of metals than prevent it. Hence it is not a method of preventing the corrosion of metals.