Describe two changes that are harmful. Explain why you consider them harmful? How can you prevent them?

(i) Rusting: If a piece of iron is open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting.

Rusting is harmful because it destroys the iron objects. Iron is the most widely used metal and so rusting is such a serious problem.

Prevention of rusting: Rusting can be prevented by preventing iron articles from coming in contact with oxygen, or water, or both.

• One simple way is to apply a coat of paint or grease. In fact, these coats should be applied regularly to prevent rusting .

• Another way is galvanization, i.e., to deposit a layer of metal like chromium or zinc on iron.

(ii) Spoilage of food: Food item when kept carelessly, get spoiled. This is a chemical change and obviously harmful for us.

Food is spoiled by microorganisms.

Prevention of food spoilage: Microorganisms do not survive at high or low temperature. So, food items stored in refrigerator do not spoil. Also we should keep them covered so that microorganisms do not get any chance to enter and spoil them.