Chapter 2: Is Matter around us Pure?

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Is Matter around us pure?


Q.11: Which of the following are chemical changes?

(a)     Growth of a plant

(b)     Rusting of iron

(c)     Mixing of iron filings and sand

(d)     Cooking of food

(e)     Digestion of food

(f)     Freezing of water

(g)     Burning of a candle


(a) The plant grows from seed. Each day the number of plant cells increases which cause the growth of the plant. The growth of plants occurs due to biochemical synthesis reactions regulated by plant hormones and enzymes. Thus, it is a chemical change.

(b) Rusting of substance is the chemical process in which the substance reacts with air in the presence of moisture to form a coating called rust. The rusting of iron is a chemical change in which the chemical composition of iron changes. It is an irreversible change as the damaged iron cannot be rectified.  Thus, it is a chemical change.

(c) When iron is mixed with sand, there is no chemical reaction takes place.  Thus, it is not a chemical change.

(d) The food is cooked at high temperatures by adding certain additives like oil, butter, etc. The food is combined with these additives at a specific temperature to form a new substance. Here, the chemical composition of the food changes. Thus, it is a chemical change that cannot be reversed.

(e) During the digestion process, the food is broken down into similar molecules by the enzymes present in the stomach and intestines. Here, the chemical composition of the food changes when the larger molecules of food break down into smaller molecules. Thus, the digestion of food is a chemical change.

(f) The water freeze to form ice. Here, the physical appearance of water changes, but the chemical composition remains the same. So, it is a physical change that can be reversed. Thus, it is not a chemical change.

(g) When the candle is burned, the candle melts, and hence, its physical composition change. When the candle burns, the substance present in the candle reacts with oxygen which changes its chemical composition to form light. Thus, the burning of a candle is both physical and chemical change.

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