NCERT 7th Class (CBSE) Science: Chemical and Chemical Changes

Question: To obtain a solid from its solution, which technique is better, evaporation or crystallisation?

Answer: Crystallisation.

Question: Which method is mostly used for purification of solids?

Answer: Crystallisation.

Question: Why burning of oil is a chemical change?

Answer: During burning of oil, oil reacts with oxygen and new substance is formed. Hence it is a chemical change.

Question: Crystallisation of a substance from its solution is a physical change. Give reason.

Answer: Crystallisation of  a substances is a physical change because there is no change in composition and chemical properties of the substance. The crystallized substance can again be made into solution.

Question: Melting of wax is a physical change but burning of wax is a chemical change. Explain.

Answer: Explanation is as follows:

1. Melting of wax change the state of wax from solid to liquid. No new substance is formed. The chemical nature of solid wax and liquid wax is the same. Hence melting of wax is a physical change.
2. Burning of wax gives new gaseous products. Both the physical and chemical changes are taking place. Hence burning of wax is chemical change.

Question: Does the energy changes take place both in physical and chemical changes?

Answer: Yes, all types of changes involve energy. During a change, energy is either evolved or absorbed. Burning of candle is chemical change and energy is evolved. Evaporation of liquid is a physical change and energy is absorbed.

Question: Why the falling and breaking of a glass tumbler is a physical change?

Answer: When a glass tumbler breaks, it forms many pieces. Each broken piece of glass tumbler is still a glass. So, during the breaking of glass tumbler only the size and shape of glass has changed but no new substance has been formed. So, breaking of glass tumbler is a physical change.

Question: Why the burning of a magnesium wire is a chemical change?

Answer: When a magnesium wire is heated, it burns in air to form a white powder called magnesium oxide. This magnesium oxide is an entirely new substance. Thus, a new chemical substance is formed during the burning of a magnesium metal wire. So, the burning of a magnesium wire is a chemical change.

Question: The burning of paper is a chemical change. Give reason.

Answer: If we burn a piece of paper with a lighted matchstick thenon  entirely new substances like carbon dioxide, water vapour, smoke and ash are produced. So, the burning of paper is a chemical change.

Question: What happens when iron nails are dipped in copper sulphate solution?

Answer: When iron nails are dipped in copper sulphate solution, then after some time a brown layer of copper gets deposited on iron nail. This change is due to chemical reaction between copper sulphate and iron. The color of copper sulphate solution also changes from blue to green color due to formation of iron sulphate. Iron nail + Copper sulphate solution → Iron sulphate (green) +  copper (brown)