NCERT Class VII Science Solutions: Chapter 6-Physical and Chemical Changes Part 4

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LONG ANSWER QUESTIONS

Question 19:

Give two examples for each of the following cases:

  1. Physical changes which are reversible.

  2. Physical changes which are not reversible.

  3. Chemical changes.

Answer:

    1. Folding of paper.

    2. Melting of ice.

      Image of reversible physical change

      Image of Reversible Physical Change

      Image of reversible physical change

    1. Tearing of paper.

    2. Breaking of glass.

      Image of not reversible exaple of physical change

      Image of Not Reversible Exaple of Physical Change

      Image of not reversible exaple of physical change

  1. Reaction between vinegar and baking soda.

  2. Burning of a match stick.

    Image of example of chemical change

    Image of Example of Chemical Change

    Image of example of chemical change

There are many other examples in each case which can be given.

Question 20:

Give an example of a chemical reaction for each of the following situations:

  1. A change in colour is observed.

  2. A gas is evolved.

  3. Sound is produced.

Answer:

  1. Reaction between copper sulphate solution and iron metal.

    Image of reaction between Copper sulphate and iron.

    Image of Reaction between Copper Sulphate and Iron.

    Image of reaction between Copper sulphate and iron.

  2. Reaction between baking soda and vinegar (carbon dioxide is evolved).

  3. Burning of crackers.

Question 21:

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.

  1. Do you think rust is different from iron?

  2. Can you change rust back into iron by some simple method?

  3. Do you think formation of rust from iron is a chemical change?

  4. Give two other examples of a similar type of change.

Answer:

  1. Yes, rust is quite different from iron.

  2. No.

  3. Yes. It is a chemical change.

  1. Setting of curd from milk.

  2. Burning of magnesium ribbon to form magnesium oxide.

Question 22:

A student took a solution of copper sulphate in a beaker and put a clean iron nail into it and left it for about an hour.

  1. What changes do you expect?

  2. Are these changes chemical in nature?

  3. Write a word equation for the chemical change, if any

Answer:

    1. Colour of the solution in the beaker changes from blue to green.

    2. A brown coloured deposit is found on the surface of the iron nail.

      Image of reaction between copper sulphate solution and iron metal

      Image of Reaction between Copper Sulphate Solution and Iron Metal

      Image of reaction between copper sulphate solution and iron metal

  1. The changes are chemical in nature as new substances, iron sulphate (green) and copper (brown) are formed.