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

Question: State four characteristics of Physical change?

Answer: Following are the characteristics:

1. It is temporary.
2. It is reversible
3. No new substance is produced.
4. No change in the chemical properties.

Question: State four characteristics of chemical change?

Answer: Following are the characteristics:

1. It is permanent.
2. It is irreversible.
3. New substance is produced.
4. Change in the chemical properties due to the formation of new substance.

Question: Why is conversion of milk into curd is a chemical change?

Answer: During the conversion of milk into curd many reactions involving proteins, organic acids and fat takes place. The taste and other properties of curd are different from those of milk. It is not possible to obtain milk from the curd. Therefore, it is a chemical change.

Question: How would you obtain pure crystals of copper sulphate from its impure sample?

Answer: The process is as follows:

1. Take about 100 ml of distilled water in a beaker and add 1-2 ml of dilute sulphuric acid in it.
2. Heat it to nearly boiling.
3. Remove the beaker from the tripod stand and keep it on the table.
4. Add powdered sample of copper sulphate to this.
5. Stir it well to dissolve.
6. Continue adding copper sulphate powder and stirring till no more copper sulphate can be dissolved. Stir it thoroughly to dissolve the remaining undissolved powder. Heat on low flame, if needed.
7. Allow it to cool undisturbed. Leave it over night.
8. Decant the mother liquid and dry the crystals by keeping them between the folds of filter paper.