G. Give a reason for each of the following.
1. Burning of fuel is a desirable change.
2. Rotation of the Earth is a periodic change.
3. Melting of butter is a physical change.
4. Generally physical changes are easier to reverse while chemical changes are not.
5. Boiling of water is a physical change but boiling of egg is a chemical change.
Answers
Answer:
(i) The burning of fuels like wood, coal or petrol is both desirable and undesirable change. The burning of fuels produce heat which is desirable but the burning also produce smoke. It pollutes the atmosphere and is undesirable.
(ii) Rotation of earth on its axis replicates its motion after a fixed period of time, so its movement is periodic. The motion of the earth revolving on its axis in every 24 hours is a kind of periodical motion because the time span of 24 hours is fixed.
(iii) No chemical bonds are broken or formed in melting butter, so it is not a chemical change. The melting process is only a physical change, the change of state from solid to liquid.
(iv) No new substances are formed in a physical change. Also, the original substance can generally be recovered again in a physical change i.e. most physical changes are reversible, as the melting of ice only includes the change in physical properties like state, shape, etc. moreover no new chemical products is formed.
(v) this is because boiled water is again converted into normal water .
but once egg is boiled it's never come into its original form.
Answer:
the burning of fuel is both desirable and undesirable change burning is burning is a desirable change fuels when it burns with smoke come out from it that is undesirable change so it is both desirable and undesirable change Ok