Give one example in each case
(a) Change which occurs on heating but can be reversed.
(b) Change which occurs on heating but cannot be reversed.
(c) Change which occurs on cooling but can be reversed.
(d) Change which occurs on mixing two substances, butcan be reversed.
(e) Change which occurs on mixing two substances, butcannot be reversed.
Tearing of paper is said to be a change that cannot be reversed.
What about paper recycling
Boojho’s sister broke a white dove, a symbol of peace, made of
Plaster of Paris (POP). Boojho tried to reconstruct the toy by
making a powder of the broken pieces and then making a paste
by mixing water. Will he be successful in his effort? Justify
your answer
Which of the following changes cannot be reversed?
(a) Blowing of a balloon
(b) Folding a paper to make a toy aeroplane
(c) Rolling a ball of dough to make roti
(d) Baking cake in an oven
(e) Drying a wet cloth
(f) Making biogas from cow dung
(g) Burning of a candle
Can we reverse the following changes? If yes, suggest the name
of the method.
(a) Water into water vapour
(b) Water vapour into water.
(c) Ice into water.
(d) Curd into milk.
Paheli had bought a new bottle of pickle from the market. She
tried to open the metal cap to taste it but could not do so. She
then took a bowl of hot water and immersed the upper end of
the bottle in it for five minutes. She could easily open the
bottle now. Can you give the reason for this?
A piece of iron is heated till it becomes red-hot. It then becomes
soft and is beaten to a desired shape. What kind of changes
are observed in this process– reversible or irreversible?
Iron rim is made slightly smaller than the wooden wheel. The
rim is usually heated before fixing into the wooden wheel,
because on heating the iron rim
(a) expands and fits onto the wooden wheel.
(b) contracts and fits onto the wooden wheel.
(c) no change in the size takes place.
(d) expands first, then on cooling contracts and fits onto
the wooden wheel.
Rolling of chapati and baking of chapati are the changes that
(a) can be reversed.
(b) cannot be reversed.
(c) can be reversed and cannot be reversed, respectively.
(d) cannot be reversed and can be reversed, respectively
Salt can be separated from its solution (salt dissolved in water),
because
(a) mixing of salt in water is a change that can be reversed
by heating and melting of salt.
(b) mixing of salt in water is a change that cannot be
reversed.
(c) mixing of salt in water is a permanent change.
(d) mixing of salt in water is a change that can be reversed
by evaporation.
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