Question

Please answer properly since i've added 10 marks each


1.Explain with the help of examples, the difference between the changes that can be
reversed and those that cannot be reversed.

2.Explain the type of changes involved in this sequence
Ice cube placed in sun → puddle of water → disappearance

3.Why are there gaps between sections of nails on the railway track

4.Why does milk in a saucepan overflow when heated?

Answer 1

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Answer:

Reversible changes : (a) Butter melts when jt becomes warm or heated ( change in state ). we can easily get back the solid butter by cooling it .

(b) We inflate a balloon by blowing air into air ( change in shape and size ) the balloon can be deflated easily by releasing the air from it .

Ireversible changes : (a) An egg of butterfly hatches into a caterpillar . The caterpillar forms a pupa . A butterfly emerges from pupa . The butterfly cannot change back to pupa .

(b) When a glass tumbler breaks into pieces cannot be changed back to it's original form

The ice cube changing into water puddle in MELTING

This process is solid ice cube changing into a liquid.

This is a PHYSICAL CHANGE. When water melts, it changes from solid state to to liquid state ;  but it is still water; it has not formed any new substance.

The water puddle disappearing is EVAPORATION

This process is liquid water changing into gas

This is a CHEMICAL CHANGE  When water evaporates, it changes from the liquid state to the gas state,  When water evaporates, it changes from the liquid state to the gas state, but it is still water; it has not formed any new substance.

The rails of railway tracks are made up of steel. They can bend sideways in summer due to linear expansion of the steel. Hence, a small gap is left between the rails at the time of laying them. This is because rails expand in summer due to increase in temperature.

Milk overflows when boiled for the first time because it contains entrapped air. The air expands on heating and tends to escape lifting the surface of milk.Once all the air is expelled, the boiling would be regular and smooth. A second boiling would not lead to overflow.