Form 1:
Classification of substances

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Physical/Temporary and Chemical changes

A physical/temporary change is one which no new substance is formed and is reversible back to original.

A chemical/permanent change is one which a new substance is formed and is irreversible back to original.

The following experiments illustrates physical and chemical changes

(a)Heating ice

Place about 10g of pure ice in a beaker.

Determine its temperature.

Record it at time 0.0 in the table below.

Heat the ice on a strong Bunsen flame and determine its temperature after every 1 minute to complete the table below:

Plot a graph of time against Temperature (y-axes)

Explain the shape of your graph

Melting/freezing/fusion/solidification and boiling /vaporization /evaporation are the two physical processes.

Melting /freezing point of pure substances is fixed /constant.

The boiling point of pure substance depends on external atmospheric pressure.

Melting/fusion is the physical change of a solid to liquid.

Freezing is the physical change of a liquid to solid.

Melting/freezing/fusion/solidification is therefore two opposite but same reversible

physical processes i.e. A (s)----- A (l)

Boiling/vaporization/evaporation is the physical change of a liquid to gas.

Condensation/ liquidification is the physical change of gas to liquid.

Boiling/vaporization/evaporation and condensation/ liquidification are therefore two opposite but same reversible physical processes i.e. B (l)---- B(g)

Practically

(i) Melting/liquidification/fusion involves heating a solid to weaken the strong bonds holding the solid particles together.

Solids are made up of very strong bonds holding the particles very close to each other (Kinetic Theory of matter).

On heating these particles gain energy/heat from the surrounding heat source to form a liquid with weaker bonds holding the particles close together but with some degree of freedom.

(ii)Freezing/fusion/solidification involves cooling a liquid to reform /rejoin the very strong bonds to hold the particles very close to each other as solid and thus lose their degree of freedom (Kinetic Theory of matter).

Freezing /fusion / solidification is an exothermic process that require particles holding the liquid together to lose energy to the surrounding.

(iii)Boiling/vaporization/evaporation involves heating a liquid to completely break/free the bonds holding the liquid particles together.

Gaseous particles have high degree of freedom (Kinetic Theory of matter).

Boiling /vaporization / evaporation is an endothermic process that require/absorb energy from the surrounding.

(iv)Condensation/liquidification is reverse process of boiling /vaporization / evaporation.

It involves gaseous particles losing energy to the surrounding to form a liquid

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