Page 44 - Chemistry ICSE Class IX
P. 44
32 ICSE Chemistry – 9
Physical change Chemical change
3. No energy in the form of heat, light or sound is 3. A chemical change is always accompanied by the
absorbed or evolved in a physical change. absorption or evolution of energy in the form of heat,
light or sound.
4. The original form of the substance can be obtained 4. The original substance cannot be obtained by simple
easily by simple physical methods. physical methods.
5. Physical change is generally temporary, in the 5. Chemical change is a permanent change in the
sense that it can be easily reversed by reversing the sense that it cannot be reversed by just reversing the
conditions. conditions.
Changes Involving both Chemical and Physical
Changes
There are certain changes which involve both chemical and physical
changes. Some typical examples are described below:
Burning of candle. During the burning of a candle, the following
changes occur:
Process Nature of change
Melting of wax and its resolidi cation Physical change
Burning of wax to produce CO , H O(g), and light and heat Chemical change
2
2
Burning of thread to give CO , H O(g), and light and heat Chemical change
2
2
Heating of zinc hydroxide. When zinc hydroxide (Zn(OH) ) is
2
heated, the following changes occur:
Decomposition of zinc hydroxide to zinc oxide
heat
Zn(OH) o ZnO + H O Chemical change
2
2
ZnO ZnO Physical change
(white) (yellow)
Action of heat on sodium nitrate. When sodium nitrate (NaNO ) is
3
heated, following changes take place:
Melting of sodium nitrate
NaNO (s) NaNO (l) Physical change
3
3
Burning of a candle
Decomposition of sodium nitrate to give sodium nitrite (NaNO )
2
and oxygen.
heat
2NaNO (l) o 2NaNO (s) + O (g) Chemical change
2
2
3
Conditions Necessary for a Chemical Reaction to
Take Place
Chemical reactions (or chemical changes) occur under certain conditions.
Some of the conditions found necessary for a chemical reaction or chemical
change to take place are described below.
Close contact
For reactions to occur, the reactants must come in contact with each other.
The reactions in solution phase are quite fast, because the reactants are
able to come in close contact with each other in solutions. For example, solid
NaCl does not react with solid AgNO , but when the aqueous solutions of
3
the two are mixed, the reaction proceeds immediately.
