Page 40 - ICSE Chemistry 8

Fast changes: Changes that occur very quickly
Natural changes: Changes that occur naturally on their own, i.e., without any human eff ort
Man-made changes: Changes that occur due to human eff ort or ac vity

Physical changes: Changes in which only the physical proper es of a substance change without any
change in its composi on
Dissolving: A process in which a solid, liquid or gas mixes uniformly with another substance to
form a solu on

Evapora on: A process by which a substance changes from the liquid state to the gaseous state at
a temperature below the boiling point
Boiling: A process by which a substance changes from the liquid state to the gaseous state
when heated to its boiling point
Freezing: A process by which a substance changes from the liquid state to the solid state on
cooling

Mel ng: A process by which a substance changes from the solid state to the liquid state on
hea ng
Condensa on: A process by which a substance changes from the gaseous state to the liquid state on
cooling
Sublima on: A process by which a substance changes directly from the solid state to the gaseous
state without forming the liquid state

Chemical changes: Changes in which two substances react chemically to form new substances with
diff erent proper es
Burning: A process by which a substance combines with oxygen to produce new substances
with heat and light
Rus ng: A process by which iron objects when le exposed to moist air develop a brown fl aky
layer of hydrated iron oxide on their surface

Fermenta on: A process by which a substance is broken down by the ac on of microorganisms with
the release of heat and carbon dioxide

Quick Review

• Changes that occur around us can be classifi ed as reversible and irreversible changes, periodic and
non-periodic changes, desirable and undesirable changes, physical and chemical changes, slow and
fast changes, and natural and man-made changes.
• Processes like dissolving, mel ng, freezing, boiling, evapora on and sublima on are physical changes
as no new substance is formed and we can get back the original substance by reversing the ac on.
• Burning, rus ng, diges on, cooking and fermenta on are chemical changes as new substances are
formed during these processes and they are irreversible in nature.
• Most of the changes involve either absorp on or release of energy usually in the form of heat.
• Changes in which energy is absorbed include mel ng of ice, boiling of water, cooking of food, etc.
• Combus on of fuel, freezing of water, diges on of food, etc. are changes in which energy is released.

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