Page 36 - ICSE Chemistry 8
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Freezing
Freezing is a process by which a substance changes from its liquid state to its solid state on cooling.
Freezing of water into ice is a physical change which could be natural or man-made and is fast, reversible
and desirable.
Melting
Mel ng is a process by which a substance changes from its solid state to its liquid state on hea ng.
Mel ng of ice is a physical change which could be natural or man-made, and is fast and reversible.
Condensation
Condensa on is a process by which a substance changes from its gaseous state to its liquid state on
cooling. The water vapour condense to form clouds. This change is natural, reversible and desirable.
Sublimation
Sublima on is a process by which a substance changes directly from its solid state to its gaseous state
without changing into its liquid state. Naphthalene balls are kept in cupboards to protect woollen clothes
from insects as naphthalene sublimes at room temperature.
Activity 2.6
Aim: To show that sublima on of camphor is a physical change cotton plug
Materials required: A funnel, camphor, co on wool, an evapora ng dish,
solid camphor
a burner, a tripod stand, a wire gauze
Procedure
• Take camphor in an evapora ng dish.
camphor
• Cover the dish by inver ng the funnel as done in the fi gure.
• Block the end of the funnel with co on.
Bunsen
• Now heat the camphor. burner
Observa on: On hea ng, fumes of camphor rise in the funnel, which
then change back to solid camphor and se le along the cooler part of
the funnel.
Conclusion: This shows that sublima on of camphor is a physical change.
Chemical Changes
Thirst for Knowledge
Changes in which two substances react to form new substances
with diff erent proper es are called chemical changes. They are • Chemical changes are the result
generally irreversible in nature. of chemical reac ons.
In chemical changes, molecules of the original substances undergo • All chemical reac ons involve
change in substances and
change to form molecules of the products, i.e., substances
energy.
undergo chemical transforma on to form new substances.
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