Changes Around Us

- Some changes can be reversed and some cannot be reversed.

- A change may occur by heating a substance or by mixing it with some other.

- Types of changes:
(a) Reversible change: A change in which the initial substance can be obtained back by reversing the action. Example: folding of paper, dissolving sugar in water, etc.
(b) Non-irreversible change: Change in which the initial substance cannot be obtained back by reversing the action. Example: burign of paper, grinding grains etc.
(c) Physical change: Changes in the form of substance but not in chemical identity. No new substance formed. Changes is sometimes reversible. Example: breaking a log of wood.
(d) Chemical changes: Changes in which substance is transformed into new substance.
Initial substance is lost. Change is always irreversible. Example: burning a log of wood.

- Ways by which changes occur:
(a) Boiling and Condensation:
(i) Boiling: The rapid vaporization of a liquid when it is heated to its boiling point.
(ii) Condensation: The change of water vapor into liquid water on cooling.

(b) Heating of metal: Process in which a metal is heated to a certain temperature and the cooled in a particular manner to alter its internal structure for obtaining desired degree of physical and mechanical properties such as brittleness, hardness, and softness.

(c) Freezing and Melting:

(i) Freezing: The process in which a liquid turns into solid when its temperature is lowered.
(ii) Melting: The process in which a solid converts to a liquid by applying heat.