Revision: Term - 2 >> Changes Around Us Science SSLC (English Medium) Class 7 Tamil Nadu Board of Secondary Education

A permanent change in which new substances are formed whose composition and properties are completely different from those of the original 
substances.

Chemical change:
(i) Burning of wood.
(ii) Breathing.

Physical change: A temporary change in which no new substance is formed, the composition of substance remains the same, though its state, shape and size may change.

Example of physical change:

1. Change of water into its vapours.
2. Heating of iron rod.

A transition from one state to another is called a change of state (or phase change).

Or

The process of change from one state to another at a constant temperature is called the change of phase.

The triple point of water is that point where water in a solid, liquid and gas state co-exists in equilibrium and this occurs only at a unique temperature and a pressure.

Sublimation is the process in which a solid changes directly into a gas on heating, without passing through the liquid state.

Heat energy is absorbed by a solid during melting and an equal amount of heat energy ‘ is liberated by the liquid during freezing, without any temperature change. It is called the latent heat of fusion.

The physical process that results in the transition of a substance from the solid-state to the liquid state on heating at a fixed temperature is called Melting.

It can be defined as the fixed temperature at which a solid start changing to its liquid state is Melting Point

Melting or fusion: The process in which a solid is converted to a liquid by absorbing heat is called melting or fusion.

The particular temperature at which vaporisation occurs is called the boiling point of liquid.

The change from liquid to gas (or vapour) phase on absorption of heat at a constant temperature, is called vaporisation.

The change from vapour to liquid phase on liberation of heat at a constant temperature is called condensation (or liquefaction) and the particular temperature at which the condensation occurs is called the condensation point of vapour.

The change of state from solid to gas directly is called sublimation.

The change of a solid substance directly into a gas or vapour without first changing into a liquid is called sublimation.

OR

The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime.

Chemical reactions accompanying the evolution of heat are called exothermic reactions whereas reactions involving absorption of heat are called endothermic reactions.

A chemical reaction in which heat is absorbed from the surroundings or must be continuously supplied is called an endothermic reaction.

The process in which heat is absorbed from the outside, is called endothermic process.

A chemical reaction in which heat is released when reactants are converted to products is called an exothermic reaction.

The processes in which heat is given out are called exothermic processes.

• A change of state occurs when heat exchange causes a substance to transition between solid, liquid, and gas phases.
• Temperature remains constant during a phase change because heat energy changes molecular arrangement (potential energy), not molecular speed (kinetic energy).
• The heating curve has flat plateaus at the melting point (0 °C) and boiling point (100 °C) for water, with rising slopes in between.