Revision: Physical and Chemical Changes Chemistry ICSE ICSE Class 7 CISCE

Changes which take place over a long period of time (hours/days/months/years) are known as slow changes.

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.

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.

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

OR

A transition from one state of matter (solid, liquid, or gas) to another is called change of state.

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

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

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.

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

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 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 substances taking part in chemical reaction are called reactants.

The cell which converts electrical energy into chemical energy is called an electrolytic cell and the process which converts electrical energy into chemical energy is called electrolysis.

A chemical reaction that occurs in the presence of light is called a photochemical reaction.

A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.

Any chemical change in matter that involves its transformation into one or more new substances is called a chemical reaction.

An electrochemical reaction is a chemical reaction involving the transfer of electrons, where oxidation and reduction occur simultaneously.

Chemical reactions which are characterised by the formation of insoluble solid substances are called precipitates.

A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the products.

A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.

The substances formed as a result of a chemical reaction by formation of new bonds are called products.

A chemical change is a process in which the composition of matter changes, resulting in the formation of one or more new substances.

When an iron nail is kept in copper sulphate solution, a reddish brown coating is seen on the iron nail after sometime. Also, the colour of the copper sulphate solution changes from blue to green. This is observed because iron displaces copper from copper sulphate solution, as iron is more reactive than copper.

\[\ce{Fe_{(s)} + CuSO4_{(aq)} -> FeSO4_{(aq)} + Cu_{(s)}}\]

• 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.