Physical and chemical changes

PHYSICAL AND CHEMICAL CHANGES

Physical Change:

A change in physical state of matter under suitable conditions.
A Change in which the shape, size, appearance or state, of a substance may alter, but its chemical composition remains same
A physical change occurs when there is change in the interparticle spaces and also interparticle forces.

Characteristics associated with physical change

There is no change in the composition of the substances during the physical change.
No new substance is formed during a physical change.
The change is temporary and can be reversed by reversing the conditions which bring about the change.
No energy change occurs during a physical change.

Example:

By using a piece of gold, a goldsmith can make a large variety of ornaments.
But all of them consist of the same substance, namely gold.
A more common example is that of water, which can be converted into solid ice, liquid water, and gaseous water vapour. It can be reconverted to its previous state by various methods. Yet, in all the three forms, the chemical composition of water is not altered.
No new substance with new chemical properties is formed. In all these, water consists of two atoms of hydrogen and one atom of oxygen.
Thus, a physical change is a change, in which the chemical composition of the substance is not affected, i.e., no new substance is formed.

Examples of Physical Changes:

Dissolving of Sugar in Water:

When sugar is dissolved in water, it seems to disappear completely, and not even a very powerful microscope can show us the molecules of sugar in water.
However, the solution obtained is evenly sweet. By evaporating the water, the sugar can be recovered in a crystalline form.

Magnetizing an Iron Rod:

An iron rod can be converted into a magnet by passing an electric current through an insulated coil wound around.
When the current flows, the iron bar gets magnetized. One end becomes North Pole and the other the South Pole.
If the direction of the current is reversed, the North Pole changes to South Pole and the South Pole changes to North Pole.

Glowing of an Electric Bulb:

When an electric current is passed through the filament of a bulb, the filament starts glowing and the glow stops as soon as the current is cut off.

Chemical Change:

A change in which the chemical composition of a substance is altered is called as chemical change.
The substances combine with each other producing a new substance with properties different from the combining components.

Characteristics associated with chemical change

A new substance is formed as the result of chemical change.
There is always a change in composition of the substances undergoing chemical changes.
The chemical change is of permanent nature and cannot be easily reversed.
Energy changes always occur in the chemical reactions responsible for these changes.

For example:

A layer of reddish brown substance called rust is formed on iron objects.
Iron is a grayish white metal and conducts electricity. It is attracted by a magnet, and reacts with dilute acids to yield hydrogen.
Sulphur is a non-metal, and is yellow in colour. It dissolves in carbon disulphide.
When powdered iron and sulphur are heated together, a completely new substance, iron sulphide is formed.
The properties of iron sulphide are entirely different from those of iron and sulphur.
It is black in colour, does not get attracted by a magnet and does not allow electric current to pass through it.
It reacts with dilute acids to from hydrogen sulphide gas.
In short, the properties of neither iron nor sulphur are exhibited by iron sulphide.

Difference between physical and chemical change:

PHYSICAL CHANGE		CHEMICAL CHANGE
1.	No new substance is formed in a physical change.	1.	A new substance is formed in a chemical change.
2.	A physical change is a temporary change.	2.	A chemical change is a permanent change.
3.	A physical change is easily reversible	3.	A chemical change is usually irreversible.
4.	Very little heat (or light) energy is usually absorbed or given our in a physical change.	4.	A lot of heat (or light) energy is absorbed or given out in a chemical change.
5.	The mass of a substance does not alter in a physical change.	5.	The mass of a substance does alter in a chemical change.

Some Reactions where both changes occurs simultaneously:

For example: When a person eats chocolate and then digests it physical and chemical changes take place.

By chewing chocolate the shape, size and appearance of chocolate changes i.e. Physical change occurs.
Once acted upon by saliva and other digestive juices, the chocolate is broken down into other simpler substances, which can be absorbed by the blood. This is a chemical change.
The necessary conditions for a chemical change are:
A minimum amount of energy is needed to initiate the chemical reaction should be supplied in the form of heat, light or electric current.
In a chemical change, the reactants combine to form new products.
For this process it is necessary to break the old bonds of the reactants and form fresh bonds, in order to give new products. This requires a certain amount of energy.