Reaction to temperature: 1 Mark
Examples: 2 Marks
Metals expand on heating and contract on cooling. This expansion and contraction of metals is reversible.
Examples:
1) Small gaps are left between the rails that form the parallel lines of a railway track. These gaps are called expansion gaps. These gaps allow the rails to expand in hot weather. If there are no gaps or gaps are too small the lines can bend.
2) While making agricultural tools, the metal ring is heated before fixing the wooden handle. On heating, the metal ring expands. When the ring becomes bigger in size, the wooden handle can fix easily. When the metal ring cools, it contracts and the handle fits in tightly.The fixing of a metallic rim to the wooden base of the wheel of bullock cart can be done in a similar manner. The metallic rim used is slightly smaller in diameter than the wheels. On heating, the rim expands and can easily lie on the wheels. When the rim cools, it contracts and presses tightly onto the wheels.

Definition : 1 Mark
Example : 1 Mark
A reversible change is a change that can be undone or reversed. These changes are observed in the physical state, size and shape of the substance. A reversible change does not create a new material.
Example - A blacksmith changes a piece of iron into different tools. For that, a piece of iron is heated to red hot. This also softens it. It is then beaten into the desired shape. It is a reversible change.

Periodic changes and examples : 2 Marks
Non-periodic changes and examples : 2 Marks
Type of change : 1 Mark
Apart from irreversible and reversible changes, there are periodic and non-periodic changes. 
Periodic changes are the changes that repeat themselves after a fixed interval of time. Example - Change of season, day and night cycle, heart beat.
Non-periodic changes are the changes that do not repeat themselves after a fixed interval of time. Example - Tornado, volcanic eruption, tsunami, etc. (most natural disasters).
A forest fire is a non-periodic change.

Each change : 1 Mark
Any three -
1. Shedding of leaves
2. Ripening of fruits
3. Growth of the body
4. Blooming of flowers

Process : 1 Mark
Type of change : 1 Mark
Reason : 1 Mark
When vinegar is mixed with bicarbonate of soda, the release of carbon dioxide gas is seen in the form of bubbles. We cannot get back our original substances since it is an irreversible change.

Physical change: 2 Marks
Chemical change: 2 Marks
Examples: 1 Mark
Physical changes are the changes where the physical property of the substance varies without any formation of new products. These changes are reversible and the products are identical to the reactants.

Examples: Melting of ice, melting of wax.

Chemical changes are the changes where the chemical property of the substance varies with the formation of new products. These changes are irreversible and the products are not identical to the reactants.

Types of changes: 1 Mark
Slow changes + example: 2 Marks
Fast changes + example: 2 Marks
Based on speed, there are two types of changes - slow and fast changes.
Slow changes:
The changes that take place slowly over a long period of time are called slow changes.
Examples- Germination of seeds, formation of day and night and rusting of iron.

Fast changes:
The changes which take place quickly are called fast changes.
Examples: Burning of paper, melting of a candle and blowing of balloons.

Deforestation is not a reversible change.
Deforestation is considered an irreversible change because once the trees are cut, the same tree cannot be planted. 

The change of a substance from solid to liquid is called melting.

Justification: 2 Marks
The formation of clouds is a physical change because there is a phase transformation cycle of natural water from liquid to gas and then, gas to liquid. Hence, the property of water never changes.