:
Anything that may undergo different look, composition, colour, position, size or shape due to some factor internal or external is called change. Changes may be permanentor temporary.

:
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 changesare the changes that repeat themselves after a fixed interval of time.Example - Change of season, day andnight cycle, heart beat.
Non-periodic changesare 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.

:
Reaction to temperature:1 Mark
Examples: 2Marks
Metalsexpand on heating and contract on cooling. This expansion and contraction ofmetals isreversible.
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 expandsand can easily lie on the wheels. When the rim cools, it contracts and presses tightly onto the wheels.

:
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.

Answer: Option C. -> Change that can be undone is irreversible change and change that can't be undone is reversible change.
:
C
All the other points mentioned are correct except this. It should be: "Changes that can be undone are reversible change while the changes that can't be undone are irreversible change.”
For example, evaporation of water to become water vapour is a reversible change as it can be condensed back to water whereas baking bread is irreversible change as baked bread can't be unbaked.

Answer: Option B. -> False
:
B
Forest fires are irreversible changes. It is not possible to get back the forest in the exact same condition as before. Forest burns to produce some new material. These are ash and some gases. We cannot see these gases but can sense them due to their pleasant smell.

Answer: Option C. -> There is no such case possible in general.
:
C
Formation of curd from milk is an irreversible change, as milk cannot be obtained back from curd.
Melting of wax in a candle is a reversible change, as wax can be, solidified and can be converted back to a candle.
In general, changes around us are either reversible or irreversible.

Answer: Option A. -> Heating the iron ring
:
A
On heating the iron ring, it expands and so it becomes bigger in size. The wooden handle can go into the bigger hole. When iron rim is cooled it contracts and fits the wooden handle tightly. This same principle is used in many applications where a snug fit is required in metallic parts.

Answer: Option C. -> Orange juice is frozen to make ice lollies.
:
C
Changes that can be undone are called reversible changes. In the above-given options, only orange juice, which is frozen into ice-lollies, can be melted to give back the orange juice. So, it is a reversible change.
All of the other changes are irreversible.