CLASS 7 SCIENCE CHAPTER 6 PHYSICAL AND CHEMICAL CHANGE NCERT NOTES

Physical and Chemical Changes :-

Properties like shape, size, colour and state of a substance are called its physical properties.
A change in which a substance undergoes a change in its physical properties is called a physical change.
A physical change is generally reversible.
In such a change no new substance is formed.

Breaking a glass,
Sharpening a pencil,
Mixing sand and water,
Wetting of clothes,
Cutting fruits, vegetables and meat,
Transformation of the state of matter(like from liquid to solid, liquid to gas),
Mixing water and oil,
Changing colour, Releasing an odour.

A change in which one or more new substances are formed is called a chemical change.
A chemical change is also called a chemical reaction.
All new substances are formed as a result of chemical changes.
For example, digestion of food in our body, ripening of fruits, fermentation of grapes, etc.,
A medicine is the end product of a chemical reactions.
Plastics and detergents, are produced by chemical reactions.
For new products, the following may accompany a chemical change:
Heat, light or any other radiation (for example, ultraviolet) may be given off or absorbed.
Sound may be produced.
A change in smell may take place or a new smell may be given off.
A colour change may take place.
A gas may be formed.

Burning of magnesium ribbon.
Burning of coal, wood or leaves.
Burning(production of heat)of any substance.
Explosion of a firework which produces heat, light, sound and unpleasant gases that pollute the atmosphere.
That’s why you are advised not to play with fireworks.
When food gets spoiled, it produces a foul smell.
Melting of sugar.
Cooking of food.
Slice of an apple forms a brown colour if not consumed immediately.
Same with a slice of potato or brinjal due to the formation of new substances.
Photosynthesis
Neutralisation of acid and base resulting in the formation of salt and water.

You know the ozone layer in our atmosphere.
It protects us from the harmful ultraviolet radiation coming from the sun.
Ozone absorbs this radiation and breaks down to oxygen.
Oxygen is different from ozone.
If radiations were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms.
Ozone is a natural shield against this radiation.
And this process is happens by chemical reaction.

Ozone formula is O₃ & Oxygen formula is O₂

If you leave a piece of ironin the openfor some time, it acquires a brownish substance.
This substance is called rust and the process is called rusting.
Iron gates of parks or farmlands, iron benches kept in lawns and gardens, almost every article of iron, kept in the open gets rusted.
At home shovels and spades getting rusted when exposed to the atmosphere for some time.
In the kitchen, a wet iron pan (tawa) often gets rusted if left in that state for some time.
Rust is not iron.
Rusting is one change that affects iron articles and destroys them slowly.
Since iron is used in making bridges, ships, cars, truck bodies and many other articles.
- The monetary loss due to rusting is huge.

The process of rusting is represented by the following equation:

Iron + Oxygen + water → rust

Rust = Ferrous oxide (Fe₂O₃xH₂O)

For rusting, the presence of both oxygen and water (or water vapour) is essential.
In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster.

Prevent iron from coming in contact with oxygen, or water, or both.
To prevent rusting, coat of paint or grease should be applied regularly
Another way is to deposit a layer of a metal like chromium or zinc on iron.
This process of depositing a layer of zinc on iron is called galvanisation.
The iron pipes in our homes to carry water are galvanised to prevent rusting.
Ships are made of iron and a part of them remains under water.
Above the water, water drops keep clinging to the ship’s outer surface.
Moreover, the water of the sea contains many salts.
The salt water makes the process of rusting faster.
Therefore, ships suffer a lot of damage from rusting in spite of being painted.
So, a fraction of ship’s iron has to be replaced every year.

Salt can be obtained by the evaporation of sea water which is not pure and the crystals shape cannot be seen clearly.
But, large crystals of pure substances can be formed from their solutions.
The process is called crystallisation.
It is an example of a physical change.