CLASS 7 SCIENCE CHAPTER 6 PHYSICAL AND CHEMICAL CHANGE NCERT NOTES

Physical and Chemical Changes :-

• Making a sugar solution,
• setting curd from milk,
• souring of milk,
• stretched rubber band,
• boiling water evaporation,
• cutting hair,
• sharpening a pencil,
• breaking a glass,
• digestion of food,
• boiling an egg,
• etc. represents a change(either physical or chemical change)

Physical Change :-

• 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.

 

Some examples of physical change :-

• 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.

Chemical Change :-

• 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.

Some examples of Chemical change :-

• 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.

A protective shield :-

• 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

Rusting of iron :-

• 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.

 

Prevention of rusting :-

• 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.

Crystallisation :-

• 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.