CBSE – 7th Standard Science

Chapter 6

Physical and Chemical Changes

• Properties such as shape, size, color 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, no new substance is formed.

• 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

• Chemical changes are very important in our lives.

•  All new substances are formed as a result of chemical changes, if a metal is to be extracted from an ore, such as iron from iron ore, a series of chemical changes will be carried out

•  A medicine is the end product of a chain of chemical reactions.

• Useful new materials, such as plastics and detergents, are produced by chemical reactions.

• Indeed, every new material is discovered by studying chemical changes.

• Heat, light or any other radiation 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, color change may take place, gas may be formed after chemical reactions.

• A change is in the rusting of iron a piece of iron left in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting

• Rusting is one change that affects iron articles and slowly destroys them.

• Since iron is used in making bridges, ships, cars, truck bodies and many other articles, the monetary loss due to rusting is huge.

• Iron (Fe) + Oxygen (O2, from the air) + water (H2O)—à rust (iron oxide Fe2O3)

• For rusting, the presence of both oxygen and water or vapor is essential.

• One simple way is to apply a coat of paint or grease, these coats should be applied regularly to prevent rusting.

•  Another way is to deposit a layer of a metal like chromium or zinc on iron.

• The salt water makes the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting in spite of being solutions.

• Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.

• This process of depositing a layer of zinc on iron is called galvanisation.

• The iron pipes we use in our homes to carry water are galvanised to prevent rusting.

• Salt can be obtained by the evaporation of sea water.

• The salt obtained in this manner is not pure and its crystals are small.

• The shape of the crystals cannot be seen clearly. The process is of crystallization, is an example of a physical change.

• Changes can be of two types, physical and chemical.

• Physical changes are changes in the physical properties of substances.

• No new substances are formed in these changes. These changes may be reversible.

• In chemical changes new substances are produced.

• Some substances can be obtained in pure state from their solutions by crystallisation.

• The equations here are different from those in mathematics. In equations of this kind, the arrow implies ‘becomes’.

• No attempt should be made to balance chemical equations at this stage.

• You must have heard of the ozone layer in our atmosphere.

• It protects us from the harmful ultraviolet radiation which come from the sun.

• Ozone absorbs this radiation and breaks down to oxygen.

• Oxygen is different from ozone.

• If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms.

• Ozone acts as a natural shield against this radiation.

• Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.

• You know that ships are made of iron and a part of them remains under water.

• On the part above water also, 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 rust formation faster.

• In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster.

• Prevent iron articles from coming in contact with oxygen, or water, or both.

• One simple way is to apply a coat of paint or grease.

•  In fact, these coats should be applied regularly to prevent rusting.

• Another way is to deposit a layer of a metal like chromium or zinc on iron

Important reactions of this chapter are