CBSE – 7th Standard Science

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Chapter 6 – Physical and Chemical Changes

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.

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  • 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

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  • 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)

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  • 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.

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  • 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.

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  • 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.

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  • 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

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