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.
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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