·
Changes are of two types
ü Physical
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. In such a change no new substance is formed.
ü Chemical
changes
§ 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.
Example, rusting of iron, burning of magnesium ribbon.
1. Magnesium
oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2 ]
2. Copper
Sulphate solution (blue) + Iron → Iron Sulphate solution (green) + Copper
(brown deposit)
3. Vinegar
(Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide +
other substances
4. Carbon
dioxide (CO2 ) + Lime water [Ca(OH)2 ] → Calcium Carbonate (CaCO3 ) +
Water (H2O)
§ Chemical
changes are very important in our lives. All new substances are formed as a
result of chemical changes. For example, if a metal is to be extracted from an
ore, such as iron from iron ore, a series of chemical changes takes place. 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.
§ In
addition to new products, the following may accompany a chemical change:
· Heat,
light or any other radiation (ultraviolet, for example) 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
RUSTING
OF IRON
It
is a chemical 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. The process of rusting can
be represented by the following equation:
Iron
(Fe) + Oxygen (O2, from the air) + water (H2O) →
rust (iron oxide Fe2O3 )
For
rusting, the presence of both Fe, 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
Ø 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. 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.
Ø Stainless
steel is made by mixing iron with carbon and metals like chromium, nickel and
manganese. It does not rust.
CRYSTALLISATION
Ø Some
substances can be obtained in pure state from their solutions by
crystallisation.
Ø 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. However, large crystals of pure substances can be formed from
their solutions. The process is called crystallization. It is an example of a
physical change.
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