Chapter - 6 Physical and Chemical Changes

·      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.