Physical and Chemical Changes

Changes are of two kinds, physical and chemical.

Physical change: 

Properties such as shape, size, colour 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.

Chemical change: 

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. 

For example, if a metal is to be extracted from an ore, such as iron from iron ore, we need to carry out a series of chemical changes. 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.

Rusting of iron is a very familiar example of chemical change. If we leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting.

Here are some activities which illustrate chemical changes:

Activity.1 

If we burn a thin strip or magnesium ribbon , it burns with a brilliant white light. When it is completely burnt, it leaves behind a powdery ash. The change can be represented by the following equation:

Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO)

Collect the ash and mix it with a small amount of water. On dissolving the ash in water, it forms a new substance. This change can be written in the following form of equation: 

Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2]

Magnesium hydroxide is a base. So, magnesium oxide is a new substance formed on burning of magnesium. Magnesium hydroxide is another new substance formed by mixing magnesium oxide with water.

Test of Magnesium Hydroxide Solution with Litmus paper :

Magnesium Hydroxide solution will turn red litmus blue whereas blue litmus remains blue.

Activity.2 

Dissolve about a teaspoonful of copper sulphate (blue vitriol or neela thotha) in about half a cup of water and add few drops of dilute sulphuric acid to solution. This will result in a blue coloured solution. Drop a nail or used shaving blade in the solution.

Blue coloured solution is changed to green due to formation of iron sulphate, a new substance. This changes are because of reaction between copper sulphate and iron. The brown deposit on the iron nail is copper, another new substance.

We can write the reaction as:

Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)

Activity.3 

Take about a teaspoonful of vinegar in a test tube. Add a pinch of baking soda to it. There is a hissing sound produced and bubbles of a gas can be seen coming out.

When this gas, which is actually carbon dioxide, is passed through freshly prepared lime water as shown in below image, calcium carbonate is formed, which makes lime water milky. 

The turning of lime water into milky is a standard test of carbon dioxide.

Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances

The reaction between carbon dioxide and lime water is as follows:

Carbon dioxide (CO2) + Lime water [Ca(OH)2] → Calcium Carbonate (CaCO3) + Water (H₂O)

We have seen that one or more new substances are produced in a chemical change. 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.

Here are some example:

• Burning of any substance is a chemical change. Burning is always accompanied by production of heat.
• Explosion of a firework is a chemical change. Such an explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere.
• When food gets spoiled, it produces a foul smell. 
• A slice of an apple acquires a brown colour if it is not consumed immediately. Same happens with a slice of potato or brinjal. The change of colour in these cases is due to the formation of new substances.
• Plants produce their food by a process called photosynthesis, which is a chemical change.
• Even digestion is a chemical change.

Do you know about ozone layer? 

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. The breaking down of ozone is also a chemical change.

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.

Rusting Of Iron:

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 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.

How do we prevent rusting?

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

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. 

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

Crystallisation:

The process of formation of large crystals of pure substances from their solutions is called crystallisation. It is an example of a physical change. 

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

Crystals of copper sulphate.