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Chemistry-day2 physical-chemical-changes Target ssc cgl 2018

Chemistry-day2 physical-chemical-changes Target ssc cgl 2018

Chemistry-day2 physical-chemical-changes Target ssc cgl 2018

Physical Changes

Physical changes are the change, which only affect the physical properties like colour, hardness, density, melting point etc. of matter, but do not affect the composition and chemical properties of matter. A physical change is generally reversible (Things or processes which can be reversed are called reversible. For example, a folded sheet of paper can be unfolded and hence folding a sheet of paper is reversible). In such a change no new substance is formed.

Chemical Changes

Chemical changes affect the composition as well as chemical properties of matter and result in the formation of a new substance. A chemical reaction is irreversible (Things or processes which cannot be reversed are called irreversible. For example, when milk turns into curd; it cannot be changed back to milk and hence is an irreversible change).  A chemical change is also called a chemical reaction.

A change in which one or more new substances are formed is called a chemical change

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

 Magnesium hydroxide is a base. So, magnesium oxide is a new substance formed on burning of magnesium. 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.

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.

We have seen that one or more new substances are produce 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.

Burning of coal, wood or leaves is also a chemical change. In fact, burning of any substance is a chemical change. Burning is always accompanied by production of heat.

Explosion of a firework is a chemical change. Are not these changes chemical changes?  You neutralized an acid with a base. Is neutralization a chemical change?

physical-chemical
Rusting of Iron: 

Deposition of a brown layer on iron is called rusting. In rusting, a new substance is formed. The chemical structures of iron and rust are completely different. Rust is iron oxide. Iron is a grey-black material while rust is reddish brown Thus, this is a chemical and irreversible change. Reaction in rusting can be written as follows:

4Fe + 3O2  –> Fe2O3 

 When articles made of iron come in contact with moisture present in air, they get rusted. Iron is converted into iron oxide, i.e. rust. The iron article becomes weak in due course as all the iron slowly turns into rust. This is corrosion of iron. Rusting gives a huge monetary loss to the people and nation.

Prevention of rusting:

For rusting, both water and oxygen should come in contact with iron. If anyone of these is prevented to come in contact with iron, rusting can be prevented. So, rusting is prevented using following methods:

Painting:

Articles such as iron gates, grills, etc. are painted at regular intervals of time.

Applying of layer of grease:

Applying a layer of grease prevents the iron articles from coming in contact with moist air. This prevents rusting. That is why grease is applied over the chain of bicycle and also over many machine parts.

Galvanisation:

In the process of galvanization, a layer of non-reactive metal, such as zinc is deposited over iron articles. The layer of non-reactive metal prevents the iron articles from coming in contact with moisture. Thus it prevents rusting.

Crystallization

You have learnt that 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.

A protective shield

You must have heard of the ozone layer in our atmosphere. It protects us from the harmful ultraviolet radiation which comes from the sun. Ozone absorbs this radiation and breaks down to oxygen. Oxygen is different from ozone. Can we call the breaking down of ozone 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. Plants produce their food by a process called  photosynthesis. Can we call photosynthesis a chemical change? Even digestion is a chemical change.

Keywords in the chapter

Chemical change

Chemical reaction

Crystallization

Galvanization

Physical change

Rusting

What you have learnt

Changes can be of two types, physical and chemical.

Physical changes

Boiling of water.

Melting of ice-cream.

Evaporation of water

Chemical changes

Digestion of food.

Rusting of iron.

Photosynthesis.

physical-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 crystallization.

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