chemistry

There are different substances around us having different properties.Why do they have different properties?For example, why does petrol burn but water does not?What is the difference in composition of petrol and water? Matter undergoes varieties of changes. Why do such changes occur?For example,why does iron rust?Why does milk turn sour?all substances around us do not occur in nature.For example, plastic does not occur in nature but it is prepared by a chemical process.Iron does not occur in free state but it is extracted form the iron compounds that occur in mines. Washing soda is manufactured by a chemical process. If we want to learn all about them, than we should study chemistry.

Chemistry is a branch of science which deals with the composition, properties and transformation of matter.

For convenience chemistry may be sub-divided into the following major branches.

1. Inorgani chemistry

2. Organic chemistry

3. Physical chemistry

4. Analytical chemistry

5. Biochemistry

6. Industrial chemistry

7. Nuclear chemistry

Since the dawn of civilization man has been curious to know about the things around them .If we look around us we will find many things.All these things are nothing but matter.

Classification of matter

           Matter cab be classified in many different ways .One way of classification of matter is shown in the following diagram:

Matter:

1. Physical Classification

a.solid

b.liquid

c.gas

2.Chemical classification

A. Pure substance

a. Element

b. Compound

B. Mixture

Physical classification

Depending upon the conditions of temperature and pressure,matter may exist in the following three physical states.

1. SOLID

2. LIQUID

3. GAS

SOLID is that substance which at room temperature and pressure has definite shape and volume . In solid state  , the molecules constituting the solid are closly packed and there is very little space between them.The

molecules in the solids are tightly held by stroing force of attraction .So solid has definite shape and volume.

LIQUID is that  substance which at room temperature has definite volume but does not have definite shape.In liquid state , the molecules are closly packed but not tightly held like in solid. Mplecules can slide one over another easily in liquid state.Therefore , liquid has definite volume but does not have definite shape.

GAS is that  substance which has neither definite volume nor definite shape.In gaseous state ,molecules are vary far apart and free to move in all possible directions. In gaseous state the intermolecular forces of attraction is very weak. Hence gas has  definite volume nor definite shape.

Molecules

               A molecule is the smallest unit of element or a compound which is capable of independent existence.A molecule of an element will contain only one kind of atoms while a molecule of compound will contain at least two kimd of atoms. For example , a molecule of hydrogen contains two atoms of hydrogen.Hence hydrogen is an element. On the other hand

a molecule of water contains two atoms of hydrogen and one atom of oxygen. Hence water is a compound. A molecule of ozone contains three atoms of oxygen and a molecule of helium contains one atoms of helium. Are ozone and helium an element or compound? What is the difference between helium atom and helium molecule?

Molecular weight

                  Molecular mass or weight is the mass of the molecule in the atomic mass unit.The molecular mass is calculated by adding the atomic masses of constituent atoms in the molecule. The molecular mass of water is 18. This is obtained by adding the mass of two hydrogen and one oxygen.