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What are the postulates of Dalton’s Atomic theory?

(i) Matter is made up of atoms, which are extremely small particles and cannot be subdivided. Atoms retain their identify in all chemical reactions.

(ii) Atoms of the same element are similar in all respects, i.e., in shape, size, mass, etc.

(iii) Atoms of different elements are different in all respects and possess different properties. For example, sulphur and oxygen are entirely different in their properties and posses different atomic masses.

(iv) Atom is the smallest particle that can take part in a chemical combination.

(v) Atoms of the different elements combine in simple but fixed ratio to form compound atom.

(vi) Atoms are indestructible. They can neither be created nor destroyed during any physical or chemical change.


Write the chemical formulae of the following compounds:
a. Magnesium sulphide

b. Ferric chloride

a. The chemical formulae of Magnesium sulphide is MgS

b. The chemical formulae of Ferric chloride is FeCl3.


What is the valency of
a. Sulphur in SO2 
b. phosphorous in P2O5

a. The valency of Sulphur in SO2 is 2.
b. The valency of phosphorous in P2O5 is 5.


Write the significance of chemical formula.
1 .It represents the name of a compound.
2.It shows the constituent elements and the number of atoms of each element present in one molecule of the compound.
3. It represents the molecular mass of the compound.


Explain the rules for writing a chemical equation.

Certain rules have to be followed in writing a chemical equation

1. The reactants taking part in the reaction are written in terms of their symbols or molecular formulae on the left-hand side of the equation.

2. A sign of plus(+) is added between the formulae of the reactants.

3. The products of reaction are written in terms of their symbols or molecular formulae on the right-hand side of the equation.

4. A sign of plus (+) is added between the formulae of the products.

5. In between the reactants and the products an arrow sign(→) is marked to show which way the reaction is occurring.

A + BC + D

In this chemical equation, A and B are the reactants, and C and D are the products. The arrow indicates that the reaction proceeds towards the formation of C and D.


6. Special conditions, such as heat or electrical current, needed to bring about a reaction are mentioned are the arrow mask.

2KNO3 2KNO2 + O2

2H2 + O2 2H2O


Sodium chloride solution gives a white curdy white precipitate of silver chloride when it reacts with silver nitrate solution.

Reactants: Sodium chloride NaCl and silver nitrate AgNO3

Products: Silver chloride AgCl and sodium nitrate NaNO3

Condition: mixing at room temperature

Sodium chloride + silver nitratesilver chloride + sodium nitrate

NaCl + AgNO3 AgCl + NaNO3

NaCl + AgNO3AgCl + NaNO3.


Draw the structure of oxygen atom.

Oxygen has atomic number 8 and mass number 16. So its atom has

No of protons   = 8

No of electrons =8

No of neutrons =(16-8)


So 8P and 8n are present in the nucleus. 8 electrons are present outside the nucleus. Out of 8e, two are in the 1st shell (orbit) and 6 are in the 2nd shell (orbit).


What is valency of an element? How do you derive valency of Aluminium?

Valency of an element is defined as its combining capacity and it is measured by the number of hydrogen atoms with which, one atom of the element combines.

One atom of aluminium combines with three atoms of chlorine.

One atom of Al = 3 atoms of chlorine

Since the valence of chlorine is one,

Valence of Al = 1 × 3 = 3.


Write balanced chemical equations for the following reactions.

Lead nitrate, on heating, decomposes to lead monoxide, nitrogen dioxide gas and oxygen gas .

Calcium oxide reacts with water to form calcium hydroxide and heat is produced.



2Pb(NO3)2 (s) 2PbO(s) + 4NO2(g) + O2(g)
(Lead nitrate)   (Lead monoxide)


            2) CaO(s) + H2O(l)  Ca(OH)2(aq) + heat (Calcium oxide).



State the characteristics of a physical change.

The change is temporary and reversible.

There is no change in the weight of substance

There is no gain or loss of energy.

No new or different products are formed.


Define chemical change.

Chemical changes are changes that substances undergo when they become new or different substances.

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