Isomorphism

 (Iso = same + morphous = shape or form)
Phenomenon in which due to which two or more substances have same crystalline shape is called isomorphism and the substances are called isomosphous substances.
Example
NaCl, MgO, and ZnS have cubic shape.
CaCO3 and AgNO3 have Trigonal shape.

Properties of isomorphous substances:-
Isomorphous substances have definite physical properties, like melting point, boiling point, density, solubility etc. Their chemical properties are also different. Only isomorphous substances have similar arrangement of particles in crystal lattice. They have property of over growth, thus if crystal of one isomorphous substance is placed in the solution of other then the solution builds its crystal over the first crystal.

Polymorphism:-
(Poly = many + morphous = shape or form)
Phenomenon due to which a substance has more than one crystalline shapes is called polymorphism and the substance is called polymorphous substance
Example
CaCO3 has orthorhombic and trigonal shapes, CuSO4  5H2O has monoclinic and triclinic shapes.

A substance exists in different crystalline shapes due to different condition of formation of crystals.

Unit Cell:-
The Smallest repeating particle or unit of a crystal is called unit cell. A unit cell is a basic building block of a crystal. A unit cell has its axis which are represented by a, b and c and the angle between the axis are represented by alpha, beta and gamma shape of crystal depends on its unit cells and the shape of unit cell depends on the measurement of its axis and the angle between the axis.

Crystal System:-
Shape of crystal showing the measurement of its unit cell with respect to the lenght of its axis and angle between the axis is called crystal system. There are seven crystal system, given below:

1) Cubic System:-
 In this system all the three axis of unit cell are equal in lenght and all the angles are 90 i-e axis a=b=c and angles alpha = beta = gamma = 90
Example:-
NaCl, MgO, ZnS and Diamond.

2) Tetragonal System:-
 In this system two axis of unit cell are equal in lenght and all the angles are of 90.
 ‎i-e axis a (equal to) b (does not equal to) c
 ‎and angles
 ‎alpha = beta = gamma = 90

Example
SnO2 (stannic oxide) and BaSO4 5H2O (Barium sulphate pentahydrate).

3) Orthorhombic System:-
 In this system all three axis of unit are different in lenght and all the are of 90 degree.
Example
FeSO4 7H2O (Ferous sulphate hepta hydrate), ZnSO4 7H2O (Zinc sulphate hepta hydrate), CaCO3 and KNO3.

4) Trigonal or Rhombohedral System:-
 In this system all three axis of unit cell are equal in lenght and all the angles are equal but between 90 and 120 degree.
 ‎Example
 ‎CaCO3, AgNO3 and KNO3.

5) Hexagonal System:-
In this system two axis of unit cell are equal in lenght and two angles are of 90 degree amd third is of 120 degree.
Example
Graphite, Show and Quartz (Crystal of sand) SiO2.

6) Monoclinic System:-
 In this system all the three axis of unit cell are different in lenght and two angles
 ‎are of 90 degree third is unequal to 90 degree.
 ‎Example
 ‎CuSO4 5H2O (Copper sulphate penta hydrate), Na2CO3 10H2O (Sodium carbonate deca hydrate washing soda) and cane sugar.

7) Triclinic System:-
 In this system all the three axis of unit cell are different in lenght and all the angles are unequal and enequal to 90 degree.
Example
CuSO4 5H2O (copper sulphate penta hydrate) and K2Cr2O7 (Potassium dichromate).


Melting Point:-
Temperature at which solid becomes transparent liquid is called its melting point.
OR
Temperature at which solid phase of a substance is in equillibriun with its liquid state is called melting point.
   Solids which are hard, they have high melting points and the solids which are soft, they have low melting point.

Factors Affecting Melting Point:-
Following factors affect the melting point.

i) Impurity:-
Impurity decreases the melting point. Thus a pure solid has high melting point than impure solids.
Example
Pure NaCl melts at 108 celsius but jf some CaCO3 is mixed with it then it melts at 600 celsius.

ii) Pressure:-
Generally solids expand on melting and their volume increases so increase in pressure increases their melting temperature because it opposes the increase in volume and thus high temperature required to increase the volume of solid for melting.
In case of ice the phenomenon is different, because on melting volume of ice decreases so by increasing pressure melting point of ice decreases.

Boiling Point:-

Definition:-
Temperature at which vapour pressure or internal pressure of a liquid becomes equal to the atmosphere pressure or external pressure is called boiling point.

Explanation:-
When the liquid is heated its molecule change in to vapours as temperature of liquid increases the number of molecules changing into vapours also increases so the vapour pressure is increase.
When the vapour pressure and internal pressure of a liquid become equal to the atmosphere pressure and some external pressure then bubble start coming from the liquid continously and the liquid is said to be boil, at this stage temperature of liquid is called its boiling point.
Both the definition of melting and boiling point are certain different.