FSc Notes Class 11 Chemistry Three States Of Matter

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Three States Of Matter

Matter

It is defined as any thing which has mass and occupies space is called matter.

Matter is composed of small and tiny particles called Atoms or molecules. It exist in three different states which are gaseous, liquid & solid.

Properties of Gas

1. It has no definite shape.
2. It has no definite volume, so it can be compressed or expanded.
3. A gas may diffuse with the other gas.
4. The molecules of a gas are in continuous motion.

Properties of Liquids

1. A liquid has no definite shape.
2. It has a fixed volume.
3. The diffusion of a liquid into the other liquid is possible if both of the liquids are polar or non-polar.
4. It can be compressed to a negligible.

Properties of Solids

1. A solid has a definite shape.
2. It has a fixed volume.
3. The rate of diffusion of solid with each other is very slow.
4. It cannot be compressed easily.

Kinetic Theory of Gases

It was an idea of some scientist like Maxwell & Boltzmann that the properties of gases are due to their molecular motion. This motion of the molecules is related with the kinetic energy, so the postulates give by the scientist about the behaviour of gases are collectively known as kinetic molecular theory of gases.

The postulates of kinetic molecular theory are as follows.

1. All gases consists of very large number of tiny particles called molecules.
2. These molecules are widely separated from each other and are so small that they are invisible.
3. The size of the molecules is very small as compared to the distance between them.
4. There is no attractive or repulsive force between molecules so they can move freely.
5. The molecules are very hard and perfectly elastic so when they collide no loss of energy takes place.
6. The gas molecules are in continuous motion they move in a straight path until they collide. The distance between two continuous collision is called Mean Free Path.
7. During their motion these molecules are collided with one another and with the walls of the container.
8. The collision of the molecules are perfectly elastic. When molecules collide they rebound with perfect elasticity and without loss or gain of energy.
9. The pressure of the gas is the result of collision of molecules on the walls of the container.
10. The average kinetic energy of gas molecules depends upon the absolute temperature. At any given temperature the molecules of all gases have the same average kinetic energy (1/2 mv2).

Kinetic Theory of Liquids

This theory is bases on the following assumptions.

1. The particles of a liquid are very close to each other due to which a liquid has fixed volume.
2. The particles in a liquid are free to move so they have no definite shape.
3. During the motion these molecules collides with each other and with the walls of the container.
4. These molecules possess kinetic which is directly proportional to its absolute temperature.

Kinetic Theory of Solids

The assumptions of kinetic theory for solids are as follows.

1. The particles in a solid are very closely packed due to strong attractive forces between the molecules.
2. These molecules are present at a fixed position and are unable to move.
3. They have definite shape because the particles are arranged in a fixed pattern.
4. They possess only vibrational energy.

Mean Free Path

The distance which a molecule of a gas travels before its collision with the other molecule is called free path. This distance between the collision of the molecules changes constantly so the average distance which a molecule travels before its collision is called mean free path.

Boyle’s Law

A relationship of volume with external pressure was given by Boyle’s in the form of law. This law is known as Boyle’s Law which states,

For a given mass of a gas the volume of the gas is inversely proportional to its pressure provided the temperature is kept constant.

Mathematically it may be written as

V ∞ 1 / P

Or V = K / P

Or PV = K

On the bases of the relation, Boyle’s law can also be stated as

The product of the pressure and volume of a given mass of a gas is always constant at constant temperature.

Explanation

Consider for a given mass a gas having volume V1 at pressure P1, so according to Boyle’s Law we may write as

P1V1 = K1 (constant)

If the pressure of the above system is changed from P1 to P2 then the volume of the gas will also change from V1 to V2. For this new condition of the gas we can write as,

P2V2 = K2 (constant)

But for the same mass of the gas.

K1 = K2

P1V1 = P2V2

This equation is known as Boyle’s Equation.

Charle’s Law

We know that everything expand on heating and contract cooling. This change in volume is small in liquids and solids but gases exhibit enormous changes due to the presence of large intermolecular spaces.

Change of volume of a gas with the change of temperature at constant pressure was studied by Charles and was given in the form of a law. which states,

Statement

For a given mass of a gas the volume of the gas is directly proportional to its absolute temperature provided the pressure is kept constant.

Mathematically this law may be written as

V ∞ T

V = K T

OR

V / T = K

This relation shows that the ratio of volume of a given mass of a gas to its absolute temperature is always constant provided the pressure is kept constant. On this bases Charles Law may also be defined as,

If the pressure remains constant for each 1ºC change of temperature the volume of the gas changes to 1/273 of its original volume.

On the bases of this statement

V1 / T = K & V2 / T2 = K

V1 / T1 = V2 / T2

This equation is known as Charles’s equation.

The volume temperature relationship can be represented graphically. When volume of a given mass of gas is plotted against temperature, a straight line is obtained.

Absolute Scale Of Temperature

There are different scales for the measurement of temperature such as Celsius ºC and Fahrenheit ºC. Similarly another scale known as absolute scale or Kelvin scale is determined on the basis of Charles’s law.

On the basis of Charles’s law we known that the volume of the gas changes to 1/273 times of its original volume for each 1 ºC change of temperature. It suggests that the volume of a gas would theoretically be zero at -273ºC. But this temperature has never been achieved for any gas because all the gases condense to liquid at a temperature above this point. So the minimum possible temperature for a gaseous system is to be -273ºC. This temperature is referred as absolute zero or zero degree of the absolute scale or Kelvin scale.

To form an absolute scale thermometer if the equally spaced divisions of centigrade thermometer are extended below zero and when the point -273ºC is marked then this point is called as absolute zero and the scale is called as absolute scale. It shows that for the conversion of centigrade scale into Kelvin scale 273 is added to the degrees on the centigrade scale.

K = 273 + ºC

Avogadro’s Law

In 1811, a scientist Avogadro’s established a relationship between the volume and number of molecules of the gas, which is known as Avogadro’s law.

Statement

Equal volume of all gases contains equal number of molecules under the same condition of temperature & pressure.

Mathematically it may be represented as

V ∞ n

OR

V = K n

On the basis of the above statement we can say that

1 dm3 of O2 gas will contain the same number of molecules as 1 dm3 of H2 or N2 or any other gas at same temperature and pressure.

It was also observed that 22.4 dm3 of any gas at S.T.P contain 1 mole of that gas, so 22.4 dm3 volume at S.T.P is called as molar volume or the volume of 1 mole of the gas and the mass present in 22.4 dm3 of any gas will be equal to its molar mass or molecular mass. It can also be explained on the basis of following figures.

Determination of Unknown Molecular Mass of a Gas With the Help of Avogadro’s Law

Suppose we have two gases (i) Oxygen (ii) CO

The volume of these two gases are equal which are 1 dm3.

The mass of 1 dm3 of oxygen is 1.43 gm

The mass of 1 dm3 of Co is 1.25 gm

According to Avogadro’s law we know that 1 dm3 of CO at S.T.P contain the same number of molecules as 1 dm3 of O2 under similar condition. Hence a molecule of CO has 1.25 / 1.43 times as much as a molecule of O2 and we know that the molecular mass of oxygen is 32 so the molecular mass of CO would be

1.25 / 1.43 x 32 = 28 g / mole

General Gas Equation (Ideal Gas Equation)

To give a relation between the volume, pressure and number of moles of n gas, Boyle’s law, Charle’s law and Avogadro’s law are used.

According to Boyle’s law | V ∞ 1 / P

According to Charles’s law | V ∞ T

According to Avogadro’s law | V ∞ n

By combining these laws we get

V ∞ 1 / P x T x n

OR

V = R x 1 / P x T x P

OR

P V = n R T

This equation is known as general gas equation n is also known as equation of state because when we specify the four variables = pressure, temperature, volume and number of moles we define the state for a gas.

In this equation “R” is a constant known as gas constant.

Value of R

1. When Pressure is Expressed in Atmosphere and Volume in Liters or dm3

According to general gas equation

P V = n R T

OR

R = PV / nT

For 1 mole of a gas at S.T.P we know that

V = 22.4 dm3 or litres

T = 273 K (standard temperature)

P = 1 atm (standard pressure)

So,

R = PV / nT

= 1 atm x 22.4 dm3 / 1 mole x 273 K

= 0.0821 dm3 K-1 mol0-1

2. When Pressure is Expressed in Newtons Per Square Metre and Volume in Cubic Metres

For 1 mole of a gas at S.T.P

V = 0.0224 m3 ………. ( 1 dm3 = 10-3 m3)

n = 1 mole

T = 273 K

P = 101200 Nm-2

So,

R = PV / nT

= 101300 Nm-2 x 0.0224 m3 / 1 mole x 273 K

= 8.3143 Nm K-1 mole-1

= 8.3143 J K-1 mol-1

Derivation of Gas Equation

According to general gas equation

P V = n R T

For 1 mole of a gas n = 1

P V = R T

OR

P V / T = R

Consider for a known mass of a gas the volume of the gas is V1 at a temperature T1 and pressure P1. Therefore for this gas we can write as

P1 V1 / T1 = R

If this gas is heated to a temperature T2 due to which the pressure is changed to P2 and volume is changed to V2. For this condition we may write as

P2 V2 / T2 = R

P1 V1 / T1 = P2 V2 / T2 = R

P1 V1 / T1 = P2 V2 / T2

This equation is known as gas equation.

Graham’s Law of Diffusion

We know that gas molecules are constantly moving in haphazard direction, therefore when two gases are placed separated by a porous membrane, they diffuse through the membrane and intermix with each other. The phenomenon of mixing of molecules of different gases is called diffusion.

In 1881, Graham established a relationship between the rates of diffusion of gases and their densities which is known as Graham’s law of diffusion.

Statement

The rate of diffusion of any gas is inversely proportional to the square root of its density.

Mathematically it can be represented as

r ∞ 1 / √d

r = K / √d

Graham also studied the comparative rates of diffusion of two gases. On this basis the law os defined as

The comparative rates of diffusion of two gases under same condition of temperature and pressure are inversely proportional to the square root of their densities.

If the rate of diffusion of gas A is r1 and its density is d1 then according to Graham’s law

r1 ∞ 1 / √d1

OR

r1 = K / √d1

Similarly the rate of diffusion of gas B is r2 and its density is d2 then

r2 ∞ 1 / √d2

OR

r2 = K / √d2

Comparing the two rates

r1 / r2 = (K / √d1) / (K / √d2)

r1 / r2 = √d2 / d1 ………………. (A)

But density d = mass / volume

Therefore,

For d1 we may write as

d1 = m1 / v1

And for d2

d2 = m2 / v2

Substituting these values of d1 & d2 in equation (A)

r1 / r2 = √(m2 / v2) / (m1 / v1)

But v1 = v2 because both gases are diffusing in the same volume.

Therefore,

r1 / r2 = √m2 / m1

Hence Graham’s law can also be stated as,

The comparative rates of diffusion of two gases are inversely proportional to the square root of their masses under the same condition of temperature and pressure.

It means that a lighter gas will diffuse faster than the heavier gas. For example compare the rate of diffusion of hydrogen and oxygen.

Rate of diffusion of H2 / Rate of diffusion of O2 = √Mass of O2 / Mass of H2 = √32/ 2 = √16 = 4

It shows that H2 gas which is lighter gas than O2 will diffuse four times faster than O2.

Dalton’s Law of Partial Pressures

Partial Pressure

In a gaseous mixture the individual pressure exerted by a gas is known as partial pressure.

When two or more gases which do not react chemically are mixed in the same container each gas will exert the same pressure as it would exert if it alone occupy the same volume.

John Dalton in 1801 formulated a law which is known as Dalton’s Law of partial pressure and stated as.

Statement

The total pressure of a gaseous system is equal to the sum of the partial pressures of all the gases present in the system.

Suppose in a system three gases A, B & C are present. The partial pressure of these gases are

PA = Partial pressure of gas A

PB = Partial pressure of gas B

PC = Partial pressure of gas C

Then Dalton’s law may be mathematically written as

PT = PA + PB + PC

Where PT is the total pressure of the system.

To calculate the individual pressures of gases in the above example suppose the number of moles of A, B & C in the container are nA, nB and nC. So the total number of moles in the container will be

n = nA + nB + nC

Apply the general gas equation

P V = n R T

PT = n R T / V

Since R, T and V are same for gases A, B and C, therefore the partial pressure of these gases are as follows.

Partial pressure of gas A | PA = n(A)RT / V ……… (2)

Partial pressure of gas B | PB = n(B)RT / V ……… (3)

Partial pressure of gas C | PC = n(C)RT / V ……… (4)

Now divide equation (2) by (1)

PA / PT = (nA RT/V) / (nRT/V)

OR

PA / PT = nA / nT

Therefore,

P(gas) = P1 x n(gas) / n(total)

Application of Dalton’s Law

In an inert mixture of gases the individual gas exerts its own pressure due to collision of its molecules with the walls of the container but the total pressure produced on the container wall will be the sum of pressure of all the individual gases of the mixture.

On this basis the number of moles formed during a chemical reaction can be measured. For this purpose a gas produced in a chemical reaction is collected over water. The gas also contains some of water vapours. So the pressure exerted by the gas would be the pressure of pure gas and the pressure of water vapours.

Therefore the pressure of the system may be represented as

P(moist) = P(dry) + P(water vapour)

So,

P(dry) = P(moist) – P(water vapour)

In this way we can obtain the pressure of the gas and by using general gas equation we can calculate the number of moles of the prepared gas.

Ideal Gas

A gas which obeys all the gas laws at all temperatures and pressures is known as ideal gas.

It means that the product of pressure and volume must be constant at all pressures.

Similarly the rate of V/T will remain constant for an ideal gas.

But there is no gas which is perfectly ideal because of the presence of the force of attraction or repulsion between the molecules.

Gas Laws on the Basis of Kinetic Theory

Boyle’s Law

According to Boyle’s law the volume of a given mass of a gas is inversely proportional to its pressure at constant temperature.

It means that when the volume of the gas is decreased the pressure of the gas will increase.

According to kinetic molecular theory of gases the pressure exerted by a gas is due to the collisions of the molecules with the walls of the container. If the volume of a gas is reduced at constant temperature, the average velocity of the gas molecules remains constant so they collide more frequently wit the walls which causes higher pressure.

Charles’s Law

According to Charles law the volume of a given mass of a gas is directly proportional to its absolute temperature at constant pressure.

According to kinetic molecular theory the average kinetic energy of gas molecules is directly proportional to its absolute temperature so if the temperature of the gas is increased the average kinetic energy of the gas molecules is also increased due to which the sample of the gas expanded to keep the pressure constant. It is accordance with the law.

Graham’s Law

According to Graham’s Law

r1 / r2 = √m2 / m1

The rate of diffusion of a gas is directly proportional to the velocity of the molecules so,

v1 / v2 = √m2 / m1

Liquefaction

According to kinetic theory, the kinetic energy of the molecules is low for lower temperature. These slower moving molecules become subject to inter molecular attraction. At a sufficiently low temperature these attractive forces are capable of holding the molecules with one another so the gas is changed into liquid and the process is called liquefaction.

Liquid State

It is one of the state of matter. In this state, the kinetic energy of the molecule is very high due to which the molecules of the liquid are able to move but due to compact nature liquids are not compressible. On this basis we can say that the volume of a liquid is always constant but its shape can be changed.

Behaviour of Liquids

The main properties of liquids are as follows.

Diffusibility

The diffusion of one liquid into another liquid is possible but its rate is slow as compared with the rate of diffusion of gases. Example of diffusion of liquids is mixing of alcohol in water.

Explanation of Diffusion in Terms of Kinetic Energy

As the molecular of a liquid are in cluster form they are very close to each other but these molecules are movable so they can mix with the other molecules. Since the intermolecular distance are smaller due to which the rate of diffusion of liquids is slow.

Compressibility

The space between liquid molecules are very small due to strong Van der Waals forces. When the pressure is applied, they can be compressed but to a very little extent.

Expansion

When a liquid is heated, the kinetic energy of its molecules also increases so the attraction between the molecules becomes weaker due to which they go further apart and hence the liquid expands.

Contraction

When a liquid is cooled its kinetic energy is lowered and the attraction among the molecules becomes stronger so they comes close to each other and hence the liquid contract.

Viscosity

Definition

The internal resistance in the flow of a liquid is called viscosity.

Liquids have the ability to flow, but different liquids have different rates of flow. Some liquids like honey mobile oil etc. flow slowly and are called viscous liquids while ether, gasoline etc. which flow quickly are called less viscous.

Explanation

The viscosity of liquid can be understood by considering a liquid in a tube, a liquid in a tube is considered as made up of a series of molecular layer. The layer of the liquid in contact with the walls of the tube remains stationary and the layer in the center of the tube has highest velocity as shown.

Each layer exerts a drag on the next layer and causes resistance to flow.

Factors on Which Viscosity Depends

1. Size of Molecules

The viscosity of a liquid depends upon the size of its molecules. If the size of the molecules is bigger the viscosity of the liquid is high.

2. Shape of Molecules

Shape of the molecules affects the viscosity. If the shapes of the molecules are spherical they can move easily but if the shapes of the molecules are irregular such as linear or trigonal then the molecules will move slowly and its viscosity will be high.

3. Intermolecular Attraction

If the force of attraction between the molecules of a liquid is greater the viscosity of the liquid is also greater.

4. Temperature

Viscosity of a liquid decreases with the increase of temperature.

Units of Viscosity

Viscosity of a liquid is measured in poise, centipoise or millipoise & S.I unit.

1 poise = 1 N.s.m(-2)

1 centipoise = 10(-2) N.s.m(-2)

Surface Tension

Definition

The force acting per unit length on the surface of a liquid at right angle direction is called surface tension.

Explanation

Consider a liquid is present in a beaker. The molecules inside the liquid are surrounded by the other molecules of the liquid. So the force of attraction on a molecule is balanced from all direction. But the force of attraction acting on the molecules of the surface from the lower layer molecules is not balanced.

The molecules lying on the surface are attracted by the molecules present below the surface Due to this downward pull the surface of the liquid behave as a membrane which tends to contract to a smaller area and causes a tension on the surface of the liquid known as surface tension.

Factors on Which Surface Tension Depends

1. Molecular Structure of the Liquid

If the force of attraction between the molecules is greater, the surface tension of the liquid is also greater. Those liquids in which hydrogen bond formation take place will have more surface tension.

2. Temperature

Surface tension of a liquid is inversely proportional to the temperature.

Units

1. Dynes / cm

2. Ergs / cm2

Capillary Action

The fall or rise of a liquid in a capillary tube is called capillary action.

When a capillary tube is dipped in a liquid which wets the wall of the tube, the liquid will rise in the capillary tube, to decrease the surface area due to surface tension. The liquid will rise in the capillary tube until the upward force due to surface tension is just balanced by the downward gravitational pull. This is called capillary action.

Vapour Pressre

Definition

The pressure exerted by the vapours of a liquid in its equilibrium state with the pure liquid at a given temperature is called vapour pressure.

Explanation

Consider a liquid is present in a bottle as shown.

In the beginning the atmosphere above the surface of liquid is unsaturated but due to continuous evaporation the molecule of the liquid are trapped in the bottle and the air present above the surface of the liquid is becomes saturated and after it the molecules present in the vapour state may hit the liquid again and rejoin it by condensing into liquid. Thus in this closed vessel two process are going on simultaneously which are evaporation and condensation of vapours. When the rates of these two processes becomes equal at this point the pressure exerted by vapours is called vapour pressure.

Units of Vapour Pressure

The units for vapour pressure are

1. Millimeter of Hg
2. Atmosphere
3. Torr
4. Newton / m(2)

Factors for Vapour Pressure

1. Nature of Liquid

Vapour pressure of a liquid depends upon the nature of the liquid. Low boiling liquid exert more vapour pressure at a given temperature.

2. Temperature

Vapour pressure of a liquid also depends upon temperature. The vapour pressure of the liquid increases with the increase of temperature due to the increase of average of kinetic energy.

3. Intermolecular Forces

Those liquids in which the intermolecular forces are weak shows high vapour pressure.

Explanation of Evaporation on the Basis of Kinetic Theory

According to this theory the molecules of a liquid collide with each other during their motion. Due to these collisions some of the molecules acquire greater energy than Van der Walls forces which binds the molecules of the liquid together so these molecules of higher energy escapes from the surface into the air in the form of vapours.

Evaporation is a Cooling Process

In liquids, due to collision between molecules some molecules acquire higher energy and escapes from the surface of the liquid in the form of vapours. The kinetic energy of the remaining molecules decreases due to which the temperature of the liquid also decreases and hence we can say that evaporation is a cooling process.

Boiling Point

Definition

The temperature at which the vapour pressure of a liquid becomes equal to the atmospheric pressure is called boiling point.

When a liquid is heated the rate of evaporation of the molecules also increases with the increase in temperature. When the pressure of the vapours becomes equal to the atmospheric pressure the liquid starts boiling and this temperature is known as boiling point.

If the external pressure on a liquid is changed the boiling point of the liquid also change. The increase in external pressure on a liquid increases the boiling point while the decreases of external pressure decrease the boiling point.

Solid State

It is a state of matter which posses both definite shape and definite volume. In solids the particles are very close to each and tightly packed with a greater force of attraction.

Properties of Solids

1. Diffusibility

Diffusion also occurs in solids but its rate is very slow. If a polished piece of zinc is clamped with a piece of copper for a long time. After few years we will see that some particles of zinc are penetrated into copper and some particles of copper are penetrated into zinc. It shows that the diffusion in solids is possible but it occurs with a slow rate.

2. Compressibility

In solids the molecules are close to each other so it is not easy to compress a solid. In other words we can say that the effect of pressure on solids is negligible.

3. Sublimation

It is a property of some solids that on heating these solids are directly converted into vapours without liquification. This property of solids is known as sublimation.

4. Melting

When solids are heated, they are changed into liquids and the property is called melting of the solids.

5. Deformity

Solids may be deformed by high pressure. When a high pressure is applied on solids due to which some particles are dislocated the force of attraction is so strong that the rearranged atoms are held equally well with their new neighbours and hence the solid is deformed.

Classification of Solids

Solids are classified into two main classes.

1. Crystalline
2. Amorphous

1. Crystalline Solids

In a solid if the atoms are attached with each other with a definite arrangement and it also possesses a definite geometrical shape. This type of solid is called crystalline solid.

e.g. NaCl, NiSO4 are crystalline solids.

2. Amorphous Solids

In these solids there is no definite arrangement of the particles so they do not have a definite shape. The particles of such solids have a random three dimensional arrangement. Examples of amorphous solids are glass, rubber, plastic etc.

The properties of crystalline and amorphous solids are quite different from each other. These differences in properties are given below.

Difference of Geometry

1. Crystalline Solids

In crystalline solids particles are arranged in a definite order due to which it possesses a definite structure.

2. Amorphous Solids

In amorphous solids particles are present without any definite arrangement so they do not have definite shape.

Difference of Melting Point

1. Crystalline Solids

Crystalline solids have sharp melting point due to uniform arrangement.

2. Amorphous Solids

Amorphous solids melts over a wide range of temperature.

Cleavage and Cleavage Plane

1. Crystalline Solids

When a big crystal is broken down into smaller pieces the shape of the smaller crystals is identical with the bigger crystal. This property of crystalline solids is called cleavage and the plane from where a big crystal is broken is called cleavage plane.

2. Amorphous Solids

Amorphous solids do not break up into smaller pieces with an identical shape.

Anisotropy & Isotropy

1. Crystalline Solids

It is a property of crystalline solid that they show different physical properties in different direction. For example graphite can conduct electric current only through the plane which is parallel to its layers. This property is called anisotropy.

In amorphous solids the physical properties are same in all directions. This property of solids is called isotropy.

Symmetry in Structure

1. Crystalline solids are symmetric in their structure when they are rotated about an axis, their appearance remains same so they are symmetric in structure.

2. Amorphous Solids

Amorphous solids are not symmetric.

Types of Crystals

There are four types of crystals.

1. Atomic crystals
2. Ionic crystals
3. Covalent crystals
4. Molecular crystal

1. Atomic Crystals

Metals are composed of atoms. These atoms are combined with each other by metallic bond and the valency electrons in metals can move freely throughout the crystal lattice. This type of solid is called atomic crystal.

The properties of atomic crystals are

1. High melting point.
2. Electrical and thermal conductivity.
3. These are converted into sheets so these are malleable.
4. These are used as wire so these are ductile.

2. Ionic Crystals

Those solids which consists of negativity and positively charged ions held together by strong electrostatic force of attraction are called ionic crystals. Ionic crystalline solids possesses the following properties.

1. The melting and boiling point of ionic crystals is high.
2. They conduct electricity in molten state.
3. Ionic crystals are very hard.
4. Indefinite growth of crystals is also a property of ionic crystals.

3. Covalent Crystals

In covalent solids, the atoms or molecules are attached with each other by sharing of electrons. Such type of solids are called covalent solids e.g. diamond is a covalent solid in which carbon atoms are attached with each other by covalent bond. The other examples of covalent crystals are sulphur, graphite etc.

Covalent crystals possesses the following properties.

1. High melting point.
2. High refractive index.
3. Low density.
4. Molecular Crystals

Those solid in which molecules are held together due to intermolecular forces to form a crystal lattice are called molecular crystals e.g. iodine and solid CO2 are molecular crystals. The general properties of molecular crystals are as follows.

1. Low melting and boiling point.
2. Non – conductor of heat and electricity.

Isomorphism

When two different substance have same crystalline structure, they are said to be isomorphous and the phenomenon is called isomorphism.

e.g. ZnSO4 and NiSO4 are two different substances but both are orthorhombic similarly the structure of CaCO3 and NaNO3 is trigonal.

Polymorphism

If a substance exist in more than one crystalline form it is called polymorphous and the phenomenon is known as polymorphism. E.g. sulphur exist in rhombic and monoclinic form similarly CaCO3 exist in trigonal and orthorhombic form.

Unit Cell

The basic structural unit of a crystalline solid which when repeated in three dimensions generates the crystal structure is called a unit cell.

A unit cell of any crystalline solid has a definite geometric shape and distinguish from other crystals on the basis of length of the edges and angle between the edges.

Crystal Lattice

In crystalline solids atoms, ions or molecules are arranged in a definite order and form a three dimensional array of particles which is known as crystal lattice.