Chemistry Chapter (5) Liquid and Solid State

Liquid State

Points To Remember

• Molecules do not occupy fixed positions and less closely packed.
• Molecules move freely but slower gases.
• Due to absence of fixed positions,liquids have no definite shape. However, liquids have definite volume because intermolecular forces, through weaker are powerful enough to prevent the separation of molecules from one another.
• Liquids diffuse slowly than gases but rapidly than solids.
• Liquids can be compressed only under high pressure.
• The inter molecular forces among the molecules are called vander waal’s forces. These inter molecular forces bring the molecules close together and give particular physical properties to the substances in gaseous,liquid and solid state. Examples of such forces are:

 1.Dipole-dipole forces                2.Ion-dipole forces

 3.Dipole-induced dipole forces  4.Instantaneous

• Dipole-induced dipole forces or London dispersion forces.
• The relative strengths of dipole-dipole and dispersion forces depend upon the polarity, polarisability,size and shape of the molecules.
• Ionic compounds are normally soluble in polar solvent like water.The forces of attraction between ions and water molecules are known as ion-dipole forces.
• London forces are present in all types of molecules whether polar or non-polar but they are very significant for non-polar molecules and noble gases.
• The vapour pressure of liquid measures the tendency of a liquid to evaporate.
• Evaporation causes cooling.
• Boiling point is defined as the temperature at which vapour pressure of the liquid becomes equal to the atmospheric pressure.
• The amount of heat required to vapourize one mole of a liquid at its boiling point is called its molar heat of vapourization.
• A liquid can be made to boil at any temperature by changing the external pressure.
• Liquid can be made to boil at low temperature at which where they can be distilled easily. This process is called vacuum distillation.

Molar Heat of Fusion:

It is amount of heat absorbed by one mole of a solid when it melts into liquid from at its melting points. The pressure, during the change, is kept one atmosphere.

Molar Heat of Vapourization: 

It is amount of heat absorbed when one mole of a liquid is changed in to vapours at its boiling points. The pressure, during the change,is kept one atmosphere.                                                                                           
Molar Heat of sublimation: 

It is the amount of heat absorbed when one mole of solid sublimes to give one mole of vapours at a particular temperature and one atmospheric pressure.

All the above enthalpy changes are positive because they are endothermic processes.

Surface Tension:

• It is the force in dynes acting at right angle to any line on a unit length of the surface of the liquid.
• Molecules having strong hydrogen bonds have high surface tension.
• The capillary action arises due to inter-molecular forces. The two types of force i.e cohesive and adhesive are responsible for this property.
• The phenomenon of surface tension is important for understanding chromatography, colloids, catalysis, detergent action of soap etc.

Viscosity: 

• The frictional force between the layers of a liquid in motion is called viscosity.
• The coefficient of velocity of liquid may be defined as the tangential force per unit area required to maintain unit viscosity gradient normal to the direction of liquid flow. It depends on the nature of liquid.
• Hydrogen bonding affect viscosity. Due this fact, H₂O has higher viscosity that benzene and chloroform.
• Viscosity of a liquid generally decreases with rise in temperature.

Liquid Crystals: 

• many crystalline solids melt to give a turbid liquid before melting to give a clear liquid. These turbid liquids possess some degree of order and are called liquid crystals.
• Liquid crystals have the fluidity of liquids and the optical properties of solids
• A liquid crystalline state exists between two temperature i.e. melting temperature and clearing temperature.
• A crystalline solid may be isotropic or an isotropic, but liquid crystals are always isotropic.
• Like solid crystals,liquid crystals can diffract light.
• Liquid crystals are used to fine the point of potential failure in electrical circuit.
• Liquid crystalline substances are used to local the veins, arteries,infection and tumors.
• Liquid crystals are used in the display of electrical devices such as digital watches, calculators and computers.
• Oscillographic and TV displays also use liquid crystal screens.

Solid State

• Solids are those substances which are rigid, hard, have definite volume.
• The atoms, ions and molecules that make up a solid are closely packed. There exists a well ordered arrangement in solids.
• Solids are difficult to compress.
• Diffusion of a solid into another solid is very slow as compared to liquids and gases.
• Atoms, ions or molecules present in a solid possess only vibrational motion. At the melting point, the vibrational frequency of the particles increases so much that they leave their fixed positions and collapse to become a liquid.

Types of Solids

Crystalline Solids: 

In this, the arrangement of unit cell in the solid is regular.

There is definite arrangement of the constituent atoms, a sharp melting point and property of impressibility.

Amorphous Solids: 

• In this, the arrangement of unit cell in the solid is not regular.
• This type of solid is also called pseudo solids.
• These do not possess well defined surfaces called crystal planes.
• Many crystalline solids can be changed into amorphous solids by melting them and then cooling the molten mass rapidly. In this way, the constituent particles do not fine time to arrange themselves.

Anisotropy:

Physical properties of crystals such as electrical conductivity, refractive index are different in different directions. This property is called anisotropy.

Isotropy: 

when physical properties of crystals are same in all directions. This property is called isotropy.

Isomorphism: 

When two substances have same crystal structure, they are said to be isomorphous and the phenomenon is called isomorphism.

Polymorphism:

The substance which can exist in more than one crystalline from is called polymorphous and the phenomenon is called polymorphism.

Transitions Temperature:

It is that temperature at which two crystalline forms of the same substance can co-exists in equilibrium with each other.

At transition temperature, one crystalline form of a substance changes to another. Above and below this temperature only one from exists.

Crystal Lattice:

A crystal lattice or space lattice is defined as an array of point representing atoms, ions or molecules of a crystal, arranged at different sites in three dimensional space.

Unit Cell: 

• The smallest part of the crystal lattice has all the characteristic feature of the entire crystal and is called a unit cell.
• The quantitative aspects of a crystals lattice are deduced from the size and shape of unit cell. The length and angles of a unit cell are collectively known as unit cells, all the known crystals are classified into seven groups called the crystal systems.
• There are four type of crystalline solids depending upon the type of bonds present in them i.e. ionic solids, covalent solids and molecular solids
• Ionic solids are hard and brittle and have high melting points.
• Covalent solids consist of atoms held together by covalent bonds and these bonds extend throughout the solid. They are hard and have high melting points.
• Metallic solids consist of metal cations immersed in a sea of electrons and give a wide range of properties.
• Molecular solids consist of atoms or molecules held together by inter molecular forces.
• One of the most accurate methods for determining Avogadro’s number is based on the study of crystalline solids.

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