Entry Test Preparation 2015
Chemistry
BOOK 1
Chapter # 3
liquids and
solids
Properties of liquids:
·
Liquids don’t have a
definite shape. They adopt the shape of container in which they are enclosed.
·
Given mass of liquid has
definite volume at a particular temperature.
·
Molecules of liquids are in
a constant state of motion. The evaporation and diffusion of liquid molecules
is due to this motion.
·
The densities of liquids
are much greater than those of gases but are close to those of solids.
·
The intermolecular spaces
in liquids are negligibly small.
·
The intermolecular
attractive forces in liquids are intermediate to the intermolecular forces
between gases and solids.
·
Liquids can be converted
into solids by cooling i.e., by decreasing their kinetic energy.
·
Molecules of liquids
collide among themselves and exchange energy.
Properties of Solids:
·
The molecules or atoms
present in solid substance are very close to each other and they are tightly
packed, making the solid incompressible.
·
There are strong attractive
forces in solids, which hold the particles together firmly, and for this
reason, solids have definite shape and volume at a particular temperature
·
The solid particles
(molecules or atoms) possess only vibrational motion about their mean position.
Intermolecular Forces
The intermolecular forces among
the molecules of solids, liquids or gases are called Van der Wall’s forces.
These intermolecular forces bring the molecules close together and give
particular physical properties to the substances in gaseous, liquid, and solid
state.
Molecules that show the strongest
Van der Waal’s forces are highly polarisable, i.e. their electron clouds are
easily distorted. These are usually large, heavy atoms or molecules.
Examples:
·
Dipole-dipole forces
·
Ion-dipole forces
·
Dipole-induced dipole
forces / Debye forces
·
London dispersion forces
Dipole-Dipole Forces:
The forces of attraction between
the permanent diploes of polar molecules are called dipole-dipoles forces.
There is a net attraction between
the polar molecules.
These forces
are approximately one percent as effective as a covalent bond
The strength of these forces
depends upon the electro negativity difference between the bonded atoms and the
distance between the molecules.
Ion-Dipole Forces:
The forces of attraction between
ions and dipole of a covalent polar molecule like water are known as Ion-dipole
forces. Ionic compounds like M +X- are normally soluble
in polar solvent like water. Water molecules break the crystal lattice and the
ions are set free. These positive and negative ions are then surrounded by
water molecules. The negative ends are attracted towards the anion (X-).
The
dissolution of most of the ionic compounds in water is due to this reason.
Dipole-Induced Dipole Forces:
In a mixture of polar and
non-polar molecules, the poles of polar molecule induce plarity in non polar
molecules, which then have forces of attraction. These forces are called
dipole-induced dipole forces or as Debye forces.
Instantaneous Dipole Induced Dipole Forces or London
The London dispersion force is the weakest
intermolecular force. The London
dispersion force is a temporary attractive force that results when the
electrons in two adjacent atoms occupy positions that make the atoms form
temporary dipoles. This force is sometimes
Symmetrical
distribution unsymmetrical
distribution
called an induced dipole-induced
dipole attraction. London
forces are the attractive forces that cause non polar substances to condense to
liquids and to freeze into solids when the temperature is lowered sufficiently.
Hydrogen Bonding:
Hydrogen boding is defined as the
electrostatic force of attraction between an electronegative atom and a
partially positive charged hydrogen atom. The electronegative atoms may be
fluorine, oxygen, nitrogen and rarely chlorine. The strength of hydrogen bond
is generally twenty times less that that of a covalent bond.
Hydrogen bonding is responsible
for:
·
The relatively high melting
and boiling points of liquids like water and hydrogen fluoride.
·
Holding the strands of DNA
together.
·
Intermolecular hydrogen
bonds occur between water and alcohol molecules or between ethanoic acid
dimmers.
·
Intramolecular hydrogen
bonds occur between groups in the same molecule, e.g. in 2-nitrophenol.
…………………………
Constitutive Properties:
Any physical or chemical property
that depends on the constitution of structure of the molecule.
Evaporation:
Evaporation is the process
whereby atoms or molecules in a liquid state enter the gaseous state.
The equivalent
process in solids is sublimation. It is the opposite process of condensation.
Evaporation is exclusively of
surface phenomenon and should not be confused with boiling. Most notably, for a
liquid to boil, its vapor pressure must equal the ambient pressure, whereas for
evaporation to occur, this is not the case.
Evaporation
causes cooling.
Vapour Pressure
Vapour pressure of liquid is the
partial pressure of the vapour over the liquid surface measured at equilibrium.
It is the tendency of a liquid to evaporate.
H2O (I) → H2O (g)
V.P does not depend upon amount
of liquid, volume of container and surface area of liquid.
Measurement by Manometric method:
P = Pa
+ ∆h
P = V.P of liquid at 1 atm
Pa = atmospheric pressure
Molar Heat of Vapourization:
The amount of heat required to
vapourize one mole of a liquid at its boiling point.
Factors Affecting Vapour Pressure Temperature:
Evaporation increases with the
rise in temperature because increase in temperature will shift the equilibrium
towards formation of vapours, and hence vapour pressure also increases. The
vapour pressure of water increases with temperature and becomes equal to 760 mm
at the boiling point.
Surface Tension
The work (energy) required to
expand the surface of the liquid by unit area.
Unit of surface tension is either
joule/m or Newton/meter.
There are several methods to
employ for measuring surface tension of a liquid.
·
The torsion method
·
The capillary method
·
The drop of stalagmometer
Molecules having strong hydrogen
bonding have high surface tension.
Viscosity
Viscosity is defined as the
resistance of a liquid to flow. It is usually represented by 
.
.
It is usually expressed in
dynes/cm2 sec (poise). In
M.K.S system, it is expressed as kgm-1s-1.
1 poise = 10-2kgm-1s-1
The reciprocal of viscosity is
called fluidity and is denoted by 
= 
=
The relative viscosity is defined as the ratio of viscosity of a
liquid to the viscosity of water taken as standards.
The viscosity
of water is taken as 1 centipoise at 25oC.
………………………………….
Boiling
When a liquid is heated, the
vapour pressure goes on increasing. A stage reaches when the vapour pressure of
the liquid becomes equal to the external atmospheric pressure. At this point
the liquid starts boiling and is called boiling point of that liquid.
Boiling Points of Some Common
Liquids
|
Liquids
|
B.P. (oC)
|
|
Acetic acid
|
118.50
|
|
Acetone
|
56.00
|
|
Aniline
|
184.4
|
|
Benzene
|
80.15
|
|
Carbon disulphide
|
46.30
|
|
Carbon tetrachloride
|
76.50
|
|
Ethanol
|
78.26
|
|
Naphthalence
|
218.00
|
|
Phenol
|
181.80
|
|
Water
|
100.00
|
Boiling Point and Pressure:
Vacuum
distillation and pressure cooker
Molar Heat of Vaporization:
The amount of heat required to
vapourize one mole of a liquid at its boiling point is called its molar heat of
vaporization.
The molar heat
of vapourization of water is 40.6 kj mole-1.
Dynamic Equilibrium:
The state at which the rate of
evaporation becomes equal to the rate of condensation is called the dynamic
equilibrium.
Cohesive Forces or Cohesion:
The forces of attraction among
the particles of liquid are called cohesive forces or cohesion.
Adhesive Forces or Adhesion:
The forces of attraction between
a liquid and another surface (usually the container) are called adhesive forces
or adhesion. For example, attraction between water molecules and the walls of
the glass containing it is called adhesion.
Meniscus:
The shape of the surface of a
liquid in a cylindrical container is called meniscus. It may be either convex
or concave.
Liquids like
water which wet glass forms a concave meniscus.
This is due to strong
interactions between liquid such as water and the surface of the glass. Thus a
liquid with stronger adhesive forces than cohesive forces shows concave
meniscus.
Liquids like
mercury which do not wet glass form a convex meniscus.
Capillary Action:
The rise of liquid in a capillary
tube is called capillary action.
Enthalpy Change
If a physical or a chemical
change takes place at a constant pressure, heat change during the process is
called, enthalpy change, denoted by ∆H. These enthalpy changes are usually
expressed per mole of the substances. Three types of enthalpy changes are
associated with usual physical changes:
·
Molar heat of fusion
·
Molar heat of vapourization
·
Molar heat of sublimation
Molar Heat of Fusion (∆Hf):
It is the amount of heat absorbed
by one mole of a solid when it melts into liquid form at its melting point. The
pressure, during the change, is kept one atmosphere.
Molar Heat of Vapourization (∆Hv):
It is the amount of heat absorbed
when one mole of liquid is changed into vapours at its boiling points. The
pressure during the change, is kept one atmosphere.
Molar Heat of Sublimation (∆H s):
It is the amount f heat absorbed
when one mole of a solid sublimes to give one mole of vapours at a particular
temperature and one atmospheric pressure.
Energy Changes and Intermolecular Attraction:
When a solid substance melts,
atoms, molecules or ions undergo relatively small changes in intermolecular
distances and the potential energy also undergoes a small change. But when a
liquid evaporates, then larger changes in intermolecular distances and in
potential energy take place.
So ∆H of vapourization of substance is greater than ∆H of fusion.
The values of Hs are
even larger than ∆Hv,
because attractive forces in solids are stronger than those in liquids. The
values of ∆Hv and Hs
directly tell the energy needed to separate molecules from each other so from
these values, we can compare the strengths of intermolecular forces in
different compounds.
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 molecular order and are called liquid crystals.
The molecules of such turbid
resemble crystals in certain properties and the most important properties are
optical properties. These turbid liquids are hence called liquid
crystals. So, a liquid crystalline
state exists between two temperatures i.e., melting temperature and clearing
temperature.
1888 Frederick Reinitzer Austrian
botanist discovered them. Cholesterol Benzoate an organic compound turned milky
at 145oC and becomes clear liquid at 179oC.
·
Liquid crystals have the
fluidity of liquids and the optical properties of solids.
·
A liquid crystalline state
exists between two temperatures i.e. melting temperature and clearing
temperature.
·
Liquid crystals are always
isotropic.
Uses of Liquid Crystals:
·
Like solids crystals,
liquid crystals can diffract light.
·
Liquid crystals are used to
find the point of potential failure in electrical circuit.
·
Liquid crystalline
substances are used to locate the veins, arteries, infections, and tumors.
·
Liquid crystals are used in
the display of electrical devices such as digital watches, and calculators.
·
In chromatographic
separation, liquid crystals are used as solvents.
Liquid Crystal Display (LCD):
Liquid
Crystals are used in the display of electrical devices such as calculators,
digital watches, pH maters and many other electronic devices. The optical
characters of liquid crystals are changed by the electrical field. In this case
a thin film of liquid crystals is sandwiched between transparent electrodes
arranged on glass in special patterns.
When
the particular segment of electrode is energized, the orientation in the
molecules of liquid crystal is changed and the segment becomes opaque.
In
this way, various numbers or letters are formed.
Solids
Solids are the substances, which
are rigid, hard, have definite shape and definite volume.
·
In solids the constituting
atoms, ions and molecules are closely packed.
·
There exists a well defined
arrangement of molecules, atoms or ion 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.
Classification of Solids
Crystalline Solids:
A crystalline solid is a
substance whose constituent particles possess a regular orderly arrangement.
Examples:
Sodium chloride (rock salt),
sucrose (sugar), diamond and quartz.
Anisotropy:
The property of a crystal in
which physical properties such as electrical conductivity, refractive indices
are different in different directions.
Isotropy:
The property of a crystal in
which physical properties are independent of direction.
Properties of Crystalline Solids
Geometrical Shape:
All the crystalline solid have a
definite, distinctive geometrical shape due to definite and orderly arrangement
of atoms, ion or molecules in three dimensional spaces.
Melting Points:
Crystalline solids have sharp
melting points and can be identified by their definite melting points.
Cleavage Planes: The crystalline solids are broken along definite
planes. These planes are called cleavage planes and hey are inclined to one
another at a particular angle for a given crystalline solid. The value of this
angle varies from one solid to another solid.
Symmetry:
The repetition of faces, angles,
or edges when a crystal is rotated by 360o along its axis is called
symmetry.
Habit of Crystal
The shape of a crystal in which
is usually grows is called habit of a crystal.
Isomorphism:
When two substances have same
crystal structure, they are said to be isomorphous and the phenomenon is called
isomorphism.
|
Isomorphs
|
Crystalline form
|
Atomic Ratio
|
|
NaNO3
|
Rhombohedral
|
1 : 1 : 3
|
|
K2SO4
|
Orthorhombic
|
2 : 1 : 4
|
|
ZnSO4
|
Orthorhombic
|
1 : 1 : 4
|
|
Naf, Mgo
|
Cubic
|
1 : 1
|
Polymorphism:
The substance with can exist in
more than one crystalline form is called polymorphous and the phenomenon is
called polymorphism. Polymorphs have same chemical properties but they differ
in the physical properties.
|
Substance
|
Crystalline Forms
|
|
AgNO3
|
Rhombohedral, Orthorhombic
|
|
CaCO3
|
Trigonal, Orthorhombic
|
Allotropy:
The existence of an element in
more than one crystalline form is known as allotropy and these form of the
element are called allotropes or allotropic forms. Sulphur , Phosphorus, Carbon, and Tin are some
important examples of element, which show allotropy.
|
Element
|
Crystalline Forms
|
|
|
Rhombic, Monoclinic
|
|
Carbon
|
Cubic (Diamond) Hexagonal
(graphite)
|
|
Tin
|
Cubic (grey tin), Tetragonal
(white tin)
|
Transition Temperature:
The temperature at which two crystalline
forms of the same substance can co-exist in equilibrium with each other is
known as transition temperature. At this temperature, one crystalline form of a
substance changes to another. Above and below this temperature only one form
exists.
Crystal Lattice:
A crystal lattice or space
lattice defined as an array of point representing atoms, ion or molecules of a
crystal, arranged at different sites in three dimensional spaces.
·
Each lattice point has the
same environment as that of any other point in the lattice.
·
The constituent particle
has always to be represented by a lattice point.
Classification of Crystals
A French mathematician, August
Bravis in 1850, observed that crystal lattice of substances can be categorised
into seven types:
Cubic System:
In this system
·
two axes are of equal
length
·
the third axis is not equal
to the other two
·
all angles are of 90o
Orthorhombic or Rhombic System:
·
All the three axes are of
unequal length
·
All the axes are at right
angle to one another
Monoclinic System:
·
All the three axes are of
unequal length
·
Two axes are at right angle
to each other
·
The third angle is greater
than 90o
Hexagonal System:
·
Two axes are of equal
length
·
These axes lie in one plane
making an angle of 120o with each other
·
The third axis which is different
in length than the other two is at right angle to these two axes
Rhombohedral System or Trigonal System:
·
All the three axis are of
equal length
·
The three angles are not
equal and lie between 90o and 120o
Triclinic System:
·
All the three axes are unequal
·
The three angle are unequal
and none of them is of 90o
UNIT CELL
A unit cell may be defined as a
three dimensional group of lattice points that generates the whole lattice by
translation or staking.
Types of Unit Cells
Simple:
A unit cell having lattice points
only at the corners is called simple, primitive or basic unit cell. A crystal
lattice having primitive unit cell is called a simple crystal lattice.
Face Centered:
Unit cell having lattice point at the centre
of each face in addition to the lattice points at the corners is called face
centered unit cell.
Body Centered:
A unit cell having a lattice
point at the centre of the body in addition to the lattice points at the
corners is called body centered unit cell.
Bravis lattices:
There are seven crystal types for
which fourteen (14) different types of lattices (seven primitive, three
body-centered, two face-centered and two end-centered) are possible. These
fourteen different types of lattices are known as Bravis lattices.
Types of Crystalline Solids
There are four types of
crystalline solids depending upon the type of bonds present in them i.e.
·
Ionic solids
·
Covalent solids
·
Metallic solids
·
Molecular solids
Ionic Solids:
Crystalline solids formed by ions
are known as ionic solids. These ions are held together by strong electrostatic
forces of attraction. The crystals of NaCl, KBr, etc are ionic solids.
·
Ionic solids are very
stable compounds.
·
They do not exist as
individual neutral independent molecules.
·
Ionic crystals do not
conduct electricity in the solid state.
·
They are highly brittle.
Covalent Solids:
Covalent solids are of high
density. They are called atomic solids because they are called atomic solids
because they are compounds of neutral atoms of the same or of different
elements.
·
Covalent crystals are
extended in three dimensions.
·
These crystals are very
hard and considerable amount of energy is required to break them.
·
They are bad conductors of
electricity.
·
Mostly covalent crystalline
solids are insoluble in polar solvents like water but they are readily soluble
in non-polar solvents.
Covalent solids are of two types:
·
The covalent bonds give
joint molecules like diamond, silicon carbide or aluminium nitride.
·
Atoms join to form the
covalent bond and separate layers are produced like that of graphite, cadmium
iodide and boron nitride.
Molecular Solids:
Molecular solids are defined as
the solid substances in which the particles forming the crystals are polar or
non-polar molecules or atoms.
·
Ice and sugar are the
examples of crystals having polar molecules.
·
Iodine, sulphur, phosphorus
and carbon dioxide form the molecular crystals containing non-polar molecules.
·
Polar molecular solids have
usually higher melting and boiling points as compared to non-polar molecular
solids.
·
The forces, which hold the
molecules together in molecular crystals, are very weak so they are soft and
easily compressible.
·
They are mostly volatile
and have low melting and boiling points.
·
They are bad conductors of
electricity, have low densities and are, sometimes, transparent of light.
·
Polar molecular crystals
are mostly soluble in polar solvents, while non-polar molecular crystals are
usually soluble in non-polar solvents.
Metallic Solids:
The metallic bond consists of a
series of metal atoms that have all donated their valence electrons to an
electron cloud that permeates the structure. This election cloud is frequently
referred to as an electron sea.
Each atoms in a metal crystal
loses all of its valence electrons. These valence electrons form a pool or a
gas a gas of electrons. The positively charged metal ions are believed to held
together by electron pool or gas. These positively charged ions occupy definite
positions at measurable distances from each other in the crystal lattice.
Valence electrons are not attached to any individual ion or a pair of ions
rather belongs to the crystal as a whole. These electrons are free to move
about from one part of the crystal to the other.
The force,
which binds metal cation to a number of electrons within its sphere of
influence, is known as metallic bond.
In Pauling’s theory, the metallic
bond is treated essentially as covalent in character. However, it is assumed
that the covalent bonds are not localized but are highly delocalized in metal
structure.
Lattice Energy:
The lattice energy is the energy
released when one mole of the ionic crystal is formed from the gaseous ions. OR
The energy required to break one mole of solid into isolated gaseous atoms.
Lattice energy decreases with the
increase in the size of the cation keeping the anion of same size. It also
decreases with the increase in the size of anion.
Elasticity:
A material is said to be elastic
if it deforms under stress (e.g., external pressure), but returns to its
original shape when the stress is removed.
Amorphous Solids:
An amorphous solid is a substance
whose constituent particles do not possess a regular orderly arrangement.
Examples:
Glass, plastics, rubber, starch,
and proteins. The do not possess well-defined crystal planes so these are also
called pseudo solids.
·
The substances have solid
state properties of extremely slow diffusion and virtually complete maintenance
of shape and volume but do not have and ordered crystalline state.
·
Many crystalline solids can
be changed into amorphous solids by melting them and then cooling the molten
mass rapidly.
·
Long range regularity does
not exist in amorphous solids but they can possess small regions of orderly
arrangements. These crystalline parts of otherwise amorphous solids are known
as Crystallites.
·
Amorphous solids don’t have
sharp melting points.
Properties of Solids
Electrical properties
Conductors:
Conductors are the substance with
low resistance. They conduct electricity in normal conditions.
Metals are good conductors and
have conductivities in the order 107 
m-1
m-1
Semiconductors:
Semiconductors are solids with
very low conductivities, ranging between10-10 to 10-20 m-1
m-1
Insulators:
Insulators are solids with very
high Resistivity. They do not conduct electricity in normal conditions.
Magnetic Properties:
Solids can be classified into
different types depending upon their behavior towards magnetic fields.
Diamagnetic substance:
Diamagnetic substances are weakly
repelled by magnetic field. They have no intrinsic magnetic dipole. However, a
strong external field can induce a dipole, but in the opposite direction to the
applied field (Lenz’s Law). A diamagnetic substance will therefore move away
from the strong field. Diamagnetic effects are weak (all substances are
diamagnetic, but it is swamped by any Para or ferromagnetic property), so its
movement towards the flat pole is subtle. Diamagnetic substances have all their
electrons paired.
Examples:
TiO2 and NaCl.
Paramagnetic substances:
Paramagnetic substances are
weakly attracted by magnetic field. They have permanent but unaligned magnetic
diploes (thermal random motion de-aligns them, that’s why the effect is
stronger at low temperatures). An external magnetic field does align the
dipoles, however, the substance will move toward the strong field and attach
itself to the pointed pole. Paramagnetic substances lose their magnetism in the
absence of a magnetic field.
Examples:
Olivine and pyroxene
Ferromagnetic substances:
Ferromagnetic substances have
permanently aligned magnetic dipoles. In the presence of the external field the
sample moves toward the strong field, and attaches itself to the pointed pole.
Examples:
Iron, Cobalt, Nickel, and CrO2.
…………………………………
Piezoelectricity:
Piezoelectricity is the ability
of a mineral or crystal to acquire opposing electrical charges on opposing
surfaces when mechanical stress (such as bending, stretching, or compression)
is applied to the crystal. the piezoelectric effect is caused by the
displacement of ionic charges within a crystal structure, and the magnitude of
the charge generally is proportional to the amount of stress applied.
Removal of the stress reverses the
effect.
Piezoelectricity was first
discovered in quartz crystals. This effect allows them to be used in certain
sorts of radio turner (first done in 1921, but now largely replaced by other
types), in the timing mechanisms of quartz watches, and in the electronics
industry. Tourmaline, another strongly piezoelectricity mineral, is used in
gauges to measure transient blast pressure.
Superconductivity:
The electrical resistance of
metal depends on temperature. Electrical resistance decreases with decreases in
temperature and becomes almost zero. Materials in this stare are said to
possess superconductivity. The electrical power dissipation in superconductors
is zero.
Examples:
YBa2Cu3O7 90 K
Bi2Ca2Sr2Cu3O10105
K
Ti2Ca2Ba2Cu2O10125
K
Plasma
If the temperature is high
enough, the electrons (at least those of the outermost orbit) acquire enough
kinetic energy to escape the atom’s potential. In this situation the electrons
are no longer rapped in orbits around the nucleus. This is the plasma state,
where a gas becomes a collection of negatively charged electrons, which have
escaped the pull of the nucleus and ions, which are positively charged because
they have lost one or more electrons.
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