L. R. & S. M. VISSANJI ACADEMY
SECONDARY SECTION - 2016-17
CHEMISTRY - GRADE: VIII
Elements, Compounds and Mixtures
ELEMENTS
 An element is a pure substance which cannot be converted into anything simpler
than itself by any physical or chemical process.
 Thus, elements are the basic substance from which all other substances are
made.
 An element is a pure substance composed of only one kind of atom.
CLASSIFICATION OF ELEMENTS
 Elements are classified on the basis of their properties.
METALS
1. Metals are monoatomic elements.
2. They are hard solids.
3. They have lustre.
4. They are good conductors of heat and electricity.
5. They are ductile i.e. they can be drawn into wires.
6. They are malleable i.e. they can be beaten into sheets.
7. Most of them have high melting point and high boiling point.
8. They produce a sound when they are struck i.e. they are sonorous substances.
Examples: Magnesium, Mercury, Copper, Silver, Gold, Aluminium, Lead, Tin, Iron
etc.
Exceptions:
i. Mercury, Gallium and Caesium are liquids ( at 30 oC )
ii. Zinc is non-ductile and non-malleable. It is brittle in nature.
iii. Sodium and Potassium are soft solids.
iv. Tungsten is a poor conductor of electricity.
v. Lead has low melting point.
NON-METALS
1. Non-metals are either monoatomic elements or polyatomic elements.
2. They exist in all the three physical states i.e. solid, liquid and gas.
Example
Solid – Carbon, Sulphur, Phosphorus and Iodine are solids.
Liquid – Bromine is a liquid.
Gases – hydrogen, oxygen, Nitrogen, Chlorine etc are gases.
3. They do not have lustre.
4. They are bad conductors of heat and electricity.
5. They are neither malleable nor ductile, rather they are brittle.
6. They have low melting point and low boiling point.
7. They do not produce a sound when they are struck i.e. they are not sonorous
substances.
Exceptions
i. Graphite and Iodine have lustre.
ii. Graphite is a good conductor of electricity. It also had high melting point.
iii. Diamond has high melting point and high boiling point.
METALLOIDS
 Elements which show the properties of metals as well as non-metals are called
as Metalloids.
 Examples: Boron, Silicon, Germanium, Arsenic, Antimony and Tellurium.
INERT GASES OR NOBLE GASES
 Elements that are chemically inactive or inert are known as Noble Gases or Inert
Gases. They occur in traces in the atmosphere.
 Examples: Helium, Neon, Argon, Krypton, Xenon and Radon.
COMPOUNDS
 A compound is a pure substance made up to two or more elements combined
chemically in a fixed proportion.
CHARACTERISTICS OF A COMPOUND
1. A compound contains atoms of two or more elements combined by chemical
forces.
2. It has a homogenous composition.
3. The elements in a compound are present in a definite proportion.
4. The properties of compounds are different from those of the elements of which
they are made.
5. Compound can be broken down into their constituent elements only by chemical
means, not by physical means.
6. During the formation of a compound by a combination of elements, energy is
either liberated or absorbed.
MIXTURES
A mixture is made up of two or more elements or compounds or both mechanically
mixed together in any proportion.
TYPES OF MIXTURES
1. HOMOGENOUS MIXTURE
They have same composition and properties throughout their mass.
Example: sugar solution.
2. HETEROGENOUS MIXTURE
They have different composition and properties in different parts of their mass.
Example: sand mixed with salt.
METHODS OF SEPARATION
I.
SOLID – SOLID MIXTURES
1. SUBLIMATION
Sublimation is the process of conversion of a solid into vapour and back to the
solid state, without passing through the liquid state.
For this method, one of the components must be capable of subliming on
heating.
Examples: A mixture of sand and iodine (sublimes ) or of common salt and
ammonium chloride (sublimes) can be completely separated by this method.
2. MAGNETIC SEPARATION
This is possible only when one of the components is magnetic.
Examples: a mixture of iron filings and powdered sulphur can easily be
separated by using this method.
3. SOLVENT EXTRACTION
In this process one of the components of mixture dissolves in a particular
liquid, either water or any other solvent, and the other component, which does
not dissolve, is separated as residue by filtration.
Examples: A mixture of charcoal and sulphur is separated by filtration. Carbon
disulphide or carbon tetrachloride is used as a solvent in which sulphur
dissolves and carbon gets separated as a residue.
Note: dissolved substance can be separated from solution by evaporation
where solvent evaporates leaving behind solute.
4. CHROMATOGRAPHY
It is a method in which the separation of a mixture of substances is done by
the flow of solvents on a special type of paper known as Whatman
chromatographic paper. A good quality filter paper may be used for separating
dyes present in ordinary black ink.
A thin strip of rectangular shaped filter paper is used as absorbent. A pencil
line is drawn about 3 cm from
one edge. This is called base
line. A small drop of water
soluble black ink is applied with
the help of a capillary tube at the
centre of the line and is allowed
to dry. The filter paper strip is
now suspended into a large size
gas jar containing water as the
solvent in such a way that the
drop of ink on the paper is just above the water level. The gas jar is covered
with a lid and is kept undisturbed for about one hour.
Water is drawn by capillary action and moves slowly upwards. As the water
rises up on the filter paper, it takes along with it the dye particles. Different
dyes present in the ink spot move with different velocities and so different
dyes settle at different heights on the paper. The strip of filter paper is called
chromatogram.
Thus, components of black ink are separated by this method.
II.
SOLID - LIQUID MIXTURES
1. FILTRATION
This method is used when the solid is insoluble in the liquid and thus forms a
heterogenous mixture.
Example: A mixture of sand and water can be separated by this method.
2. SEDIMENTATION
This method is used to separate a heterogenous mixture containing an
insoluble solid in a liquid. The mixture is allowed to stand. The insoluble solid
substance settles down, and a clear liquid is left standing. It is called
supernatant liquid. The solid substance that settles down is called sediment,
and the whole process is known as sedimentation.
The clear liquid is then carefully poured out into another beaker, leaving the
sediment undisturbed. This process is known as decantation.
3. EVAPORATION
This method is suitable if one of the
components is a soluble solid and the
other liquid that forms a homogenous
solution with the solid component. The
liquid can easily be evaporated, and its
loss can be ignored.
4. DISTILLATION
Distillation is the process of converting a
liquid into vapour (by heating) and the
subsequent condensation of the vapour
back into the liquid.
This method is used to separate a solid
from a liquid in which it is dissolved.
Unlike evaporation, distillation permits
separation and recovery of both the
dissolved solid and the solvent liquid.
III.
LIQUID - LIQUID MIXTURES
1. SEPARATING FUNNEL
If the liquids are immiscible, put the mixture in a separating
funnel and leave it undisturbed for a while. It will be
separated the mixture of the two liquids into two distinct
layers; the heavier liquid will form the lower layer, while the
lighter one will form the upper layer. Open the stop-cock and
collect the heavier liquid in a beaker. Now place another
beaker under the funnel, open the stop cock again, and let
the lighter liquid run into it.
Example: A mixture of carbon tetrachloride and water can
be separated with the help of separating funnel.
A mixture of oil and water can be separated with the help of separating funnel.
2. FRACTIONAL DISTILLATION
If the liquid in a mixture are miscible and have different boiling points, they
can be separated by fractional distillation.
Here, an advantage is taken of the fact that there is a difference in the boiling
points of the two liquids. On heating the
mixture in a distilling flask, the liquid with
the lower boiling point changes into
vapour first, and this vapour, on being
cooled by the condenser, collects in the
receiver. The temperature remains
stationary till all the liquid with the lower
boiling point distils over. The receiver is
then changed, and heating is continued
so as to allow the second liquid to distil.