Mixtures
A mixture is a combination of two or more elements or compounds in any proportion so that the components do not lose their identity. Air is an example of a mixture (several gases mix to form air).
Mixtures are of two types, homogeneous and heterogeneous.
- Homogeneous mixtures have the same composition throughout the sample. The components of such mixtures cannot be seen under a powerful microscope. They are also called solutions. Examples of homogeneous mixtures are air, sea-water, gasoline, brass etc.
- Heterogeneous mixtures consist of two or more parts (phases), which have different compositions. These mixtures have visible boundaries of separation between the different constituents and can be seen with the naked eye e.g., sand and salt, chalk powder in water etc.
Separation of Mixtures
The separation of mixtures into its constituents in a pure state is an important process in chemistry. The constituents of any mixture can be separated on the basis of their differences in their physical and chemical properties e.g., particle size, solubility, effect of heat, acidity or basicity etc.
Some of the methods for separation of mixtures are:
- Sedimentation or Decantation method
Aim : To separate the mixture of coarse particles of a solid from a liquid e.g., muddy river water.
Principle :The coarse particles of the solid being heavier than the liquid (usually water), settle down due to gravity. The clear
upper layer of the liquid is then gently poured out into another container. Settling down of the coarse particles due to the effect of
gravity is called sedimentation. The mechanical transfer of the clear upper liquid without disturbing the settled solid particles is
called decantation.
Process :
The mixture is taken in a container and allowed to stand for sometime. The solid particles settle down with time. Settling down of the particles leaves the upper layers of the liquid clearer. Bigger particles settle down faster than the finer particles. Sometimes the sedimentation process is hastened by adding a small quantity of alum. Al3+ ions in alum cause the coagulation of the fine particles by undergoing hydrolysis to Al(OH)3 that has a strong tendency for adsorption.
Fig: 1.1 - Separation of the coarse particles of a solid from a liquid by sedimentation and decantation
- Filtration method:
Aim : To separate the insoluble solid component of a mixture from the liquid completely i.e. separating the precipitate (solid phase) from any solution.
Principle : The solvent molecules and the molecules/ions present in the solution can pass through the porous membranes while
the suspended particles cannot and are retained on the porous membrane.
Process :The solution containing the suspended impurities is made to pass through the porous membrane such as filter paper,
filter cloth etc. The solvent or solution containing dissolved substances passes through the porous membrane, which is called
filtrate. The insoluble solid suspended particles that remain on the porous membrane is termed residue. When the suspended
impurities are very fine, a small amount of alum added to the suspension makes filtration faster.
Fig:1.2 - Separation by filtration
- Evaporation method :
Aim: To separate a non-volatile soluble salt from a liquid or recover the soluble solid solute from the solution. The solvent is lost
into the surroundings.
Principle : Liquids evaporate at all temperatures. Evaporation becomes faster at higher temperatures.
Process :The solution containing the mixture is taken in a china dish and heated gently. Gradually the solvent evaporates and
the solution containing the dissolved solute becomes thicker. The semi-solid mass left on the china dish is slowly heated to
dryness.
Fig: 1.3 - Evaporation of a solution
- Crystallization method :
This method is used to separate a solid compound in pure and geometrical form. A nearly saturated solution of an impure
substance is prepared in a hot solvent. The prepared solution is quickly filtered and the filtrate is then allowed to cool slowly in a
china dish. The resulting pure crystals that form are removed with the help of a spatula. They are dried by pressing them
between the folds of filter papers and finally put into a dessicator.
- Sublimation method :
This method is used to separate volatile solids, from a non-volatile solid. The mixture is taken in a china dish covered with a
perforated filter paper on which an inverted glass funnel is placed to collect the vapours. Upon heating, the substance vapourizes
and gets deposited on the walls of the funnel. The non-volatile substances are left in the dish.
- Distillation method :
Distillation is used for separating the constituents of a liquid mixture, which differ in their boiling points. Depending upon the
difference in the boiling points of the constituents, different types of distillation like fractional distillation, steam distillation etc.
are employed.
- Magnetic Separation Method :
Aim : To separate a magnetic component from a mixture containing non- magnetic components.
Principle :The magnetic component of the mixture is separated with the help of the magnetic attraction.
Process : A magnet is moved over the mixture containing the magnetic substance e.g., iron filings. These get attracted to the
magnet. The process is repeated until the magnetic material is completely separated from the mixture.
Fig: 1.4 - Separation of a magnetic substance by a magnet
- Gravity method:
Aim :To separate mixtures in which components have different densities.
Principle : Particles with higher density settle to the bottom while the lighter particles are separated through various processes
such as winnowing (grain separation) and washing (panning of gold in the river bed)..
Process : Riverbed sand containing fine particles of gold is repeatedly washed in a pan with flowing water. Gold particles settle
to the bottom of the pan because of higher density while lighter sand particles are washed over the edge of the pan.
- Solvent Extraction Method :
Organic compounds, which are easily soluble in organic solvents but insoluble or immiscible with water forming two separate
layers can be easily separated. The aqueous solution of the mixture is mixed with a small quantity of organic solvent in a
separating funnel. The separating funnel is stoppered and shaken strongly. It is allowed to stand for some time. The organic
liquid and water form separate layers, which are collected by opening the stop-cock. The aqueous layer is again transferred to
the separating funnel. The process is repeated with more organic solvent.
- Chromatography
This technique is based on the differential adsorption of various components of a mixture on a suitable adsorbent called the
stationary phase while the liquid in which the substance is dissolved is called the mobile or moving phase. Depending on the
nature of the two phases there are various types of chromatography. In the method of adsorption chromatography used in
column chromatography the adsorbent alumina is packed in a column, which acts as a stationary phase. The mixture is
dissolved in a suitable solvent and the solution is poured on top of this column. The mixture moves down and the different
components of the mixture get adsorbed in different strengths on the alumina surface. The adsorbed components are then eluted
out by the mobile phase (solvent).
