There are several separation methods employed in separating mixtures that are very simple to understand and practise in Chemistry.
Mixtures unlike compounds can be separated using several separation techniques based on the difference in the physical properties of the constituents of the mixture.
Why can’t we separate compounds?
Mixtures are physically combined, unlike compounds that are chemically combined. It is difficult to separate compounds because there is a chemical bond holding the component elements.
We are going to look at the different mixtures and ways we can separate the constituents based on the physical properties of the constituents.
Differences between compounds and mixtures
|Mixtures are homogeneous or heterogeneous
|Compounds are always homogeneous
|The properties of a mixture are the sum of the individual constituents
|The properties of a compound are different from the properties of the component
|The constituents are added in any ratio
|The components are added in a fixed ratio
|The constituents are added chemically bound together
|The component elements are chemically bound together
15 Separation Techniques for Mixtures
Industrial Sieving Method
This is a separation technique used to separate solids of different sizes. The mixture to be separated is first placed on a sieve having a mesh of a particular size. The solid particles smaller than the mesh size of the sieve will pass through the sieve while the bigger particles remain on the sieve.
The method is extensively applied in the mining industries (separating gold and diamond), Garri making industries across west Africa. Sand and gravel or pebbles can also be separated using this method
Also, it can be used in separating sand from gravel.
Separating Funnel Method
This is a separation method used to separate immiscible liquids due to the difference in their densities. Immiscible liquids means liquids that do not mix together to form a uniform mixture but separate out into layers. Using a separating funnel, the denser liquid stays at the bottom while the less dense liquid stays on top, the tap is opened slowly to allow the denser liquid to come out first before the less dense liquid. Examples are petrol and water, oil and kerosene, etc.
This is a separation method used to separate insoluble particles e.g precipitates from a liquid.
Examples are the suspension of chalk particles in water, sand, and water. The chalk suspension is poured onto the filter paper; the water passes through while chalk remains on the filter paper.
The principle is that the solid must not dissolve in the liquid. Filtration is the first stage in water recycling. Filters like zeolites, sand beds have holes so tiny that they can remove microscopic particles from water. Filtration is applied in water purification industries and breweries
Evaporation to dryness Method
This is a method of separation used to separate salts that do not decompose easily on heating i.e salts that can withstand high heating.
This method is suitable to separate a soluble solid from a liquid. When the solution is heated, the liquid evaporates leaving the solid behind. It is used to recover solid solute from solution. In this process, the solvent is usually sacrificed. Examples of mixtures separated by this method include common salt and water. Others include KCl, K2CO3, Na2CO3.
This is an old and inaccurate method of separating an insoluble solid from a liquid. The mixture of the liquid and solid separates out in two distinct layers on standing. A lower solid layer and an upper liquid layer can be carefully decanted and poured out into a container. Examples of mixture that can be separated by decantation include sand and water
This is a method used to separate mixtures that involve a substance that is able to sublime e.g camphor, iodine, ammonium chloride.
Examples of such mixtures include impure NH4Cl, iodine, and Sulphur etc. The substance that is able to sublime can easily be separated from the other when the mixture is heated leaving the other behind.
Simple distillation method
This is a separation method used in separating miscible liquids with far apart boiling points example water (1000C) and ethanol (780C), Acetone (560C) and water (1000C), etc.
For accurate distillation, the difference in boiling point must be up to 200C. Simple distillation can also be used to distill water from a salt solution.
During distillation liquids with lower boiling points are distilled first followed by liquids with higher boiling points when the heat starts increasing the temperature of the mixture.
This is applied in Gin distilleries (Breweries)
Fractional Distillation method
This is a method used in separating miscible liquids with small differences in their boiling points e.g ethanol and octanol, also the fractions of petroleum can be separated using fractional distillation.
Industrial magnetic separation
This is a method used in separating magnetic materials from non-magnetic ones. The principle behind this separation is that one of the materials must be magnetic. It is applied in mining industries in separating magnetic ore from non-magnetic ones.
Magnetic materials include iron, nickel, cobalt, and steel.
Crystallization is a method used to separate salts that decompose easily on heating from their solutions. The salt solution is first heated to evaporate some of the solvents, the solution then becomes saturated. When the saturated solution is cooled, crystals of the salt begin to form. The salts are obtained purely as crystals containing water of crystallization.
Examples are CuSO4.5H2O, FeSO4.7H2O, Cu(NO3)2.3H2O etc
Fractional crystallization is used to separate more salts based on the difference in their solubilities The solubilities of different salts in a given solvent must differ at different temperatures. During the cooling process, at a particular temperature, crystals of salts start forming as cooling continues.
This is a separation method used for separating mainly colored mixtures.
Examples include ink, dye, and pigment.
However, it can also be used for colorless mixtures like leaf colors, amino acids etc.
In this method, a strip of filter paper or the chromatographic paper is spotted near one end with the solution containing the mixture of solute to be separated.
As the solvent travels up, the different solutes in the mixture also move along at different speeds and are hence separated.
The principle is that the solutes must be soluble in the solvent but as they move up, they move with different migration speeds
Diffusion is a method of separating liquids or gases with different diffusion gradients. The liquids or gases with higher diffusion gradients will diffuse faster than liquids with lower diffusion gradients.
The precipitation method is a method used to solid from two miscible liquids. For example, if a salt X is soluble in water but not in ethanol. If ethanol is added, the ethanol is miscible with water and thus will then precipitate the salt out of the solution.
This is sometimes referred to as solvent extraction.
The centrifugation process is a separation method based on the spinning action of a centrifuge. The spinning action of the centrifuge causes the heavier solid particle to settle at the bottom of the test tube while the clear liquid separates out as the upper liquid layer.
Many metallic ores are found in nature in low concentrations when mixed with earthly materials. These metals can only be extracted by concentrating it via grinding the materials and then mixed with water to form a foamy mixture.
Summary of the Separation Methods
|Physical property employed or principle of separation
|Solids of different sizes
|Insoluble solid from liquid
|Immiscible liquids of different densities
|Miscible liquids with a small difference in boiling points
|Miscible liquids with large differences in boiling points
|Difference in solubility of salts
|Ability to withstand high heating
|Ability to sublime
|Different migration speeds of the soluble solutes
Separation techniques used in Chemistry are many and easy to understand the pattern and process. every separation method has a principle of separation which is always based on the physical property of the constituents.