A pure substance is one that contains only one type of substance, i.e. only one type of atom or molecule.
A mixture contains two or more different substances physically combined together. They can be separated by different methods of purification.
Dissolving, Filtering and Evaporating:
This method is used when a mixture of solids (both soluble and insoluble) are dissolved in a particular liquid.
For example, take a mixture of salt, sand and water. On stirring, the salt dissolves in water, whereas the sand remains as it is. If we filter this mixture, the sand will be collected in the filter paper as the residue, while the dissolved salt and water will pass through the filter paper as filtrate.
To separate the salt from water, we put the solution in an evaporating dish and heat gently. The water evaporates, leaving behind salt crystals. The slower the evaporation, the larger the crystals. This process of evaporating some of the solvent and letting the remainder dissolved solute to cool and turn into crystals is known as crystallization.
Separating Funnel:
This method is used to separate two immiscible liquids (liquids that do not mix together).
For example, we could take the mixture of oil and water and place them in a separating funnel. The lighter liquid (oil) collects on the top and the heavier liquid (water) collects at the bottom. When the tap is opened, the water at the bottom starts to flow out. It can be collected in a container placed below the tap.
Fig 3.1. A separating funnel.
Simple Distillation:
This method is used to separate a pure liquid from a solution.
For example, take salt water, with the objective of obtaining the pure water from it and removing the impurities. The flask containing the mixture is heated and when the solution boils, steam is given off. This is cooled down and condensed in a condenser, which consists of a jacket of cold water with the coldest water entering at the bottom of the jacket and circulating out through the top. This way, it makes sure that the coldest part of the condenser is just before the vapour escapes. The condensed water is called the distillate and is collected in a receiver.
The water which collects in the flask is very pure as all its impurities are left behind in the flask. It is called distilled water. A thermometer at the top of the flask shows the temperature at which the vapour distills over (100°C).
Fig 3.2. Simple Distillation Apparatus.
Fractional Distillation:
This technique is used to separate two mixtures which dissolve in each other. These liquids are known as miscible liquids, and their separation depends on their different boiling points.
For example, let’s take a mixture of alcohol and water, having boiling points of 78°C and 100°C respectively. A fractionating column is used for this distillation. The fractionating column is packed with glass beads, which provide a large surface area for condensation.
Method:
When the flask is heated, the vapour coming off from the mixture will contain both ethanol and water molecules, but will be richer in ethanol molecules due to their lower boiling point. At first the vapours will condense back, but as the coloumn warms up the ethanol molecules will reach the top of the coloumn and will be distilled over, while the water molecules, due to their higher boiling point, will continue to condense and fall back in the flask. While the ethanol is being distilled over, the thermometer at the top shows a constant temperature of 78°C, the boiling point of ethanol. The ethanol passes through the condenser, in which the cool running water cools and condenses the hot ethanol vapour, and the liquid ethanol then flows down the inner tube of the condenser into the receiver. This ethanol is now the distillate.
Fig 3.3. Fractional Distillation Apparatus.
Paper Chromatography:
The technique of using a solvent to separate a mixture into its components is called chromatography. A drop of the mixture is placed on a chromatography paper, and one end of the paper is dipped in a solvent. The solvent travels up, separating the different components of the mixture and spreading them on the chromatography paper. This technique is useful in separating dyes in ink, pigments in plants, etc, and to detect traces of banned substances in food.