Experimental Chemistry

Separating Mixtures 
1. Filtration, Decanting and Centrifuging are used to separate a solid from a liquid.

• Filtration - Sand can be separated from a mixture with water by filtering through filter paper. The sand gets trapped in the filter paper and the water passes through it. The sand is called the residue and the water is called the filtrate

• Decanting - Vegetables do not dissolve in water. When you have boiled some vegetables it is easy to separate them from the water by pouring it off. Decanting is a technique used quite often to separate an insoluble solid, which has settled at the bottom of a flask, from the liquid.

• Centrifuging - This technique involves a suspension being spun round very fast in a centrifuge so that the solid gets flung to the bottom of the tube. The pure liquid can be decanted after the solid has been forced to the bottom of the tube. This method is used to separate blood cells from blood plasma.

2.   Evaporation is used to obtain a solute from a solution.
If a solid has dissolved in a liquid, the solution can be heated so that the liquid evaporates completely  and the leaves the solid behind. The simplest way to obtain salt from its solution is by slow evaporation.

    















3. Crystallisation
Salt is obtained from sea water on a vast scale. This is done by using the heat of the sun to evaporate the water and leave a saturated solution known as brine.
When the solution is saturated the salt begins to crystallise, and it is removed using large sccops

4. Simple Distillation is used when you want to obtain a solvent from a solution.
Water can be obtained from salt water using this method. The solution is heated in the flask until it boils. The steam rises into a condenser, where it condenses back into water. The salt is left behind in the flask.

5. Separating Funnel is used to separate two liquids that are immiscible (do not mix).
The mixture is poured into the funnel and the layers allowed to separate. The lower layer can be run off by opening the tap.

6. Fractional Distillation is used if miscible liquids (can mix) are to be separated.

This could be used to separate a mixture of ethanol and water.

Fractional distillation relies on the liquids having different boiling points.

Ethanol boils at 78 degrees Celsius where as water boils at 100 degrees Celsius.

This means they can be condensed and collected separately.

This technique is used to separate crude oil and can also be used to separate individual gases such as nitrogen from air.

7. Chromatography

There are several types of chromatography. 

The simplest kind is paper chromatography.

To separate the different coloured dyes in a sample of black ink is put onto a piece of chromatography paper. 

This paper is then set in a suitable solvent.

As the solvent moves up the paper, the dyes are carried with it and begin to separate.

Numerical measurements

Retardation factors (Rf value) can be obtained from chromatograms.

An Rf value is defined as the ration of the distance traveled by the solute to the distance traveled by the solvent.

8. Solvent Extraction

Sugar can be obtained from crushed sugar cane by adding water.

The water dissolves the sugar from the sugar cane.

The substances are extracted from a mixture by using a solvent which dissolves only those substances required.

 CRITERIA FOR PURITY
The purity of a substance can be gauged by:

1. Its melting point - if it is a pure solid it will have a sharp melting point. If an impurity is present then melting takes place over a range of temperatures.
2. Its boiling point - if it is a pure liquid the temperature will remain steady at its boiling point. If he substance is impure then the mixture will boil over a temperature range.
3. Chromatography - If it is a pure substance it will produce only one well-defined spot on a chromatogram. If impurities are present then several spot will be seen on the chromatogram.