Pure substances and impurities: Methods Of Separation

  

Pure substances and impurities:

What is a pure substance?

A pure substance has no particles of any other substance mixed with it. In real life, very few substances are 100% pure. For example tap water contains small amounts of many different particles (such as calcium ions and chloride ions). The particles in it are not usually harmful – and some are even good for you.

Distilled water is much purer than tap water, but still not 100% pure. For example it may contain particles of gases, dissolved from the air.

Does purity matter?

Often it does not matter if a substance is not pure. We wash in tap water, without thinking too much about what is in it. But sometimes purity is very important. If you are making a new medical drug, or a flavoring for food, you must make sure it contains nothing that could harm people.

An unwanted substance, mixed with the substance you want, is called an impurity.

How can you tell if a substance is pure?

Chemists use some complex methods to check purity. But there is one simple method you can use in the lab: you can check melting and boiling points.

·         A pure substance has a definite, sharp, melting point and boiling point. These are different for each substance. You can look them up in table

                                      

·      When a substance contains an impurity:
*Its melting point falls and its boiling point rises
*It melts and boils over a range of temperatures, not sharply.

·         The more impurity there is:
*the bigger the change in melting and boiling points
*the wider the temperature range over which melting and boiling occur.

Separation: (The first step in obtaining a pure substance)

        When you carry out a reaction, you usually end up with a mixture of substances. Then you have to separate the one you want. The table below shows some separation methods. These can give quite pure substances.

Separating a solid from a liquid:

Which method should you use? It depends on whether the solid is dissolved, and how its solubility changes with temperature.

           1)      By filtering:

        For example, chalk is insoluble in water. So it is easy to separate by filtering. The chalk is trapped in the filter paper, while the water passes through. The trapped solid is called the residue. The water is the filtrate.

2) By crystallization:

        You can obtain many solids from their solutions by letting crystals form. The process is called crystallization. It works because soluble solids tend to be less soluble at lower temperatures. For example:

3) By evaporating all the solvent:

                For some substances, the solubility changes very little as the temperature falls. So crystallization does not work for these. Salt is an example

  

        ·          To obtain salt from an aqueous solution,
          you need to keep heating the solution, to
          evaporate the water.                    

       ·      When there is only a little water
         left, the salt will start to appear.
          Heat carefully until it is dry.

Separating a mixture of two solids:

To separate two solids, you could choose a solvent that will dissolve just one of them. For example, water dissolves salt but not sand. So you could separate a mixture of salt and sand like this:

·         Add water to the mixture, and stir. The salt dissolves.

·         Filter the mixture. The sand is trapped in the filter paper, but the salt solution passes through.

·         Rinse the sand with water, and dry it in an oven.

·         Evaporate the water from the salt solution, to give dry salt.

Water could not be used to separate salt and sugar, because it dissolves both. But you could use ethanol, which dissolves sugar but not salt. Ethanol is flammable, so should be evaporated over a water bath, as shown here.

 

 

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