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Separating and Purifying Substances

Grade 8
Aug 20, 2022

Introduction: Separating and Purifying Substances 

Anything that occupies some space and has mass is called matter. Based on chemical composition, matter can be categorized into three major categories: elements, compounds, and mixtures. The study of matter involves the study of the properties of various categories of elements. It also includes the study of mixtures and techniques of separation of different types of mixture. 

A mixture is nothing but the combination of two or more substances where every substance retains its own chemical identity. Air, salt and sugar solutions, are a few examples of mixtures. Mixtures can be categorized into homogeneous and heterogeneous mixtures. 

Homogeneous mixtures have a uniform composition throughout all substances present in them. The mixtures that are not uniform are called heterogeneous mixtures. 

The various constituents of a mixture have different physical properties such as density, solubility, size of particles, boiling point, magnetic property, volatility, etc.  The difference in the physical properties of components is used to separate them from the mixture. In the previous classes, we have learned about a few separation techniques such as evaporation, decantation, sieving, hand picked and many more. 

In this session let us learn some more techniques used to separate the mixtures. 


Separation techniques can be used to separate two different states of matter, such as solids and liquids. Separation is an important asset to purify components of interest from a mixture. 

Separation techniques are classified based on the type of mixtures. These are 

  1. A mixture of two solids 
  1. A mixture of solid and Liquid 
  1. A mixture of two liquids 

Mixture of Two Solids: 

A Mixture contains both substances as solid only, and they can be separated by the following methods. 

  1. By using suitable solvent 
  1. By sublimation 
  1. By using magnet 

By Using Suitable Solvent 

In this method, a mixture of two solids can be separated by using a solvent that makes any one of the substances soluble in that. 

  1. The mixture can be separated by taking a solvent in which any one of the substances is soluble. 
  1. Therefore, one substance can separate from another. 
  1. Differences in the solubilities of mixture substances can be used to separate them. 

For example, Mixture of Sand and Salt 

Sand is insoluble in water, whereas salt can be completely soluble in water. This difference of salt and sand solubilities in water separates them in the mixture. 

  1. The mixture of sand and salt was taken into the 
  1. beaker, and water was added to it, as shown in this figure. 
  1. Then the mixture is stirred to dissolve all the salt in the water. 
  1. The sand remains undissolved in the beaker. 
  1. Then the salt solution which contains sand is also filtered by pouring 
  1. over the filter paper kept in a funnel.  
  1. The residue in the solution is sand which remains on the filter paper, and salt solution is obtained as filtrate in a beaker kept below the funnel. 
  1. Finally, the filtrate is heated until the water is gone, and it is left with salt. 
Mixture of Sulphur and sand 

Sulphur and sand are insoluble in water. So, water cannot be used, but Sulphur is soluble in carbon disulphide, salt is insoluble in CS2

By Sublimation: 

The conversion of a substance directly from solid-state into vapors state on heating and then from vapors state into solid when it is cooled is called sublimation. 

The substance which undergoes Sublimation is known as sublime. 

The substance obtained after cooling is called a sublimate. 

This process separates those substances from the mixture, which sublime on heating. 

Examples: At normal conditions, Iodine, Camphor, Naphthalene Balls, Ammonium chloride etc., show Sublimation. 

 Let us make an activity to separate a mixture of salt and ammonium chloride. 

  1. Firstly, take a mixture of salt and ammonium chloride in China dish. Now, Cover this China dish with an inverted conical funnel. 
  1. Put a cotton plug to another end of this funnel so the  vapor could not escape. Now place this China dish on a burner. 
  1. Ammonium chloride is sublime and therefore, after heating, it will change into vapor. 
  1. This vapor will again condense at the upper colder part of the funnel to form solid ammonium chloride.  


A white solid of pure ammonium chloride is obtained. 


The mixture of salt and ammonium chloride is separated by the sublimation method.

Separation by using Sublimation 

By Using Magnet:

If a mixture contains iron as one of the components, it can be separated using a magnet. 

  1. This separation technique is useful for only those minerals that show magnetic properties and minerals with paramagnetic properties. (The less affected substances are called paramagnetic) 
  1. Not every metal is magnetic; gold, silver and aluminum are some examples. Whereas iron, cobalt, nickel all are magnetic.  

For example, a mixture of iron filing and sulphur powder can be separated using a magnet. 

How to Separate Iron Filing from Powder of Sulphur: 

A horse-shoe type of magnet is moved over the surface of the given mixture. A magnet attracts the iron filling; they cling to the magnet’s poles and get separated. 

Separation of the mixture by Magnet 

Mixture of Solid and Liquid 

The following process separates a mixture containing solid and liquid. 

  1. By Evaporation 
  1. By Crystallization 
  1. By Centrifugation 
  1. By Chromatography 
  1. By Distillation 
  1. By using a Separating Funnel 

We have learned about filtration and evaporation in our previous classes. Evaporation separates the solid substance dissolved in water (or any other liquid). Whereas the filtration 

is used for separating insoluble substances from a liquid by using filter paper. 

Let’s learn more about the other separation technique like Crystallization in this session. 


The process in which the hot, concentrated solution of a substance is cooled to obtain crystals is called crystallization. 

This process is used to obtain the pure solid substance from the impure sample. 

Note: Crystals are solid particles with definite shapes and sizes. They are lustrous too. 

Application of Crystallization 

  • Separation of salt from seawater 
  • Separation of pure copper sulphate (CuSO4) from an impure sample. 
  • Separation of crystals of pure Alum from an impure sample. 

Crystallization is used to separate liquid-solid compounds. In this process, the liquid is evaporated by heating, leaving the solid remains in crystalline form. 

To Obtain the Pure Copper Sulphate from the Given Sample of Copper Sulphate: 


  1. An impure solid sample of copper sulphate is dissolved in the minimum amount of water. 
  1. Then this solution is filtered to remove any insoluble impurities. 
  1. The clear solution is heated gently on a water bath until the concentrated or saturated solution is obtained. 
  1. Now allow this hot, saturated solution to cool slowly. In this way, the crystals of pure solid are formed. Impurities will remain dissolved in the solution. 
  1. The last step will be to separate the crystal of pure substance by filtration and dry. 

Why is the crystallization technique better than the simple evaporation technique? 

It is because:  

  • Some solid substances get decomposed on heating, such as sugar, and may get charred on heating to become dry. 
  • Some impurities might remain dissolved in the solution even after filtration. On evaporation, these contaminate the solid. 
  • Therefore, we cannot get the pure substance by using evaporation. 


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