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Salts

The salt we use in our food is sodium chloride. It is also known as common salt. But common salt is not the only salt. A salt is a general name given to the chemical produced during the neutralisation reaction between an acid and a base. So, we have a large number and variety of salts.

Preparation of Salts

When an acid reacts with a base, it results in the formation of another compound, called salt, and water. This is called as neutralisation reaction. In this reaction, the acid neutralises the base and the base neutralises the acid.
 
When sodium hydroxide a base, is added to dilute sulphuric acid, sodium sulphate is formed. In this reaction phenolphthalein is added as an indicator. The end point is the disappearance of pink colour.
 
2NaOH + H2SO4  Na2SO4 + 2H2O
 
Ferric chloride is prepared by the direct combination of iron and chlorine.
 
2Fe + 3Cl2  2FeCl3
 
Zinc sulphate is produced by displacement of hydrogen from sulphuric acid.
 
Zn + H2SO4  ZnSO4 + H2

Neutralisation Reaction Between a Strong Acid and a Weak Base

If sulphuric acid (H2SO4) reacts with a weak base, say ferric hydroxide (Fe(OH)3), the reaction can be represented as:
 
 
3H2SO4 + 2Fe(OH)3   Fe2(SO4)3 + 6H2O
 
When all traces of excess reactants i.e. H2SO4 and  Fe(OH)3 are removed and we isolate the pure salt  Fe2(SO4)3. Then, we dissolve the salt Fe2(SO4)3 in distilled water and check its pH. What do we observe? We find that the pH of the solution is less than 7, indicating that the salt obtained in the above reaction is acidic in nature.
 
We conclude that the salt obtained from the neutralisation reaction between a strong acid and a weak base is acidic in nature.
 
Neutralisation reaction between strong base and weak acid: If sodium hydroxide (NaOH) reacts with a weak acid, say carbonic acid (H2CO3), the reaction can be represented as:
 
2NaOH + H2CO3  Na2CO3 + 2H2O
 
Suppose all traces of excess reactants are removed and we isolate the pure salt Na2CO3. Then, we dissolve the salt Na2CO3 in distilled water and check its pH. What do we observe?
 
We find that the pH of the solution is more than 7, indicating that the salt obtained in the above reaction is basic in nature. We conclude that the salt obtained from the neutralisation reaction between a strong base and a weak acid is basic in nature.
 
Thus, if the acid and the base are both strong, the salt produced is neutral. If the acid is strong and the base is weak, the salt will be acidic in nature. If the acid is weak and the base is strong, the salt will be basic in nature.




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