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Question

State the change noticed when blue litmus and red litmus are introduced in the following solution

(a) Na_2CO_3 solution (b) NaCl solution

(c) NH4NO3 (d) MgCl2 solution

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Solution

(i) If we dip red litmus in a solution of Na2CO3 it will turn blue. That is because the solution of Na2CO3 is basic.

So to understand it you need to know that there are acidic, basic and neutral salts.

An acidic salt is one which upon hydrolysis yields a strong acid and a weak base. Similarly a basic salt will yield a strong base and a weak acid, and finnaly a neutral salt yields an acid and a base of comparable strength.

In the above case, we have the salt Na2CO3. It’s hydrolysis will result into this:

Na2CO3 + 2H2O → 2NaOH + H2CO3

So we see that it gives NaOH, a strong base, and H2CO3, a weak acid. Now we can say that it is a base salt, so its solution will be basic. And the colour of the litmus will turn blue, because there will be OH- ions(coming from the dissociation of NaOH: NaOH → Na+ + OH- ) in the solution.

(ii) NaCl is the salt of a strong acid and a strong base, so a solution of the salt would be neutral. Whichever colour of litmus you use would not change colour.


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