Question

Explain giving reasons why,
(i) Ionic solids conduct electricity in molten state but not in solid state.
(ii) Sodium Chloride has no effect on litmus, but a solution of zinc chloride turns blue litmus red.

Answer 1

(i) Hint: We can use the idea that in molten states the ionic solids get disassociated to form constituent anions and cations that are the positive and the negative ions. These free ions are capable of conducting electricity. This approach can be used to solve the above question. Complete step-by-step answer:
In the ionic compounds, electricity is conducted by free ions. When present in solid state, ions are held together by strong electrostatic forces and hence are not free to move around within the solid. Hence, when present in molten state or in solution form, the ions are free to move and can therefore conduct electricity.
Example when NaCl is in solution state(By heating or dissolving) it gets disassociated to form a Cation and an Anion as follows -
$NaCl\left( s \right) + {H_2}O\left( l \right) \to N{a^ + }\left( {aq} \right) + C{l^ - }\left( {aq} \right)$

Note – When the ionic solid is present in the solid/firm state, the constituent ions are held at fixed sites by the electrostatic forces of attraction in the crystal structure of the solid.
As a result, ions become unable to move and cannot conduct electricity even when electric potential is applied.

(ii) Hint – Aqueous solution of salt of a strong acid and a weak base is acidic, and hence it will turn blue litmus paper to red whereas aqueous solution of salt of a weak acid and a strong base is basic in nature, hence it turns red litmus paper to blue in color.

Complete step-by-step answer:
As NaCl (sodium chloride) is a salt of a strong acid and a strong base it does not undergo any hydrolysis when it is dissolved in water and hence the solution remains neutral.
Therefore NaCl is considered to be a neutral electrolyte which has no effect on the litmus paper and the solution of NaCl conducts electricity as NaCl ionises to its constituent ions as we have already seen in part (i)
$NaCl + {H_2}O \rightleftharpoons N{a^ + } + C{l^ - } + {H^ + } + O{H^ - }$
On the other hand Zinc Chloride is a salt of strong acid and a weak base so it undergoes hydrolysis in water which is particularly Cationic Hydrolysis and forms less disassociated (weak base) and a strong acid.
\[ZnC{l_2} + 2{H_2}O \rightleftharpoons Zn{\left( {OH} \right)_2} + 2{H^ + } + 2C{l^ - }\].
Hence thereby Zinc Chloride turns the litmus paper red.

Note – Hydrolysis is a form of chemical reaction where water is used to break down the chemical bonds that exist between a particular substance in order to disassociate it. So with respect to the branch of chemistry, hydrolysis basically means the act of setting apart chemicals by adding water to get the constituents. The reaction of water with another chemical compound results in the formation of two or more products/compounds.