Question

# Describe in each case, one chemical test that would enable you to distinguish between the following pairs of chemicals. Describe what happens with each chemical or state 'no visible reaction'. (i) Sodium chloride solution and sodium nitrate solution. (ii) Sodium sulphate solution and sodium chloride solution. (iii) Calcium nitrate solution and zinc nitrate solution.

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Solution

## (i) Sodium chloride and sodium nitrate solutions can be distinguished by adding silver nitrate solution in the following manner: Chemical test Sodium chloride solution Sodium nitrate solution Addition of silver nitrate solution It reacts with silver nitrate to form a white precipitate of silver chloride, which dissolves in excess of ammonium hydroxide. $\mathrm{NaCl}+{\mathrm{AgNO}}_{3}\to \mathrm{AgCl}+{\mathrm{NaNO}}_{3}$ It does not react with silver nitrate solution. (ii) Sodium sulphate and sodium chloride solutions can be distinguished by adding barium chloride solution in the following manner: Chemical test Sodium sulphate solution Sodium chloride solution Addition of barium chloride solution It reacts with barium chloride to form a white precipitate of barium sulphate, which is insoluble in mineral acids. ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}+{\mathrm{BaCl}}_{2}\to {\mathrm{BaSO}}_{4}+2\mathrm{NaCl}$ It does not react with barium chloride solution. (iii) Calcium nitrate and zinc nitrate solutions can be distinguished by adding sodium hydroxide in the following manner: Chemical test Calcium nitrate solution Zinc nitrate solution Addition of sodium hydroxide solution It reacts with sodium hydroxide solution to form a white precipitate of calcium hydroxide, which on further addition of excess sodium hydroxide is sparingly soluble. $\mathrm{Ca}{\left({\mathrm{NO}}_{3}\right)}_{2}+2\mathrm{NaOH}\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_{2}+2{\mathrm{NaNO}}_{3}$ It reacts with sodium hydroxide solution to form white gelatinous precipitate of zinc hydroxide, which on further addition of excess sodium hydroxide reacts to form soluble, complex sodium zincate. $\mathrm{Zn}{\left({\mathrm{NO}}_{3}\right)}_{2}+2\mathrm{NaOH}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_{2}+2{\mathrm{NaNO}}_{3}\phantom{\rule{0ex}{0ex}}\mathrm{Zn}{\left(\mathrm{OH}\right)}_{2}+2\mathrm{NaOH}\to {\mathrm{Na}}_{2}{\mathrm{ZnO}}_{2}+2{\mathrm{H}}_{2}\mathrm{O}$

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