Question

The solution which has pH less than 7 is:
A.Solution of caustic soda in water
B.Solution of common soda in water
C.Solution of blue vitriol in water
D.Solution of baking soda in water

Answer 1

Hint: pH of a solution accounts for the free \[{H^ + }\] ions available in the solution. It is a scale used to determine the acidity or basicity of an aqueous solution. A weak acid is a substance that partially dissociates when dissolved in a solvent.

Complete step by step answer:
pH of a solution is a scale used to specify the acidity or basicity of an aqueous solution. It is known as the potential of hydrogen. Lower pH values correspond to solutions which are more acidic in nature, while higher values correspond to solutions which are more basic in nature. If the pH of a solution < 7, it is acidic. If the pH of a solution = 7, it is neutral and if the pH > 7, it is basic. The pH of a solution is defined as the negative of logarithm of the molar concentration of hydrogen ions in the solution. Mathematically,
pH = - log[\[{H^ + }\]]
 In the given question, we will need to investigate each option individually:
A.Solution of caustic soda in water
Sodium hydroxide is known as caustic soda. When sodium hydroxide is added to water, it breaks into its constituent ions.
\[NaOH + {H_2}O \to N{a^ + } + O{H^ - }\]

The solution becomes basic with pH > 7.

B.Solution of common soda in water
Common soda is known as sodium carbonate.
\[N{a_2}C{O_3} + {H_2}O \to NaOH + {H_2}O + C{O_2}\]

The solution becomes basic with pH > 7.
C.Solution of blue vitriol in water
Blue vitriol is basically hydrated copper sulphate.
\[CuS{O_4}.5{H_2}O + {H_2}O \to Cu{(OH)_2} + {H_2}S{O_4}\]
The solution becomes acidic with pH < 7.
D.Solution of baking soda in water
Baking soda is sodium bicarbonate
\[NaHC{O_3} + {H_2}O \to NaOH + {H_2}O + C{O_2}\]

The solution becomes basic with pH > 7.
Hence, the correct answer is (C).

Note: Remember that only ions which further react with water are the ones which produce a weak acid or a weak base. Strong acids or strong bases are not formed by reaction with water.