Question

The pair of the solutions which have $pH$ value less than $7$ :
A.Solution of washing soda and vinegar
B.The solution of soap and solution of washing soda
C.Solution of copper sulphate and solution of washing soda
D.Solution of copper sulphate and vinegar

Answer 1

$pH$ is the scale which is used to measure the acidity and basicity of an aqueous solution or a liquid. If the $pH$ is less than $7$ then, it is an acidic solution and if the $pH$ is more than $7$ then, it is a basic solution.

Complete answer:
A.Washing soda is prepared from the combination of carbonic acid, which is a weak acid and sodium hydroxide, which is a strong base. As sodium hydroxide is a strong base, it is fully ionized to give large amounts of hydroxide ions, whereas carbonic acid is a weak acid, and it is slightly ionized to give small amounts of hydrogen ions. Therefore, in this solution, hydroxide ions will be more than the hydrogen ions, making the solution basic. Hence washing soda is alkaline.
Vinegar consists of acid radicals, that is, $CH{{ }_{3}}CO{{O}^{-}}$ ions. Hence, in the solution of washing soda and vinegar, they will not produce acids, having a $pH$ value not less than $7$ . So, option (A) is not correct.
B.In the solution of soap and solution of washing soda, soap is a combination of fatty acids which is a weak acid and a strong base. Therefore, in the solution of soap, hydroxide ions will be more than the hydrogen ions, which makes the solution basic. And as we discussed earlier that washing soda is basic in nature, therefore the solution of soap and solution of washing soda will not produce acids. Hence the option (B) is incorrect.
C.In the solution of copper sulphate and solution of washing soda, the solution of copper sulphate contains acidic radicals $(S{{O}_{4}}^{-})$ and as we discussed that solution of washing soda is basic in nature, therefore the solution of copper sulphate and solution of washing soda will not produce acids. Hence the option (C) is incorrect.
D.The solution of copper sulphate and vinegar contains acidic radicals, that is, $S{{O}_{4}}^{-}$ and $CH{{ }_{3}}CO{{O}^{-}}$ ions. Therefore, this solution will produce acids. Hence the $pH$ of solution of copper sulphate and vinegar will be less than $7$ . Therefore, the option (D) is correct.

Note:
-In the conclusion, the pair of solutions that contain acid radicals will have $pH$ value less than $7$ , that means, their solution will produce acids.
-If the $pH$ value is equal to $7$ , then the solution is considered neutral.
-acids are defined as hydrogen containing substances that have the ability to donate protons.
-bases are defined as hydroxide containing substances that have the ability to accept protons.