9701 MCQ Solution 16

Question 16: [Inorganic > Group 2]:

River water in a chalky agricultural area may contain Ca²⁺, Mg²⁺, CO₃^2-, HCO₃^-, Cl^- and NO₃^- ions. In a waterworks, such water is treated by adding a calculated quantity of calcium hydroxide.

What will be precipitated following the addition of calcium hydroxide?

A CaCl₂

B CaCO₃

C MgCO₃

D Mg(NO₃)₂

Reference: Past Exam Paper – November 2002 Paper 1 Q16

Solution 16:

Answer: B.

The question is asking what will be precipitated followed by the addition of calcium hydroxide so you can expect there will be a reaction with calcium hydroxide with the other ions, so you have two options calcium chloride which is A and calcium carbonate which is B.

Since question is saying what will be precipitated, a precipitate is a solid insoluble, so you are looking for something that is insoluble. If we look at A its CACl2 which is soluble in water so this is will not precipitate it will dissolve, because all chlorides are soluble in water except silver and lead chlorides.

If we look at D its magnesium nitrate and all nitrates all soluble so this is not an option.

If we look at C its MgCO3 which is also insoluble so we can take that as an answer but since question asked what will be precipitated followed by addition of calcium hydroxide so we are expecting a reaction of calcium hydroxide with the other ions.

Calcium hydroxide will react with HCO3 which is acid base reaction and calcium carbonate will form which is insoluble, all carbonates are insoluble except group 1 carbonates and ammonium carbonates. Therefore answer is B.