Chemistry, Third edition

98 6 · REACTIONS OF IONS IN SOLUTION

Identification of unknowns

(i)A colourless solution CD contains an anion C and a cation D. Addition of sodium hydroxide

solution to CD gave a white precipitate H which dissolved in excess hydroxide to produce a clear

solution. H turns yellow when strongly heated in a flame. Addition of CD to a mixture of silver

nitrate solution and dilute nitric acid gave a white precipitate. Identify C and D.

(ii)A colourless solution is either magnesium carbonate, sodium nitrate or dilute sulfuric acid.

Addition of sodium sulfite solution to the unknown (with warming) produces a sharp smelling

gas. What is the identity of the colourless solution? Suggest a test which would confirm your

answer.

(iii)The labels have fallen off four bottles. One of the bottles contains copper(II) sulfate

solution, another sodium hydroxide solution, the third sodium carbonate solution and the

fourth dilute hydrochloric acid. Using only these bottles (and clean test tubes) plan tests which

would enable you to identify the contents of each bottle.

Exercise 6L

Revision questions

6.1.(i)Pure nitric acid (HNO 3 ) is a covalent liquid which

does not react with dry sodium carbonate. Yet, dilute nitric

acid does. Explain this.

(ii)Explain why ammonia gas turns damp red litmus paper blue.

(iii)Why should concentrated sulfuric acid not be used as a

drying agent for ammonia gas?

6.2.Write equations showing the ions formed when the

following dissolve in water:

(i)icopper(II) sulfate crystals (CuSO 4 · 5H 2 O)

(ii)lithium hydroxide (LiOH, i.e. Li,OH).

6.3.Oxalic acid is a toxic acid found in rhubarb leaves. It

possesses the structure

which may also be written as (COOH) 2. Both hydrogen atoms

are acidic. Write down a chemical and ionic equation for the

reaction of oxalic acid with: (i)potassium hydroxide solution,

(ii)zinc metal.

Example 6.3

A solution, containing a cation Z and an anion Y, fizzed with

sodium carbonate solution and gave a cream coloured

precipitate when added to a mixture of silver nitrate solution

and dilute nitric acid. Identify Z and Y.

Answer

The fizzing suggests that Z is probably H(aq). The cream coloured precipitate is

probably silver bromide. Therefore, the unknown is a solution of hydrobromic acid.

Z = H(aq); Y = Br(aq); ZY = HBr(aq)

Table 6.3Ions discussed in this chapter

Cations Anions

H, NH 4 , K, Na, Ca^2 ,OH, Cl, SO 42 , Br, I,

Cu^2 , Fe^2 , Fe^3 , Pb^2 , Al^3 ,SO 32 , S^2 , CO 32 , NO 3 

Zn^2 , Mg^2 , Ba^2 , Ag

For more about ion

identification see

Appendix 6 on the

website.