Chemistry Paper 3A WASSCE (SC), 2018

Question 2A

 

C and D are inorganic salts. Carry out the following exercises on them. Record your observations and identify any gas(es) evolved. State the conclusions you draw from the result of each test.

(a) Put all of C in a test tube and add about 5 cm3 of distilled water. Shake thoroughly and test the resulting solution with litmus paper. Divide the solution into three portions

 

  1. To the first portion, add NaOH (aq) in drops, then in excess.
  2. To the second portion, add NH3 (aq) in drops, then in excess.
  3. To the third portion, add AgNO3 (aq) followed by HCl (aq).

 

(b)(i)   Put all of D in a test tube and add about 5 cm3 of distilled water. Shake thoroughly and feel the test tube.
(ii) To about 2 cm3 of the solution add HCl (aq)
[23 marks]

 

Observation

 

In part (a), majority of the candidates could not correctly deduce inferences from the observations. They wrote wrong cations.

In part (b), majority of the candidates did not write their observations when they felt the test tube.

The expected answers include:

 

TEST

OBSERVATION

INFERENCE

(a)

C+ distilled water +shaking

Resulting solution + litmus paper

C dissolved to form a
blue/greenish  solution

Solution turns blue litmus paper red 

 

 

Solution is acidic 

(i)

1st portion + NaOH(aq) in drops

then in excess

Blue precipitate 

 

Precipitate insoluble in excess 

 

 

Cu2+ present

(ii)

2nd portion + NH3(aq) in drops

then in excess

(Light) blue precipitate
(Do not accept deep blue ppt)

Precipitate dissolves in excess to form a deep blue solution

 

 

Cu2+ confirmed

(iii)

3rd portion + AgNO3(aq)

 

+ HCl(aq)

White precipitate formed

 

Precipitate insoluble
(accept no visible reaction)

Cl-or CO32-
S2- or SO32-

Cl-  present
(Cl- must  have been mentioned above)

(b)(i)

D + distilled water

 

touching/feel

D dissolved to form a colourless solution 

Test tube feels cold on touching

 

 

dissolution is
endothermic 

  (ii)

D(aq) + HCl(aq)

Effervescence a colourless odourless gas evolved.

Gas turned lime
water milky 

 

CO2 from
CO32- or
HCO3-