The question was well attempted by candidates.
In (a), candidates correctly tested sample F with both red and blue litmus paper with correct observations and inference as follows:
Test |
Observation |
Inference |
| F + litmus |
Turns blue litmus paper red but had no effect on red litmus paper |
F is acidic |
In (b)(i), some of the candidates showed poor knowledge of the colour change of methyl orange in acidic solution. However, majority of the candidates correctly carried out the required test and made correct observations and inferences in b(ii)-(iii) as follows:
|
Test |
Observation |
Inference |
(b)(i)
(ii)
(iii) |
F + methyl orange
F + Na2CO3
F + Mg ribbon |
Turns methyl orange red/pink
Effervescence/ evolution/bubbles of a colourless gas that turned lime water milky/turn blue litmus paper red.
Effervescence/evolution of colourless gas which gives pop sound with burning splint |
F is acidic
CO2 liberated from Na2CO3.
F is an acid.
H2 liberated |
In (c)(i), some of the candidates could not deduce correctly the colour change of methyl orange in acidic solution. However, a good number of them correctly carried out the required test and made correct observations with logical inferences in c(ii)-(iii) as tabulated below.
|
Test |
Observation |
Inference |
(c)(i)
(ii)
(iii) |
H2SO4(aq)+ methyl orange
H2SO4(aq) + Na2CO3
H2SO4(aq) + Mg ribbon |
Methyl orange turned red
Effervescence/ evolution/bubbles of a colourless gas that turned lime water milky/turn blue litmus paper red.
Effervescence/evolution of colourless gas which gives pop sound with burning splint |
CO2 liberated.
Gas is H2 |
In (d), candidates correctly deduced that the reaction of sample F is similar to that of dilute H2SO4 but majority of them did not mention that it is more vigorous in H2SO4 than F hence, lost some marks. 
