Non-metals & their compounds — questions and answers
4 high-yield, syllabus-aligned questions on Non-metals & their compounds, covering 14 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Describe4 marks
Describe the laboratory preparation of ammonia, and state how the gas is dried and collected.
TopMarks model answer
Ammonia is prepared by heating a mixture of ammonium chloride and slaked lime, calcium hydroxide, in a hard-glass test tube. The tube is inclined with its mouth downwards so that condensed water does not run back onto the hot glass and crack it. The gas is dried by passing it over quicklime, calcium oxide, because ammonia reacts with the usual drying agents. It is collected by upward delivery, with the mouth of the jar downwards, because ammonia is lighter than air and very soluble in water.
Suggested marking guide
1Correct reagents
2Tube inclined with the mouth downwards
3Dried over quicklime
4Collected by upward delivery, with a reason
Why it scores
Ammonia is the exception in every part of this procedure, which is exactly why it is asked. It cannot be dried with concentrated tetraoxosulphate(VI) acid because it is basic, nor with anhydrous calcium chloride because it forms a complex — quicklime is the only common drying agent left. And it cannot be collected over water because it is extremely soluble.
A common incomplete answer
“Heat ammonium chloride with calcium hydroxide and collect the ammonia gas produced.”
What it costs: Three marks of four. The reagents are right, but the inclined tube, the drying agent and the method of collection with its reason are all missing.
Give two chemical tests, with their observations, by which ammonia gas may be identified.
TopMarks model answer
Hold a piece of moist red litmus paper in the gas: it turns blue, because ammonia is the only common alkaline gas. Hold a glass rod dipped in concentrated hydrochloric acid near the mouth of the jar: dense white fumes of ammonium chloride are formed.
Suggested marking guide
1Moist red litmus paper used
2It turns blue
3Glass rod dipped in concentrated hydrochloric acid
4Dense white fumes observed
Why it scores
The word MOIST is a mark. Dry litmus paper does not change colour in any gas, because the gas must dissolve to form ions before it can act as an alkali. Every gas test using litmus in WAEC carries this mark, and it is dropped constantly.
A common incomplete answer
“It turns red litmus paper blue and gives white fumes with hydrochloric acid.”
What it costs: One mark of four at risk and lost. The litmus is not described as MOIST, so the first mark falls; the observations and the acid test are otherwise credited.
Key ideas to include: moist red litmus, alkaline gas, concentrated hydrochloric acid, dense white fumes.
Explain3 marks
State three differences in the physical properties of diamond and graphite, and explain TWO of them in terms of their structures.
TopMarks model answer
Diamond is very hard while graphite is soft and slippery, because in diamond each carbon atom is joined to four others in a rigid tetrahedral network, whereas graphite consists of layers held together only by weak forces, which slide over one another. Diamond does not conduct electricity but graphite does, because every valence electron in diamond is used in bonding while each carbon in graphite uses only three, leaving one delocalised electron free to move. Diamond is transparent and lustrous while graphite is black and opaque.
Suggested marking guide
1Hardness explained by the bonding arrangement
2Conductivity explained by delocalised electrons
3A third difference stated — appearance, density or use
Why it scores
Read what the question asks for and give exactly that: three differences, but only two explained. The two explanations must pair the property WITH the structural reason in the same breath — soft BECAUSE the layers slide, conducting BECAUSE one electron per atom is delocalised. That pattern earns marks in every structure-and-property question in Chemistry. The third difference only has to be stated, so do not spend time on it.
A common incomplete answer
“Diamond is hard because its atoms are joined to four others, while graphite is soft because it has layers.”
What it costs: One mark of three under this guide. Hardness is properly explained and credited, but the second explanation is missing and no third difference is stated at all.
State the test for carbon(IV) oxide and explain the observations.
TopMarks model answer
Bubble the gas through lime water, that is calcium hydroxide solution. The lime water turns milky, because insoluble calcium trioxocarbonate(IV) is precipitated. If the gas is then passed in excess, the milkiness disappears, because the insoluble trioxocarbonate reacts further to form soluble calcium hydrogen trioxocarbonate(IV).
Suggested marking guide
1Lime water used as the reagent
2It turns milky because a precipitate forms
3In excess the milkiness disappears
Why it scores
The third mark is the confirmatory half of the test and almost nobody gives it. Sulphur(IV) oxide also turns lime water milky, so the milkiness alone does not prove carbon(IV) oxide. The redissolving in excess is what distinguishes it — which is why the examiner asks for the explanation, not just the observation.
A common incomplete answer
“It turns lime water milky.”
What it costs: Two marks of three. The reagent is credited but the observation is not explained — no precipitate is named — and the behaviour in excess is missing.
Practise Non-metals & their compounds on real questions
Reading a full-mark answer is the first half. Writing one under time is the other. A free account opens exam-standard practice in Chemistry with the full solution on every question.