Topic 7 Practice Paper: Acids, bases and salts — mark scheme (print)

Back to Topic 7

Question paper PDF

QuizQuipPractice Series

QQ/IGCSE-CHEM/T7

IGCSE · Cambridge IGCSE Chemistry (Extended (Core + Supplement))

Mark scheme

Acids, bases and salts

Topic 7 Practice Paper · Maximum mark: 30

Notes for markers

Award one mark for each point marked [1], up to the marks shown for the part.
Accept answers worded differently that show the same understanding.
Units are required where the answer line shows one. ecf: allow an error carried forward.

Section A: Multiple-choice questions[10 marks]

Question	Answer
1	AAmmonia dissolves in water producing hydroxide ions, so its solution is alkaline. Copper(II) oxide is an insoluble base, and the other two are acidic.	1
2	CMethyl orange is red below about pH 3.1 and only turns yellow above about pH 4.4, so at pH 3 it is red.	1
3	BA carbonate plus an acid gives a salt, water and carbon dioxide, and the equation must balance: two HCl supply the two chloride ions and both hydrogens.	1
4	DThe middle of the universal indicator colour range, pH 7, is green. Red is strongly acidic, purple is strongly alkaline.	1
5	DThe carbonate was added in excess, so the unreacted solid left on the filter paper is copper(II) carbonate; the nitrate stays dissolved in the filtrate.	1
6	BEthanoic acid is weak and only partially ionised, so at equal concentration it releases the fewest hydrogen ions and has the highest pH.	1
7	CFiltration separates the unreacted excess magnesium (residue) from the magnesium sulfate solution (filtrate). Hydrogen has already escaped as a gas.	1
8	BEvery sodium, potassium and ammonium salt dissolves, and all nitrates are soluble. Lead and silver chlorides, the sulfates listed and the carbonates listed are all insoluble.	1
9	AHeating drives off the water of crystallisation, converting blue hydrated copper(II) sulfate into the white anhydrous salt.	1
10	APrecipitation needs two solutions that together supply Pb²⁺ and Cl⁻ ions. Lead nitrate solution with hydrochloric acid releases insoluble lead chloride immediately.	1

Section B: Structured questions[20 marks]

Question	Answer and marking points	Marks
11(a)	Calcium oxide (quicklime), calcium hydroxide (slaked lime) or calcium carbonate (limestone powder). calcium oxide / calcium hydroxide / calcium carbonate [1]	1
11(b)	The substance neutralises the acid in the soil. Hydroxide or carbonate ions remove H⁺(aq) ions from the soil solution, so the hydrogen ion concentration falls and the pH rises. the added base removes / neutralises H⁺(aq) ions [1] so the hydrogen ion concentration decreases and pH rises [1]	2
11(c)	pH 7 [1]	1
	Total for Question 11	4
12(a)	neutralisation [1]	1
12(b)	Because both reactants are soluble, no excess solid can be filtered off afterwards. The titration finds the exact volumes that neutralise each other, so the salt made afterwards is free from leftover acid or alkali. no excess reactant can be filtered off when both are soluble [1] titration finds the exact neutral volumes [1]	2
12(c)	Evaporating to dryness involves strong heating, which can decompose ammonium salts and produces poor crystals. Concentrating and cooling lets large, pure crystals form. strong heating may decompose the ammonium salt / gives poor crystals [1] cooling a concentrated solution forms better, purer crystals [1]	2
	Total for Question 12	5
13(a)	Any soluble barium salt (for example barium chloride or barium nitrate) with any soluble sulfate (for example sodium sulfate, potassium sulfate or dilute sulfuric acid). a soluble barium compound such as barium chloride or barium nitrate [1] a soluble sulfate such as sodium sulfate or dilute sulfuric acid [1]	2
13(b)	Filter the mixture to collect the barium sulfate residue. Wash the residue with distilled water to remove soluble impurities, then dry it with filter paper or in a warm oven. filter and keep the residue [1] wash the residue with distilled water [1] dry the solid [1]	3
	Total for Question 13	5
14(a)	The cation is Zn²⁺. A white precipitate with sodium hydroxide that redissolves in excess could be Al³⁺ or Zn²⁺, but the precipitate also dissolves in excess ammonia, which is true only for zinc ions. Zn²⁺ identified [1] precipitate soluble in both excess NaOH(aq) and excess NH₃(aq) is specific to zinc ions [1]	2
14(b)	Chloride, Cl⁻. chloride ion [1]	1
14(c)	Ag⁺(aq) + Cl⁻(aq) → AgCl(s) Ag⁺(aq) + Cl⁻(aq) → AgCl(s) with correct species [1] correct state symbols [1]	2
14(d)	Sodium zincate, Na₂Zn(OH)₄ (sodium zincate solution). [1]	1
	Total for Question 14	6

— End of mark scheme —