Topical Hub, IGCSE Chemistry 0620 exam-style questions: mark scheme
Mark scheme and worked solutions for 30 questions: 15 multiple choice, 15 structured.
Answer: A
Learn each indicator's colour in acid and in alkali. Litmus: red in acid, blue in alkali. Thymolphthalein: colourless in acid, blue in alkali. Methyl orange: red in acid, yellow in alkali. Dilute nitric acid is an acid, so the colours are red, colourless and red, which is row A.
All acids in water release hydrogen ions, H+, and all alkalis in water release hydroxide ions, OH−. A base is any oxide or hydroxide of a metal. An alkali is a base that dissolves in water. Copper(II) oxide is a metal oxide, so it is a base, but it does not dissolve in water, so it cannot be an alkali. Every alkali is a base, but not every base is an alkali.
Answer: B
pH is a measure of hydrogen ion concentration: a low pH means many H+ ions, a high pH means few. To put the solutions in order of increasing H+ concentration, start with the highest pH and finish with the lowest: 12 (W), 8 (Z), 6 (X), 2 (Y). A common mistake is to list them in order of increasing pH, which gives the reverse order (A).
Answer: C
An acid is a proton donor and a base is a proton acceptor. Look at what happens to the hydrogen: NH3 becomes NH4+, so it has gained a proton (H+). The HCl lost that proton, leaving Cl−. So HCl is the proton donor (acid) and NH3 is the proton acceptor (base). This definition works even though no water is present.
Answer: A
Being shiny, malleable and a good electrical conductor are typical properties of a metal. The type of oxide an element forms is linked to its metallic character: oxides of metals (such as CaO and CuO) are basic, while oxides of non-metals (such as CO2 and SO2) are acidic. So element E is a metal that forms a basic oxide.
An acid reacts with a metal to give a salt and hydrogen: magnesium + hydrochloric acid → magnesium chloride + hydrogen. The salt from hydrochloric acid is a chloride. In symbols, magnesium chloride is MgCl2 because Mg forms Mg2+ and chloride is Cl−, so two HCl are needed: Mg + 2HCl → MgCl2 + H2. Check: 1 Mg, 2 H and 2 Cl on each side. An acid reacts with a carbonate to give a salt, water and carbon dioxide, so zinc carbonate gives zinc chloride, water and carbon dioxide. An observation is something you can see: the carbon dioxide makes bubbles (fizzing), and the powder disappears as it reacts.
"Strong" and "weak" describe how much of an acid splits into ions in water, not how concentrated it is. A strong acid, like hydrochloric acid, is completely dissociated: HCl(aq) → H+(aq) + Cl−(aq). A weak acid, like ethanoic acid, is only partially dissociated, so the equation uses the reversible sign: CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq). Because only some ethanoic acid molecules release H+, a solution of ethanoic acid has fewer H+ ions than hydrochloric acid of the same concentration. Fewer H+ ions means a higher pH, so the pH 3 solution is ethanoic acid and the pH 1 solution is hydrochloric acid.
Bases react with ammonium salts when warmed to give off ammonia gas. The full word equation is: ammonium sulfate + calcium hydroxide → calcium sulfate + ammonia + water. Ammonia is an alkaline gas, so damp red litmus paper turns blue. A gardener adds ammonium sulfate to supply nitrogen to plants. If calcium hydroxide is added at the same time, the two react in the soil and the nitrogen escapes as ammonia gas, so the fertiliser is wasted.
Copper(II) oxide is a base, so it neutralises sulfuric acid: CuO + H2SO4 → CuSO4 + H2O. Adding the oxide until some is left over (excess) guarantees that no acid remains, so the only dissolved substance is copper(II) sulfate. Copper(II) oxide is insoluble, so the leftover black solid is removed by filtration, leaving a blue filtrate. To get crystals, heat the filtrate to evaporate some water until it is saturated, then let it cool so crystals form. Filter off the crystals and dry them between filter papers. Do not boil it to dryness: the crystals would lose their water of crystallisation and any soluble impurities would stay in the solid.
Answer: D
Use the solubility rules. All sodium, potassium and ammonium salts and all nitrates are soluble. Chlorides are soluble except those of lead and silver. Sulfates are soluble except those of barium, calcium and lead. Carbonates are insoluble except those of sodium, potassium and ammonium. Barium sulfate and silver chloride are both exceptions to the "soluble" rules, so both are insoluble.
Answer: C
First decide whether the solution is acidic or alkaline. Thymolphthalein is colourless in acid and blue in alkali, so this solution is an alkali. Then recall the alkali colours of the other two indicators: litmus turns blue and methyl orange turns yellow.
A base reacts with an acid to form a salt and water. This is called neutralisation. Hydrochloric acid makes chloride salts, so magnesium hydroxide gives magnesium chloride. Magnesium hydroxide is Mg(OH)2, with two hydroxide groups, so it needs two HCl and makes two water molecules: Mg(OH)2 + 2HCl → MgCl2 + 2H2O. Check: 1 Mg, 2 Cl, 4 H and 2 O on each side. A carbonate reacts with acid to give a salt, water and carbon dioxide, so calcium carbonate tablets also make carbon dioxide gas. Gas produced inside the stomach can make a person feel bloated or cause burping.
Answer: A
Nitric acid contains H+ and NO3− ions. Potassium hydroxide solution contains K+ and OH− ions. When they are mixed, H+ and OH− combine to make water molecules. At exact neutrality, both H+ and OH− have been removed (apart from the tiny amounts present in pure water). The ions that did not take part, K+ and NO3−, stay in the solution, which is potassium nitrate solution.
Answer: C
Compare two acids of equal concentration. A strong acid is completely dissociated, so it releases the most H+ ions and has the lowest pH. A weak acid is only partially dissociated, so it releases fewer H+ ions and has a higher pH. P (pH 1) behaves as a strong acid and Q (pH 4) as a weak acid, partially dissociated. Remember that any acid solution has a pH below 7, so pH below 7 on its own cannot tell you whether an acid is strong or weak.
Answer: C
Basic oxides (metal oxides such as CaO and CuO) react only with acids. Acidic oxides (non-metal oxides such as CO2 and SO2) react only with bases. Amphoteric oxides react with both acids and bases, forming a salt and water each time. The two amphoteric oxides you need to know are aluminium oxide, Al2O3, and zinc oxide, ZnO.
An acid is a proton (H+) donor and a base is a proton acceptor. A single arrow in a dissociation equation means that all of the HCl molecules split into ions: hydrochloric acid is completely dissociated, so it is a strong acid. In H+ + OH− → H2O, the OH− ion picks up the H+ to become H2O, so OH− is the proton acceptor, the base. For ethanoic acid, the forward reaction has CH3COOH donating a proton. In the reverse reaction, the CH3COO− ion picks up an H+ to form CH3COOH again, so CH3COO− is acting as a base. Looking at who gains the H+ always tells you which species is the base.
Whether an oxide is acidic or basic depends on the element it comes from. Calcium and copper are metals, so CaO and CuO are basic oxides. Carbon and sulfur are non-metals, so CO2 and SO2 are acidic oxides. A basic oxide reacts with an acid to form a salt and water. Copper(II) oxide is insoluble, so you cannot test it with indicator in water. Instead, warm it with dilute acid: the black solid disappears and a blue solution of a copper(II) salt forms, showing it has neutralised the acid.
An insoluble salt is made by mixing two solutions: one containing its positive ion and one containing its negative ion. For barium sulfate you need soluble barium ions (barium chloride or barium nitrate solution) and soluble sulfate ions (sodium sulfate solution or dilute sulfuric acid). Barium sulfate forms as a solid precipitate: Ba2+(aq) + SO4 2−(aq) → BaSO4(s). The other ions stay in solution. Filter to collect the solid on the filter paper, wash it with distilled water to remove the soluble ions left on it, then dry it. The salt you want is the residue, not the filtrate.
Answer: A
"Hydrated" means chemically combined with water, as in blue copper(II) sulfate crystals, CuSO4•5H2O. "Anhydrous" means containing no water, such as white CuSO4 made by heating the hydrated crystals. The prefix "an-" means "without" and "hydr-" refers to water.
pH 7 is neutral, so the salt solution is neutral. Above 7 is alkaline and the highest pH (13, drain cleaner) is the most alkaline, meaning it has the highest concentration of OH− ions. Below 7 is acidic and the lower the pH, the higher the H+ concentration. Increasing H+ means decreasing pH: salt solution (7), rainwater (5), vinegar (3). To measure pH, put a drop of the solution on universal indicator paper (using a clean glass rod) and match the colour to the pH colour chart.
Answer: B
Learn the three general reactions of acids. Acid + metal → salt + hydrogen. Acid + base → salt + water. Acid + carbonate → salt + water + carbon dioxide. Only the carbonate reaction makes all three: a salt, water and a gas. Sodium carbonate + sulfuric acid → sodium sulfate + water + carbon dioxide.
Answer: D
All acids release hydrogen ions, H+, in water. The rest of the ethanoic acid molecule becomes the ethanoate ion, CH3COO−. Because ethanoic acid is weak, only some of its molecules dissociate at any moment, and the ions can join back together, so we use ⇌. For a strong acid such as hydrochloric acid, we use a single arrow instead: HCl(aq) → H+(aq) + Cl−(aq).
Answer: D
Choosing a method for a soluble salt depends on the solubility of the other reactant. If the base, carbonate or metal is insoluble, you add it in excess and filter off the leftover solid. Sodium, potassium and ammonium compounds are all soluble, so any excess would stay dissolved in the product. For these salts you use titration: measure exactly how much acid neutralises a known volume of alkali (using an indicator), then mix those same volumes without the indicator and crystallise. Always check that the acid gives the right salt: nitric acid makes nitrates.
Answer: B
A hydrated substance is chemically combined with water. The water molecules that are part of the crystal are called water of crystallisation, and the formula shows how many there are: CuSO4•5H2O has five for every CuSO4. The crystals feel dry, but the water is still there, built into the crystal. Only heating drives it off, leaving white anhydrous copper(II) sulfate, CuSO4, which contains no water.
Answer: B
Bases are oxides or hydroxides of metals. Alkalis are the bases that are soluble in water. Of the hydroxides, only those of sodium, potassium, ammonium and (partially) calcium dissolve, so potassium hydroxide is an alkali. Copper(II) oxide and iron(III) hydroxide are bases but insoluble. Carbonates react with acids too, but they are not oxides or hydroxides, so they are not classed as bases here.
Thymolphthalein is blue in alkali and colourless in acid (and in neutral solutions). At the start the flask contains sodium hydroxide, an alkali, so it is blue. Once all the alkali is neutralised and a little acid is in excess, the indicator becomes colourless. In every acid-alkali neutralisation, hydrogen ions from the acid combine with hydroxide ions from the alkali to form water: H+(aq) + OH−(aq) → H2O(l). The other ions, Na+ and Cl−, remain as sodium chloride solution. When exactly neutral the pH is 7.
Magnesium oxide is a basic oxide: it reacts with acids but not with bases. Aluminium oxide is amphoteric: it reacts with both acids and bases, making a salt and water each time. P reacts with both reagents, so P is aluminium oxide; Q reacts only with the acid, so Q is magnesium oxide. The other amphoteric oxide you need to know is zinc oxide. Magnesium is a metal, and metal oxides are typically basic, which fits Q's results.
Use the typical reactions of acids and alkalis. Only an acid reacts with magnesium to give bubbles (of hydrogen), so J is hydrochloric acid. Thymolphthalein turns blue only in alkali, and an alkali (a base) warmed with an ammonium salt gives off ammonia, the gas that turns damp red litmus blue. So K is sodium hydroxide. L shows no acid or alkali reactions, so it is the neutral sodium chloride solution. When K exactly neutralises J, the H+ ions and OH− ions combine to form water, leaving only sodium chloride solution. With no H+ ions left, magnesium has nothing to react with, so no bubbles are seen.
A hydrated substance has water chemically combined with it; the water molecules that are part of the crystal are called water of crystallisation. Heating drives this water off as steam: MgSO4•7H2O → MgSO4 + 7H2O. The mass falls because water leaves the basin. When the mass stops changing, no more water is being lost, which shows all of it has gone. The white powder left is MgSO4, which contains no water, so it is anhydrous.
Use the solubility rules to decide. Lead(II) sulfate is one of the insoluble sulfates, so it is made by precipitation (method 3). To precipitate it, mix a solution containing lead ions with a solution containing sulfate ions. Lead(II) nitrate is used because all nitrates are soluble, while lead(II) chloride and lead(II) carbonate are insoluble. Any soluble sulfate, such as sodium sulfate or sulfuric acid, provides the sulfate ions. Potassium sulfate and zinc sulfate are both soluble. Zinc, zinc oxide and zinc carbonate are insoluble, so they can be added in excess to sulfuric acid and the leftover solid filtered off (method 2). Potassium hydroxide and potassium carbonate are soluble, so any excess would stay dissolved and contaminate the product. Instead, titration (method 1) is used to find the exact amounts that react.