Acids, Bases and Salts: Chemistry 0620 (Cambridge O Level / IGCSE)
Syllabus 7.1, 7.2, 7.3 · Strand 7 Acids, bases and salts
- Questions
- 10
- Total marks
- 54
- Tier mix
- 6 Core · 4 Extended
0 of 10 questions completed
Syllabus coverage
- 7.1 8 questions completed
- 7.2 6 questions completed
- 7.3 6 questions completed
Acids, bases and salts (syllabus sections 7.1–7.3) supplies some of the most predictable marks in 0620: the general reactions of acids come up year after year. Acid + metal gives a salt and hydrogen; acid + base gives a salt and water; acid + carbonate gives a salt, water and carbon dioxide. You should recognise the colour responses of litmus, thymolphthalein and methyl orange, use universal indicator and the pH scale to compare acidity, and know that alkalis are soluble bases containing OH⁻ ions in solution.
Section 7.2 classifies oxides as acidic, basic or (for Extended) amphoteric, aluminium and zinc oxide react with both acids and alkalis. Salt preparation (7.3) is a favourite practical question: choose between the excess-insoluble-base method, titration, or (Extended) precipitation, and justify each step from filtering to crystallising.
Extended candidates also define acids as proton donors and bases as proton acceptors, and distinguish strong acids (fully dissociated, like HCl) from weak acids (partially dissociated, like ethanoic acid), a distinction about dissociation, not concentration. The questions below are original with complete worked solutions.
Question 1
A laboratory technician dips a strip of universal indicator paper into each of four unlabelled beakers of solution and records the colour produced.
| Beaker | Colour of universal indicator paper |
|---|---|
| P | red |
| Q | green |
| R | purple |
| S | orange |
Which beaker most likely contains a strong alkali?
Question 2
AquaClear Maintenance services industrial hot-water tanks. Their technicians use a descaling solution of dilute hydrochloric acid to clean deposits found inside the tanks. During one visit, the acid is tested separately against three samples removed from a tank:
- Sample 1: a strip of zinc metal from a corroded fitting
- Sample 2: white scale, which is mostly calcium carbonate
- Sample 3: a grey-black deposit of copper(II) oxide from an old valve
(a) Describe what would be observed when the descaling acid is added to Sample 1, and write a word equation for the reaction. [2]
(b) Write a word equation for the reaction between the descaling acid and Sample 2, and name the gas produced. [2]
(c) Sample 3 is a base. Write a word equation for its reaction with the descaling acid, and describe the colour change of the solid as it reacts. [2]
Question 3
A campus safety office is comparing two spill-neutralising rinse solutions of the same concentration, : Rinse A is hydrochloric acid, and Rinse B is ethanoic acid. Both rinses are tested with a pH probe and a conductivity meter.
(a) Define the term proton donor, and use it to explain why both hydrochloric acid and ethanoic acid are classified as acids. [2]
(b) Write an equation to show hydrochloric acid dissociating in aqueous solution, and an equation to show ethanoic acid dissociating in aqueous solution. [2]
(c) The results show that Rinse A has a lower pH and a higher electrical conductivity than Rinse B, even though both rinses have the same concentration. Explain this observation in terms of dissociation. [3]
Question 4
A metal-recycling plant recovers aluminium from crushed drink cans. Before melting a batch, technicians test flakes of a white oxide coating removed from the aluminium surface, to confirm what it is. Small samples of the coating are added separately to dilute hydrochloric acid and to dilute sodium hydroxide solution. In both tests, the white solid dissolves completely, forming a colourless solution.
(a) State the term used to describe an oxide that behaves in this way, and name one other metal oxide (other than aluminium oxide) that behaves the same way. [2]
(b) Write a word equation for the reaction of aluminium oxide with hydrochloric acid, and identify the two types of product formed when aluminium oxide reacts with sodium hydroxide. [2]
(c) Explain why this same test result would not have been observed if the coating had been magnesium oxide instead of aluminium oxide. [2]
Question 5
Riverstone Bath Co. wants to manufacture magnesium sulfate crystals (commonly called Epsom salts) to add to bath-soak products. Magnesium oxide (an insoluble base) and dilute sulfuric acid are the two raw materials available.
(a) Explain why the excess-insoluble-base method, rather than titration, is the suitable method for preparing magnesium sulfate here, and describe how the technicians would carry out this method to obtain pure, dry crystals. [4]
(b) The magnesium sulfate crystals produced have the formula . Define the terms hydrated and anhydrous, and describe what would happen if a sample of these crystals were strongly heated. [3]
Question 6
A pool-maintenance technician tests a sample of pool water using three different indicators. The results are shown in the table.
| Indicator | Colour observed |
|---|---|
| Litmus | blue |
| Methyl orange | yellow |
| Thymolphthalein | blue |
What can be concluded about the pool water sample?
Question 7
A school science technician is preparing example reactions of acids for a lesson. She reacts (i) magnesium ribbon with dilute hydrochloric acid, (ii) sodium carbonate powder with dilute nitric acid, and (iii) aluminium turnings with dilute hydrochloric acid.
(a) Write the balanced symbol equation, including state symbols, for the reaction between magnesium and dilute hydrochloric acid. [2]
(b) Write the balanced symbol equation, including state symbols, for the reaction between sodium carbonate and dilute nitric acid. [2]
(c) Aluminium also reacts with dilute hydrochloric acid, releasing hydrogen gas. Write the balanced symbol equation, including state symbols, for this reaction. [2]
Question 8
NutriGrow Labs wants to produce pure, dry crystals of potassium nitrate for use as a laboratory-grade fertiliser standard. They have potassium hydroxide solution (a soluble alkali) and dilute nitric acid available.
(a) Explain why titration, rather than the excess-insoluble-base method, is the appropriate technique for preparing potassium nitrate here, and name a suitable indicator for carrying out the titration. [2]
(b) Describe how the technicians would use the results of the indicator titration to obtain a pure, dry sample of potassium nitrate crystals, including why the titration is repeated without the indicator present. [3]
(c) Write the balanced symbol equation, including state symbols, for the reaction between potassium hydroxide and nitric acid. [2]
Question 9
A fertiliser manufacturer tests three oxide powders recovered from a raw mineral blend: potassium oxide, sulfur trioxide and lithium oxide. Each oxide is tested separately with dilute nitric acid and with sodium hydroxide solution.
(a) Potassium oxide reacts with dilute nitric acid, forming a salt and water, but does not react with sodium hydroxide solution. State the type of oxide potassium oxide is, and write a word equation for its reaction with nitric acid. [2]
(b) Sulfur trioxide does not react with nitric acid, but reacts with sodium hydroxide solution to form a salt and water. State the type of oxide sulfur trioxide is, and write a word equation for its reaction with sodium hydroxide. [2]
(c) Lithium oxide gives the same pattern of results as potassium oxide: it reacts with the nitric acid but not with the sodium hydroxide solution. State the type of oxide lithium oxide is, and explain, in terms of metals and non-metals, why potassium oxide and lithium oxide both behave this way. [2]
Question 10
A water-treatment engineer neutralises a spillage of dilute hydrochloric acid by adding sodium hydroxide solution until the mixture is exactly neutral.
(a) Define the term proton acceptor, and explain why sodium hydroxide is classified as a base according to this definition. [2]
(b) Write the full balanced symbol equation, including state symbols, for the reaction between sodium hydroxide and hydrochloric acid. [2]
(c) Write the ionic equation for this neutralisation reaction, and explain why sodium ions and chloride ions do not appear in it. [3]