Acids & Bases

2019 O Level, Open-ended Question 9(a)

State two chemical properties of all acids.

  1. When acids dissolve in water, they dissociate to produce hydrogen ions.
  2. Acids turn moist blue litmus paper red.
2018 O Level, MCQ 10

Which statement about acids and bases is not correct?

A. Acids react with metals to give hydrogen.
B. Acidic oxides are formed from metals reacting with oxygen.
C. Alkalis dissolve in water forming hydroxide ions.
D. Hydrogen ions react with hydroxide ions to produce water.

Answer: B

Statement A is correct, usually. Acids do react with metals to form salt and hydrogen gas. However, there are exceptions. Acids do not react with unreactive metals like silver and gold.

Statement B is not correct. When metals react with oxygen, the metal oxides formed are basic oxides, like sodium oxide and calcium oxide. On the other hand, acidic oxides are non-metal oxides, like carbon dioxide.

Statement C is correct. All alkalis are soluble. They dissolve in water and dissociate to form hydroxide, OH ions.

Statement D is correct. During neutralisation, hydrogen ions from acids react with hydroxide ions from alkalis to form water. A salt is produced as the by-product.

2018 O Level, Open-ended Question 9(a)

Describe how you would distinguish between an acidic solution and an alkaline solution.

Method 1: add a few drops of Universal Indictor (UI) into each solution
Observation: acid turns UI red or orange while alkali turns UI blue or violet

Method 2: test solution using moist blue and red litmus paper
Observation: acid turns blue litmus paper red while alkali turns red litmus paper blue

Method 3: add a magnesium strip into each solution
Observation: only the acidic solution will react to give effervescence of colourless, odourless hydrogen gas

Method 4: add calcium carbonate into each solution
Observation: only the acidic solution will react to give colourless, odourless carbon dioxide gas that forms white precipitate when passed into limewater

2018 O Level, Open-ended Question 9(c)

Ammonia and hydrogen chloride are both compounds. When mixed with water, one of these compounds produces an alkaline solution and the other produces an acidic solution.

(i) Explain why one solution is alkaline and the other is acidic.
(ii) These two solutions can neutralise one another. Write either an ionic equation or a chemical equation to represent this neutralisation.

Ammonia forms the alkaline solution, as it dissolves and ionises in water to form hydroxide ions. Hydrogen chloride forms the acidic solution, as it dissolves and ionises in water to form hydrogen ions instead.

Chemical equation: HCl(aq) + NaOH(aq)  ⟶ NaCl(aq) + H2O(l)
Ionic equation: H+(aq) + OH(aq) ⟶ H2O(l)

pH in Soils

2019 O Level, Open-ended Question 1(b)

Name the substance that is used to raise the pH of soil in a farmer’s field.

Calcium hydroxide. It is an alkali that neutralises acid to increase the pH.

2018 O Level, MCQ 11

Why is calcium hydroxide added to soil?

A. to decrease pH and neutralise acidity
B. to decrease pH and neutralise alkalinity
C. to increase pH and neutralise acidity
D. to increase pH and neutralise alkalinity

Answer: C

Calcium hydroxide is an alkali. It undergoes neutralisation with acid to “neutralise acidity”. As acid is reacted away, the pH increases.

Types of Oxides

2019 O Level, MCQ 12

The oxide of an element X is added separately to hydrochloric acid and aqueous sodium hydroxide. The word equations for the reactions are shown:

X oxide + hydrochloric acid ⟶ X chloride + water
X oxide + sodium hydroxide ⟶ no reaction

Which statement best describes element X and its oxide?

A. X is a metal and X oxide is acidic.
B. X is a metal and X oxide is basic.
C. X is a non-metal and X oxide is amphoteric.
D. X is a non-metal and X oxide is acidic.

Answer: B

X oxide reacts with acid, but not base. This suggests that X is basic and not amphoteric. Given this, a basic oxide must be a metal oxide (like carbon dioxide and sulfur dioxide). Therefore, X is a metal, that forms an oxide that is basic.

2018 O Level, Open-ended Question 1

State the chemical formula and nature of the following oxides:
1. Sulfur dioxide
2. Potassium oxide
3. Aluminium oxide

  1. Sulfur dioxide, SO2 is an acidic oxide
  2. Potassium oxide, K2O is a basic oxide
  3. Aluminium oxide, Al2O3 is an amphoteric oxide

Salt Preparation

2019 O Level, Open-ended Question 9(b)(i)

Sodium sulfate is soluble in water. To prepare crystals of this salt, a neutral and colourless solution of sodium sulfate must be prepared.

Describe how you would prepare this sodium sulfate solution from sodium hydroxide solution, a suitable indicator and an acid.

  1. Using a pipette, transfer 20.0 cm3 of sodium hydroxide solution into a conical flask.
  2. Add a few drops of methyl orange indicator into the conical flask.
  3. Using a burette, add dilute sulfuric acid into the conical flask until the indicator colour changes from red to yellow. Record the volume of dilute sulfuric acid used.
  4. Repeat steps 1 to 3 using the recorded volume of dilute sulfuric acid, but without the indicator.

Marker’s note: you can also place the reagents the other way round, with dilute sulfuric acid in the conical flask and sodium hydroxide solution in the burette.

2019 O Level, Open-ended Question 9(b)(ii)

Write the chemical equation for the above reaction between the acid and sodium hydroxide solution.

H2SO4 + 2NaOH ⟶ Na2SO4 + 2H2O

2018 O Level, Open-ended Question 9(b)

The salt magnesium sulfate forms soluble crystals. Describe how a pure sample of these crystals can be prepared from insoluble magnesium oxide.

  1. Add an excess of insoluble magnesium oxide into 25 cm3 of dilute sulfuric acid
  2. Filter the resulting mixture, and collect the filtrate of magnesium sulfate solution
  3. Heat the filtrate to drive off some water, hence forming a hot, saturated solution
  4. Cool the solution to allow crystals of magnesium sulfate to form
  5. Filter the resulting mixture, and collect the residue of magnesium sulfate crystals
  6. Wash the residue and dry between sheets of filter paper