Common Errors:
Question 1 : Suggest a test for ammonia gas.
Wrong answer: Use red litmus paper and it will turn blue.
Explanation: Water is needed for ammonia to ionize to give hydroxide ions, which accounts
for alkaline properties that turn litmus paper from red to blue.
Correct answer: Moist red litmus paper is used and it will turn from red to blue.
Question 2 : Suggest a way to prepare calcium sulphate.
Wrong answer: Calcium is added to sulphuric acid The reaction mixture is heated to make it
more concentrated. The solution is then left to evaporate to form crystals. Explanation:
Calcium sulphate is an insoluble solid, which forms a layer on calcium and prevent further
reaction.
Correct answer: Calcium nitrate solution is mixed with sodium sulphate solution. White
precipitate is formed and filtered. Large amount of distilled water is used to wash the
precipitate and dry it with filter paper.
Question 3 : Suggest a test for ammonium ion.
Wrong answer: Add sodium hydroxide solution and use moist red litmus paper from red to
blue.
Explanation: Heating is needed to release ammonia gas.
Correct answer: Add sodium hydroxide solution and heat the solution. Use moist red litmus
paper to test the gas formed and it will turn from red to blue.
Question 4 : A student is asked to prepare magnesium carbonate. Suggest an appropriate
method for the student.
Wrong answer: Dissolve carbon dioxide in a beaker of water to produce carbonic acid.
Magnesium reacts with carboxylic acid to give magnesium carbonate and hydrogen.
Explanation: Carbon dioxide is only slightly soluble in water, so concentration of carbonic
acid is low. Magnesium will react with carbonic acid and magnesium carbonate is an
insoluble solid, which forms a layer on magnesium, which prevents further reaction.
Correct answer: Magnesium nitrate solution and sodium carbonate solution. White precipitate
is formed and filtered. Large amount of distilled water is used to wash the precipitate and dry
it with filter paper.
Question : Explain why sodium hydroxide is a strong alkali.
Wrong answer: Sodium hydroxide can completely ionises to give hydroxide ions.
Explanation: Ionization is the physical process of converting an atom or molecules into an
atom. Sodium hydroxide is originally an ionic compound, which consists of ions rather than
molecules. Hence, the production of sodium ions and hydroxide ions when sodium hydroxide
is dissolved in water is not ionisation as there is no conversion of molecules into ions. It is
called dissociation.
Correct answer: Sodium hydroxide can completely dissociate to give hydroxide ions.
Question : Can sodium hydroxide solution be used to do titration directly? Explain.
Wrong answer: Yes, as sodium hydroxide is an alkali.
Explanation: Sodium hydroxide cannot do titration directly as the concentration is hard to
determine.
Correct answer: No, sodium hydroxide will absorb water, which makes the concentration
hard to determine/ sodium hydroxide will react with carbon dioxide, which reduces the
concentration.
Question: Suggest an indicator when we do titration of sodium hydroxide and hydrochloric
acid.
Common error: litmus solution can be used.
Explanation: The change of colour will not be sharp enough to determine the titration end
point, which may lead to great error in calculations.
Correct answer: Methyl orange / phenolphthalein.
Question : What is the observation when hydrochloric acid and sodium hydroxide are mixed
together?
Common error: No observable change as both hydrochloric acid and sodium hydroxide are
colourless liquid.
Explanation: Observation is not limited to colour change of solution.
Correct answer: The solution becomes warmer/ heat is evolved.
Question: Which indicator can be used to determine the titration end point if potassium
hydroxide solution and ethanoic acid are reacted.
Common error: Methyl orange.
Explanation : Using methyl orange cannot give an accurate titration end point when strong
alkali and weak acid are used.
Correct answer: phenolphthalein
Common error: Reaction between sodium hydrogen carbonate and hydrochloric acid is
neutralisation.
Explanation: Definition of neutralisation is reaction of acid and alkali to give salt and water
only. Since reaction between sodium hydrogen carbonate and hydrochloric acid will give
carbon dioxide too, it is not considered as neutralisation.