Chemistry

13.11 - Common Question Types

3 min

note 1 of 3

"Suggest why / explain why / state why" a step is done

When asked why an experimental step is carried out, identify what would go wrong or be lost if the step were skipped.
The credited reason is the consequence the step prevents or ensures, not a restatement of the step itself.
Frame your answer as: [the step happens] so that [something specific is kept accurate, fair, or complete].

The step	Why it's done
The first few bubbles of gas aren't collected	Air in the apparatus must be flushed out first, so the collected gas isn't diluted with air
A mixture is stirred while being heated	Keeps the temperature and concentration even throughout, preventing hot spots near the flame
A conical flask is placed on a white tile (or dark paper) for a titration	Makes a colour change in a pale or colourless solution easier to see against the background
Small pieces of a solid catalyst are used instead of one large piece	Smaller pieces have a larger surface area for the same mass, making the reaction faster
An experiment is repeated and results are compared	Allows anomalous results to be identified and a mean to be calculated, making the result more reliable

note 2 of 3

To explain differences or similarities between two experiments, find what changed between them and connect that variable directly to the result.

Identify what changed and what was kept constant between the experiments.
State the effect of the variable, linking it to the specific observation or measurement asked about.
If the results are the same, explain why the variable did not affect the outcome (e.g. a reagent was already in excess, or the limiting reagent was unchanged).

Why a volumetric pipette can't measure the aluminium chloride - The volume of aluminium chloride varies between experiments, whereas a volumetric pipette only delivers one fixed volume.
Why distilled water is increased as another reagent is decreased - Keeps the total volume of liquid constant, ensuring any difference in results is caused solely by changing the reagent concentration.
Why the same mass of precipitate forms in several experiments - In those experiments, one reagent is in excess, so the amount of precipitate is determined by the limiting reagent, which is kept at a fixed amount.

note 3 of 3

Match the properties, observations, or behaviour in the description directly to the test, gas, or substance they indicate.

A colourless gas that relights a glowing splint - Oxygen (O₂).
A pungent, alkaline gas that turns damp red litmus blue - Ammonia (NH₃).
Gas produced when heating hydrated aluminium chloride - Hydrogen chloride (HCl) or steam (H₂O).

A negative test result rules specific substances out rather than naming what is present.
Example - No precipitate forming on adding dilute nitric acid followed by aqueous silver nitrate shows that chloride, bromide, and iodide ions are not present.

end of notes