13.7 - Evaluating a Method
Evaluation questions
Evaluation questions usually take one of two forms.
Paired error-and-improvement questions
- Part (i) asks you to identify a source of error; part (ii) asks for an improvement that reduces it.
- The two parts are marked together: the improvement only scores if it actually fixes that specific error, rather than giving a vague answer like "be more careful".
Advantage-and-disadvantage questions
- Gives two pieces of apparatus or methods and asks you to compare them.
- Requires a balanced comparison: one advantage and one disadvantage of choosing one over the other.
A typical question
During a neutralisation experiment, the temperature of the reaction mixture was measured in an open beaker.
(i) Identify a source of error in this experiment. [1]
(ii) Suggest an improvement that would reduce this error. [1]
Sources of error and improvements
- A source of error is something in the method that makes a reading less accurate.
- Every error must be paired with an improvement that actually reduces it - naming an error alone doesn't score.
Common errors and their fixes
| Source of error | Why it affects the result | Improvement that fixes it |
|---|---|---|
| Heat lost to the surroundings in a temperature-change experiment | The mixture cools faster, so the temperature change measured is smaller than the true value | Use a lid or a polystyrene cup instead of an open beaker - polystyrene is a better insulator |
| A burette read from the wrong position | Looking down on or up at the scale makes the reading appear higher or lower than it really is | Read the burette at eye level, with the bottom of the meniscus level with your eye |
| Gas escapes before the bung is replaced | Some gas is lost before it's collected, so the volume recorded is too small | Use a flask with a divided compartment for one reactant, so it's tipped in with the bung already in place |
| The reaction is too fast to time accurately | Human reaction time becomes a large part of the total time recorded | Slow the reaction with a more dilute reactant or a lower temperature |
Anomalous results
- An anomalous result doesn't fit the pattern of the rest of the data.
- Leave it out when calculating a mean or drawing a line of best fit.
Advantages and disadvantages of apparatus
Some evaluation questions ask you to weigh up two pieces of apparatus that could do the same job.
Burette against measuring cylinder
- Advantage - Measures a more accurate volume, and can deliver a variable volume, stopping exactly at a colour change or end point.
- Disadvantage - Takes longer to fill, rinse, and read, for a task where that accuracy isn't needed.
Gas syringe against upturned measuring cylinder over water
- Advantage - Gives a direct reading off its own scale, and works for a gas that dissolves in water (which an upturned cylinder over water can't measure properly).
- Disadvantage - The wrong size of syringe can push the plunger out of the barrel if the reaction is vigorous; collecting over water avoids this and needs no specialised apparatus.
Lid against no lid
- Advantage - A lid or polystyrene cup reduces heat lost to the surroundings, so the temperature change measured is closer to the true value.
- Disadvantage - A lid gets in the way if you need to add a reactant partway through, or want to watch the reaction.
Common exam questions
Question 1
A reaction mixture is heated in an open beaker and loses heat to the surroundings during the experiment. Give one source of error and an improvement that would reduce it.
Heat is lost to the surroundings, so the temperature change measured is smaller than the true value; using a lid or a polystyrene cup instead of an open beaker reduces this heat loss.
Question 2
Why does "the water bath is a source of error" on its own not score full marks?
Because an error only scores when it is paired with a matching improvement; you also need to say what you would change, such as insulating the container to reduce heat loss.
Question 3
A student reads a burette by looking down at the scale from above. What error does this cause, and how should it be avoided?
Looking down on the scale gives a reading that does not match the true level of the liquid; the burette should be read at eye level, with the bottom of the meniscus level with the eye.
Question 4
A gas escapes from the flask in the moment between adding the reactant and replacing the bung. Suggest an improvement to stop this happening.
Use a flask with a divided compartment for one of the reactants, so it can be tipped into the other to start the reaction once the bung is already in place, rather than adding the reactant and then racing to seal the flask.
Question 5
State one advantage and one disadvantage of using a burette rather than a measuring cylinder to add a reagent.
A burette measures a more accurate, variable volume and can be stopped exactly at a colour change; it takes longer to fill, rinse, and read than a measuring cylinder for a task that does not need that accuracy.
Question 6
A student collects gas over water in an upturned measuring cylinder instead of using a gas syringe. Suggest one disadvantage of this choice.
Some gases dissolve in water, so the volume collected would be less than the true volume produced; a gas syringe would give a direct reading unaffected by this.
Question 7
Why is "human error" a poor answer to a source-of-error question?
Because it does not name anything specific about the method in front of you; the mark is for a precise error, such as heat loss or a misread burette, together with an improvement that actually fixes it.