Paper 6 Errors and Improvements: Write Specific Answers

Paper 6 evaluation questions are won by matching a specific problem to a specific correction. Generic phrases such as “human error”, “be more careful”, “repeat it” and “use better equipment” usually fail to explain what changed or why the result improves.

Use this structure:

Error or limitation → effect on the result → matched improvement → why it works

Accuracy, precision and reliability

Accuracy

How close a measurement or result is to the true or accepted value.

A systematic gas leak can make every measured volume too low. Repeating the experiment can produce consistent values that remain inaccurate.

Precision

How finely a quantity is measured and how close repeated readings are to one another.

Apparatus with smaller scale divisions can allow more precise readings, provided it is used correctly.

Reliability

How consistently a result is reproduced.

Repeat measurements, anomaly checks and a mean improve the evidence for reliability.

Do not treat these terms as interchangeable.

Random and systematic effects

A random effect varies unpredictably between repeats. Repeating and calculating a mean can reduce its influence.

A systematic effect shifts results in the same direction. Repeats alone do not remove it; the method or apparatus must change.

Examples:

• variable judgement of an endpoint: can introduce random variation
• balance not zeroed: systematic offset
• constant heat loss to the room: systematic underestimate of temperature change
• gas leaking through the same loose joint: usually produces volumes that are too low

Error-improvement pairs

Problem	Likely effect	Matched improvement
Heat lost to surroundings	measured temperature rise too small	use an insulated cup with a lid; use a temperature probe and record the maximum promptly
Gas escapes before bung is fitted	initial gas volume missing; rate/total volume too low	use a setup that starts the reaction after sealing, or fit the bung and add one reactant through a suitable device
Apparatus leaks	measured gas volume too low	check airtight joints before the trial and secure the bung/tubing
Gas dissolves in water	collected gas volume too low	use a gas syringe when chemically suitable
Parallax when reading a scale	reading may be too high or too low	read the scale at eye level, perpendicular to it
Measuring cylinder used for a precise fixed volume	larger volume uncertainty	use a volumetric pipette for a fixed volume or a burette for delivered volume
Endpoint judged by eye	subjective endpoint and overshoot	use a pH probe/data logger where appropriate, or add titrant dropwise near the endpoint and use a white background
Solid particles have unequal sizes	surface area changes between trials	use sieved particles of a defined size range or one prepared piece with controlled dimensions
Reaction starts before timer	early interval missed	use a consistent start trigger and apparatus that allows immediate timing
Incomplete transfer	less reactant reaches the vessel	rinse transfer containers into the reaction vessel where appropriate
Evaporation during heating	mass or concentration changes	use a lid/reflux setup where appropriate and minimise unnecessary heating time
Thermometer touches container	reading may reflect the vessel rather than solution	suspend the bulb in the liquid without touching the sides or base

An improvement only earns its full value when it addresses the named mechanism.

Heat-loss questions

Weak answer:

Some heat is lost. Use better equipment.

Better answer:

Heat is transferred from the reaction mixture to the cup and surroundings, so the measured maximum temperature is lower than the true value. Use an insulated polystyrene cup with a lid and record the temperature continuously with a probe to reduce heat transfer and avoid missing the maximum.

Repeating the same uninsulated method does not remove the heat loss.

Gas-collection questions

Possible problems:

• gas lost before the apparatus is sealed
• leaks at bung or tubing connections
• syringe plunger sticks
• gas dissolves when collected over water
• reaction too fast for manual readings
• gas-volume reading taken from the wrong part of the scale

Matched improvements:

• seal before initiating the reaction where possible
• test for leaks
• check the syringe moves freely
• use a gas syringe for a water-soluble gas where safe and appropriate
• use automated or more frequent data recording
• read at eye level

Do not claim every gas should be collected with a gas syringe. The gas, volume and apparatus provided determine suitability.

Titration questions

Overshooting the endpoint

Effect: recorded titre is too large.

Improvement: add titrant dropwise near the endpoint while swirling and observe against a white background.

Funnel left in the burette

Liquid can continue draining into the burette after the initial reading, changing the delivered volume calculation.

Improvement: remove the funnel before taking the initial reading.

Air bubble in the burette tip

Part of the apparent delivered volume fills the tip rather than entering the flask.

Improvement: run solution through the tip before the initial reading and check no air bubble remains.

Flask rinsed with water

This does not change the moles of analyte delivered by a pipette, so it is not automatically an error. The water dilutes the solution but does not alter the amount of substance present.

Burette or pipette rinsed with water immediately before use

Residual water dilutes the solution in that apparatus.

Improvement: rinse the burette with titrant and the pipette with the solution it will deliver.

Rate experiments

Disappearing-cross method

Limitation: judging when the cross disappears is subjective.

Improvement: use the same observer and viewing conditions for all trials, or use a light sensor/colorimeter to define an objective endpoint where available.

Mass-loss method

Limitation: gas may not be the only cause of mass change if evaporation also occurs.

Improvement: use conditions that minimise evaporation and a loose cotton-wool plug where appropriate to reduce spray while still allowing gas to escape.

Gas-syringe method

Limitation: the first gas can escape during bung fitting.

Improvement: assemble an airtight apparatus first and initiate the reaction after sealing, for example by releasing one reactant from a small container inside the flask where the design permits.

Qualitative-analysis questions

Errors include:

• contaminated droppers
• excessive reagent added before observing the initial result
• test tubes not cleaned between tests
• colour described imprecisely
• gas test performed too far from the tube or after the gas has dispersed

Improvements:

• use separate clean droppers
• add reagent dropwise first, then in excess only when required
• rinse apparatus thoroughly
• observe against a white background
• test the gas promptly using the correct procedure

An observation should state what was seen, such as “a light blue precipitate formed”, not only the conclusion “copper(II) ions are present”.

Repeats, means and anomalies

A complete reliability answer:

Repeat each condition at least twice more, compare the results, repeat any anomalous trial and calculate a mean from the concordant readings.

Repeats help identify random variation. They do not correct:

• a balance that was not zeroed
• heat loss built into every trial
• a calibration error
• gas leakage present in every setup

The method must change to address systematic effects.

Percentage uncertainty

When relevant:

percentage uncertainty = absolute uncertainty ÷ measured value × 100%

For the same apparatus, a larger measured quantity often has a smaller percentage uncertainty.

Example:

If a volume measurement has uncertainty ±0.5 cm³:

• for 10.0 cm³: 5%
• for 50.0 cm³: 1%

This is why using a larger measurable change can improve the relative precision, provided other aspects of the method remain valid.

Original worked evaluation

A student measures the temperature change when 25 cm³ of acid reacts with 25 cm³ of alkali in a glass beaker. The student stirs with a glass rod and records the temperature once after 60 seconds.

Question: Identify two limitations and suggest a matching improvement for each. [4]

Limitation 1

Heat is transferred to the glass beaker and surroundings, so the measured temperature change is smaller than the true value. [1]

Improvement: use an insulated polystyrene cup with a lid to reduce heat transfer. [1]

Limitation 2

The maximum temperature may occur before or after the single reading at 60 seconds, so it can be missed. [1]

Improvement: record temperature continuously or at short regular intervals using a temperature probe, then use the maximum value. [1]

“Repeat it” would not directly address either limitation.

Writing an evaluation

A complete evaluation may include:

1. whether the data show a clear trend
2. whether repeats agree
3. whether anomalies exist
4. whether the range and intervals are sufficient
5. whether the apparatus is precise enough for the change measured
6. a systematic limitation
7. a specific matched improvement
8. a conclusion limited to the evidence

Example:

The results show a decreasing time as concentration increases, and the repeat values are close except at 0.6 mol dm⁻³. That result should be repeated before calculating the mean. The visual endpoint is subjective, so a light sensor would improve objectivity. Within the tested range, the data support the conclusion that increasing concentration increases rate.

Common mistakes

1. “Human error.” Name the action and mechanism.
2. “Be more careful.” State the changed procedure.
3. “Use more accurate apparatus.” Name it and explain why.
4. Repeating as the answer to every limitation. Repeats do not remove systematic bias.
5. An improvement unrelated to the error. Pair each one explicitly.
6. Claiming a thermometer is inaccurate because it is slow without evidence. Use the apparatus information supplied.
7. Confusing observation with conclusion. Record what was seen first.
8. Calling the furthest point an anomaly automatically. Compare it with the trend.
9. Claiming more decimal places make a reading accurate. Precision must reflect the apparatus.
10. Making an unlimited conclusion. Restrict it to the tested range.

Final answer template

Use:

The result may be [too high / too low / variable] because [specific mechanism]. Use [specific change or apparatus], which [explains how the mechanism is reduced].

Continue with Paper 6 planning investigations, tables and graphs, and the main Alternative to Practical guide.