NESA Chemistry Planning Investigations
3 sample questions with marking guides and sample answers · Avg. score: 88.7%
Using large sample sizes in an experiment increases
reliability.
precision.
validity.
uncertainty.
Reveal Answer
reliability.
Reliability refers to the consistency and repeatability of experimental results. Using a larger sample size reduces the impact of random errors and outliers, making the findings more reliable.
precision.
In experimental design, precision typically refers to the exactness or resolution of the measuring instrument. Increasing the sample size does not improve the precision of the tools being used.
validity.
Validity refers to how accurately an experiment measures what it is intended to measure. A large sample size cannot fix underlying flaws, confounding variables, or biases in the experimental design.
uncertainty.
Using a larger sample size actually decreases uncertainty. More data points reduce the margin of error, providing a more confident estimate of the true population parameter.
A student hypothesised that polishing the zinc, Zn, electrode in an Fe–Zn galvanic cell would increase the current produced by the cell.
What would be the most valid method of testing this hypothesis?
researching the scientific literature to determine how polishing changes the structure of Zn
measuring the conductivity of a Zn electrode after polishing it
measuring the change in mass per unit time of the Fe electrode in the same Fe–Zn galvanic cell before and after the Zn electrode was polished
measuring the current produced by two different Fe–Zn galvanic cells, one using a polished Zn electrode and the other using an unpolished Zn electrode
Reveal Answer
researching the scientific literature to determine how polishing changes the structure of Zn
While researching literature can provide background information, it is not an experimental method to directly test the student's specific hypothesis about the cell's current.
measuring the conductivity of a Zn electrode after polishing it
Measuring the conductivity of the zinc electrode alone does not measure the overall current produced by the electrochemical reactions in the galvanic cell.
measuring the change in mass per unit time of the Fe electrode in the same Fe–Zn galvanic cell before and after the Zn electrode was polished
The change in mass per unit time of the electrode is directly proportional to the cell's current. Using the same cell before and after polishing controls for confounding variables, making it the most valid experimental design.
measuring the current produced by two different Fe–Zn galvanic cells, one using a polished Zn electrode and the other using an unpolished Zn electrode
Although it directly measures current, using two different cells introduces confounding variables, such as slight differences in electrolyte concentration or internal resistance, which could affect the results.
A student wanted to investigate how changing temperature would influence how rapidly oxalic acid solution would decolourise an acidified potassium permanganate solution.
The student was provided with the following chemicals and equipment:
- 0.1 mol L acidified potassium permanganate solution
- 0.1 mol L oxalic acid solution
- 250 mL conical flasks
- Bunsen burner
- tripod and gauze mat
- thermometer
- stop watches
- 5.00 mL, 10.00 mL, 20.00 mL and 25.00 mL pipettes
- distilled water
- 25.0 mL measuring cylinders.
State a hypothesis for this investigation.
Reveal Answer
Answer could include:
Increasing the temperature will decrease the time taken for the acidified potassium permanganate to decolourise (change from purple to pale pink/colourless as the rate of reaction increases with increasing temperature).
| Descriptor | Marks |
|---|---|
Writes a hypothesis that gives the relationship between the independent and dependent variables. | 2 |
Writes a hypothesis that includes the independent and dependent variables without giving their relationship. | 1 |
None of the above | 0 |
Identify the independent and dependent variables.
Reveal Answer
The independent variable is the temperature of the solution.
The dependent variable is the time taken for the potassium permanganate solution/mixture to decolourise.
| Descriptor | Marks |
|---|---|
Identifies the independent variable as temperature (of solution) | 1 |
Identifies the dependent variable as time taken (for potassium permanganate solution/mixture) to decolourise | 1 |
Identify two control variables.
Reveal Answer
Answers could include (any 2 of):
- concentration of acidified potassium permanganate solution
- concentration of oxalic acid solution
- volume of acidified potassium permanganate solution
- volume of oxalic acid solution
- stopwatch/timer
- person timing/observing.
| Descriptor | Marks |
|---|---|
1 mark for each correct point (any 2 of): concentration of acidified potassium permanganate solution, concentration of oxalic acid solution, volume of acidified potassium permanganate solution, volume of oxalic acid solution, stopwatch/timer, person timing/observing. | 2 |
Describe a procedure for this investigation.
Reveal Answer
A possible answer:
Fixed volumes of oxalic acid and acidified potassium permanganate are used. Temperature must be varied and measured, and time must be measured from mixing. Repeated trials should be conducted.
Additionally, an appropriate method for determining the end point of the reaction (decolourisation) is used, e.g. a white paper base.
| Descriptor | Marks |
|---|---|
Recognises that fixed volumes of both oxalic acid and acidified potassium permanganate are used (1 mark for recognising fixed volume of only one) | 2 |
Recognises that temperature must be varied and measured | 1 |
Recognises that time must be measured from mixing | 1 |
1 mark for each correct point (any 2 of): recognition of appropriate method for determining end point of the reaction (decolourisation) e.g. use a white paper base, recognition of the use of trials, recognition of the use of appropriate glassware, recognition that solutions are mixed in appropriate proportions e.g. 2:5 ratio of solutions | 2 |
Outline the difference between systematic and random errors. Use an example of each from this investigation to support your answer.
Reveal Answer
An answer could be:
Systematic errors produce consistently high or consistently low measurements compared to the true value. Random errors produce measurements that can fluctuate around the true value.
An example of a systematic error is only heating one solution. An example of a random error is not using the same measuring equipment during the reaction.
| Descriptor | Marks |
|---|---|
Recognises that systematic errors produce consistently high or consistently low measurements compared to the true value | 1 |
Recognises that random errors produce measurements that can be either high or low/fluctuate around the true value | 1 |
Provides an example of a systematic error (e.g. only heating one solution, using an inappropriate proportion of reactants, errors in calibration with equipment, inappropriate rinsing of glassware) | 1 |
Provides an example of a random error (e.g. parallax (reading of meniscus), judging the end point, use of stopwatch, reading thermometer, not using the same measuring equipment during the reaction, using measuring cylinder rather than pipette) | 1 |