Chemical measurements — AQA GCSE Chemistry
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Chemical measurements explained
Chemical measurements covers 2 specification points, including Whenever a measurement is made there is always… and Represent the distribution of results and make….
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Open a point below for its explanation and revision guidance.
Whenever a measurement is made there is always some uncertainty about the result obtained.
Every measurement has uncertainty because instruments have limited resolution, conditions vary and the person reading the scale may make small errors. Uncertainty is the range of values within which the true value is likely to lie. For example, a balance reading to the nearest 0.01 g has an uncertainty of about ±0.01 g, while a thermometer read to the nearest 1 °C has an uncertainty of about ±0.5 °C if you estimate between marks. Uncertainty is not the same as a mistake: it is unavoidable and can be reduced but not removed. Repeating measurements and using finer-scale instruments can reduce uncertainty, and the mean of repeated readings is usually more reliable.
Students should be able to: represent the distribution of results and make estimations of uncertainty use the range of a set of measurements about the mean as a measure of uncertainty.
When you repeat a measurement, the results form a distribution. Plot them, for example as a dot plot or a small bar chart, to show how they cluster around the mean. The mean is the sum of the measurements divided by the number of measurements. The range is the difference between the highest and lowest values. A simple estimate of uncertainty is half the range, often written as ± half the range about the mean. For example, if four titres are 24.10 cm³, 24.20 cm³, 24.15 cm³ and 24.30 cm³, the mean is 24.19 cm³ and the range is 0.20 cm³, so the uncertainty is about ±0.10 cm³. A smaller range usually indicates more precise measurements.
Your focus
- Define uncertainty as the range within which a true value is likely to lie.
- Identify sources of uncertainty in a chemical measurement.
- Describe how repeating measurements or using finer instruments can reduce uncertainty.
Show all 6 objectives
- Calculate the mean and range of a set of repeated measurements.
- Estimate uncertainty as half the range about the mean.
- Represent a distribution of results using a table, dot plot or bar chart.
Chemical measurements exam tips
Marking Points
- State that all measurements have some uncertainty.
- Identify a source of uncertainty, such as instrument resolution or reading the scale.
- Distinguish uncertainty from a mistake or a gross error.
- Explain that repeating measurements or using a finer scale can reduce uncertainty.
- Use the resolution of an instrument to estimate the uncertainty in a single reading.
- Calculate the mean of a set of repeated measurements.
- Find the range by subtracting the lowest value from the highest value.
- Estimate uncertainty as half the range about the mean.
- Represent the distribution of results using a suitable table, dot plot or bar chart.
- Compare the spread of results to comment on the precision of the measurements.
Examiner Tips
- 💡Quote the instrument resolution when asked to estimate uncertainty.
- 💡Use the phrase true value when defining uncertainty.
- 💡Give a specific example, such as a balance reading to the nearest 0.01 g, rather than a vague statement.
- 💡Show the mean calculation clearly, including the sum and the number of measurements.
- 💡Write the uncertainty as a value with the same unit as the measurements, for example ±0.10 cm³.
- 💡Use a dot plot or bar chart to make the spread of results visible.
Common Mistakes
- Treating uncertainty as a mistake; correct this by explaining that uncertainty is unavoidable and affects every measurement.
- Believing that a more precise instrument removes uncertainty completely; correct this by stating that it only reduces uncertainty.
- Confusing uncertainty with accuracy; correct this by explaining that uncertainty describes the spread of possible values, while accuracy describes closeness to the true value.
- Dividing the range by the number of measurements instead of by two; correct this by using half the range as the uncertainty estimate.
- Forgetting to include the correct unit with the mean, range or uncertainty; correct this by carrying the unit through every step.
- Plotting the mean as a single value without showing the spread; correct this by representing all individual results in the distribution.