AQA Chemistry 5.3.1.4 · Combined and Triple
Chemical measurements
Every measurement carries some uncertainty. Choose the right apparatus, read it properly, repeat, and report a mean with its uncertainty: half the range.
Combined Science · Foundation tier
Start here
Two kitchen jugs.
One jug has a mark every 100 mL. The other has a mark every 10 mL. You need 30 mL of milk.
If you had to guess, which jug gets you closer to 30 mL?
Apparatus and reading it
Three ways to measure a volume.
Volumes are measured in cm3 or dm3 (1 dm3 = 1000 cm3; 1 cm3 = 1 mL). Read the bottom of the meniscus, at eye level. A measuring cylinder’s scale runs upwards; a burette’s runs downwards, and a titre is final reading minus initial reading.
You need exactly 25.0 cm3 of a solution. Which do you reach for?
Pick one.
Four words, used exactly
Accurate, precise, repeatable, reproducible.
AQA’s definitions, word for word. Tap each.
Measurements are affected by random error, where results vary in unpredictable ways; more measurements and a mean reduce it. Systematic error makes every result differ from the true value by a consistent amount, such as a balance that was not zeroed. A student’s three readings cluster closely but are all 2 g too high. Which word fits?
Pick one.
Equations · mean, range, uncertainty
Use the range of a set of measurements about the mean as a measure of uncertainty. Three lines, one after the other; none is a triangle. Leave an anomalous result out of the mean: it does not belong to the cluster, so it would drag the mean away from the true value.
Step 1 · mean = sum of readings ÷ number of readings
Step 2 · range = largest − smallest
Step 3 · uncertainty = ± range ÷ 2, written as mean ± uncertainty
Extension · not on the GCSE specification
Percentage uncertainty.
Some courses compare an uncertainty with the size of the reading. It explains why a large cylinder is a poor choice for a small volume, but it is not examined at GCSE. Tap the quantity you want to find; the × 100 is its own line.
Equation
Percentage uncertainty (extension)
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
fraction = uncertainty ÷ measured value, then × 100
- u
- uncertainty
- same unit as the reading
- f
- fraction (× 100 for %)
- no unit
- V
- measured value
- same unit
% uncertainty = fraction × 100
Worked: 25.0 cm3 measured with a cylinder of uncertainty ±0.5 cm3. % uncertainty = (0.5 ÷ 25.0) × 100 = 0.02 × 100 = 2.0%.
Think again · the whole range?
“Readings 12.4, 12.8 and 12.6: the uncertainty is 0.4.”
Your call
ChooseThree readings: 12.4, 12.8, 12.6 cm³. How should the result be written?
Sort
Which word?
Place each description.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Calculate
- A mean, leaving out anomalies; the uncertainty as half the range.
- Explain
- Why repeats reduce random error; why a result is anomalous.
- Suggest
- Suitable apparatus for a measurement.
Key fact
Whenever a measurement is made there is always some uncertainty about the result. Repeat, leave out anomalies, take the mean, and report mean ± half the range.
Key note · AQA 5.3.1.4 · Combined and Triple
Chemical measurements
- Whenever a measurement is made there is always some uncertainty about the result obtained. Use the range of a set of measurements about the mean as a measure of uncertainty: mean ± (range ÷ 2). Leave anomalies out.
- Accurate: close to the true value. Precise: cluster closely. Repeatable: same investigator, same conditions, similar results. Reproducible: different investigators, different equipment, similar results.
- Random error: results vary unpredictably; reduce it with more measurements and a mean. Systematic error: every result differs from the true value by a consistent amount; repeats do not remove it.
- Volumes: pipette for one fixed volume (smallest uncertainty), burette for a variable volume (scale runs down; titre = final − initial), measuring cylinder (scale runs up; larger graduations, larger uncertainty). Read the bottom of the meniscus at eye level. 1 dm³ = 1000 cm³.
- Worked: titres 24.10, 24.30, 24.20 cm³ give 24.20 ± 0.10 cm³. Percentage uncertainty is an extension only, not on the specification.
Photograph this card. Tomorrow, cover it and say each line first.
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Using the whole range as the uncertainty?
The readings in this lesson are made up for practice.