Chemistry • Year 11 • Module 2 • Lesson 18
Working Scientifically — Practical Investigations
Lock in the vocabulary of error analysis, equipment precision, and validity/reliability before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: random error, systematic error, validity, reliability, accuracy, precision, parallax error, titre, concordant titres, uncertainty. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | An error that varies unpredictably between measurements, affecting reproducibility. | |
| 1.2 | An error that always shifts results in the same direction, affecting accuracy; cannot be reduced by repeating. | |
| 1.3 | The degree to which a result is free from systematic error and correctly measures the intended quantity. | |
| 1.4 | The degree to which repeated measurements under the same conditions give consistent results. | |
| 1.5 | How close a measured value is to the true or accepted value. | |
| 1.6 | How closely repeated measurements agree with each other; a measure of reproducibility. | |
| 1.7 | An error caused by reading a measurement scale from an angle rather than directly at eye level with the meniscus. | |
| 1.8 | The volume of titrant delivered from the burette during a titration, recorded as the difference between initial and final readings. | |
| 1.9 | Titre readings that differ by no more than 0.1 mL; used to calculate a reliable average titre. | |
| 1.10 | The range of values within which the true measurement is expected to lie; expressed as ± half the smallest scale division. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Repeating a titration multiple times and averaging the concordant titres will fix a systematic error caused by an air bubble in the burette. T / F
2.2 A burette has a precision of ±0.05 mL, making it more suitable than a measuring cylinder (±0.5 mL) for delivering titrant in volumetric analysis. T / F
2.3 A student who writes “human error” when asked to identify a source of experimental error will receive full marks on an HSC paper. T / F
2.4 Overshooting the endpoint in a titration is a systematic error that causes the calculated concentration of the analyte to be an overestimate. T / F
2.5 A result that is highly precise is always also highly accurate. T / F
2.6 In gravimetric analysis, if a precipitate is not fully dried before weighing, the recorded mass is too high, leading to an overestimate of the analyte concentration. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
accuracy · averaging · concentration · endpoint · eye level · precision · reliability · systematic
In a titration, parallax error occurs when the burette is not read at ___________ with the meniscus. This type of error is random because it can vary between readings, reducing the ___________ of results. In contrast, an air bubble trapped in the burette tip is a ___________ error that always makes the titre appear larger than the true value. Overshooting the ___________ produces a consistently larger titre, which leads to an overestimate of the calculated ___________ of the analyte. Systematic errors reduce ___________ and cannot be fixed by repeating the experiment. Random errors, however, can be reduced by repeating trials and ___________ concordant results, which improves the ___________ of the experiment.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the purpose of performing a “rough titre” as the first titration before collecting data?
4.2 Why is an analytical balance (±0.0001 g) preferred over a top-pan balance (±0.01 g) when weighing a primary standard such as Na2CO3?
4.3 What is the effect on the calculated analyte concentration if analyte solution spills from the conical flask during a titration?
4.4 State the four-step structure an HSC student should use when answering a question asking them to identify and evaluate a source of error.
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “reduces”, “affects”, “is fixed by”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: systematic error · random error · validity · reliability · repeating · titre.
Q1 — Term–definition match
1.1 random error • 1.2 systematic error • 1.3 validity • 1.4 reliability • 1.5 accuracy • 1.6 precision • 1.7 parallax error • 1.8 titre • 1.9 concordant titres • 1.10 uncertainty.
Q2 — True / false with correction
2.1 False. Repeating and averaging reduces random errors but does NOT fix a systematic error. An air bubble in the burette affects every titre equally in the same direction; no amount of repetition removes this systematic bias. Only flushing the burette before the experiment eliminates the error.
2.2 True.
2.3 False. “Human error” receives zero marks in HSC answers. Students must name the specific source (e.g. “parallax error when reading the burette meniscus”), state the direction of effect, and explain the consequence for the calculated concentration.
2.4 True.
2.5 False. Precision (reproducibility) and accuracy (closeness to the true value) are independent properties. A systematic error — such as a mis-calibrated balance reading 0.2 g too high every time — produces precise results that are consistently inaccurate. High precision does not guarantee high accuracy.
2.6 True.
Q3 — Cloze paragraph
In order: eye level / precision / systematic / endpoint / concentration / accuracy / averaging / reliability.
Q4.1 — Purpose of rough titre
The rough titre estimates the approximate endpoint volume so that in subsequent (concordant) titrations the student can add titrant quickly at first, then dropwise near the endpoint, preventing overshooting. The rough titre is excluded from calculations because it may have been over-titrated.
Q4.2 — Analytical vs top-pan balance
An analytical balance (±0.0001 g) has 100× greater precision than a top-pan balance (±0.01 g). When weighing a small mass of primary standard (e.g. ~0.5 g Na2CO3), even a 0.01 g error represents a 2% uncertainty, which propagates into a 2% error in the calculated solution concentration. The analytical balance reduces this to 0.02%, which is acceptable for volumetric analysis.
Q4.3 — Effect of spilling analyte
If analyte spills from the conical flask, fewer moles of analyte are present than intended. Less titrant is therefore needed to reach the endpoint, making the titre smaller than it should be. Via the stoichiometric ratio, fewer moles of analyte are calculated, and the calculated concentration of the analyte is an underestimate (too low).
Q4.4 — HSC four-step error structure
Step 1: Name the specific source of error (not “human error”). Step 2: State the direction of effect on the titre or mass measurement (too large / too small). Step 3: Explain the consequence for the calculated concentration (overestimate / underestimate) via the stoichiometric chain. Step 4: Suggest a specific improvement to eliminate or reduce the error.
Q5 — Sample concept map
Correct maps should include arrows such as:
- systematic error — reduces → validity
- random error — reduces → reliability
- repeating — reduces → random error
- systematic error — inflates or deflates → titre
- random error — is NOT fixed by → repeating (for systematic)
- titre — is affected by → systematic error
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).