Working Scientifically — Practical Investigations
Knowing the formulas is only half the job. Examiners also ask you to evaluate experiments — identify what went wrong, explain the effect on results, and suggest improvements. This lesson gives you the vocabulary and frameworks to answer those questions precisely.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
When a student gets an unexpected result in a titration — say, the calculated concentration of NaOH comes out 15% higher than expected — what could have gone wrong, and how would you even begin to figure out whether the error happened consistently or just by chance?
Key facts
- Difference between random and systematic errors
- How to describe errors specifically (not "human error")
- Equipment accuracy: burette vs cylinder vs balance
- Valid result = free from systematic error; reliable = consistent/reproducible
Concepts
- Why overshooting the endpoint increases the titre and affects concentration
- How parallax error affects burette readings
- Why the first titre (rough) is excluded from calculations
Skills
- Identify a named source of error in titration or gravimetric procedures
- Explain the direction of effect on the result (too high / too low)
- Suggest a specific improvement to reduce the error
Every measurement has uncertainty. The skill is knowing what type of error you're dealing with, because the fix is completely different.
- Varies unpredictably between measurements
- Affects precision (reproducibility)
- Can be reduced by repeating and averaging
- Example: slight variations in eye level when reading a burette
- Cannot be eliminated completely
- Always shifts results in the same direction
- Affects accuracy (closeness to true value)
- NOT reduced by repeating — affects all repeats equally
- Example: an uncalibrated balance reading 0.2 g too high every time
- Must be identified and corrected
Equipment accuracy — choosing the right tool
| Equipment | Precision | Use for | Avoid using for |
|---|---|---|---|
| Burette (50 mL) | ±0.05 mL | Titrant delivery (precise volumes) | Measuring volumes for initial solution prep |
| Pipette (25 mL) | ±0.05 mL | Transferring exact volume to flask | Measuring different volumes |
| Volumetric flask | ±0.05 mL | Making solutions to exact concentration | Storing or transferring solutions |
| Measuring cylinder | ±0.5–1 mL | Approximate volumes (non-titration) | Titration setup or exact concentrations |
| Analytical balance | ±0.0001 g | Precise mass of primary standard | Rough weighing tasks |
| Top-pan balance | ±0.01 g | General mass measurements | Weighing primary standards (<1 g) |
Validity and reliability
A valid result accurately measures what it is designed to measure — the method is correct, equipment is appropriate, and systematic errors have been controlled. A reliable result can be reproduced — multiple repeats give consistent results, meaning random errors are small.
Random error varies unpredictably → affects precision/reliability → reduced by repeating and averaging. Systematic error acts in the same direction every trial → affects accuracy/validity → repeating does NOT fix it. Never write "human error" — name the specific source (parallax, calibration drift, contamination). HSC 4-step answer structure: name → direction of effect on titre/mass → effect on calculated c → how to fix.
Pause — copy the highlighted definitions into your book before moving on.
Did you get this? True or false: repeating a titration and averaging the results will reduce a systematic error.
Quick check: Which of the following would score full marks in an HSC error question?
Classify errors, predict effects and suggest improvements.
We just saw the general definitions of random vs systematic error. That raises a question: what do these errors actually look like in the specific procedures you will be examined on? This card answers it → titration and gravimetric errors, their direction of effect, and how to fix each.
Titration scenarios
Gravimetric scenarios
Key error scenarios: air bubble expelled from burette → titre too large → c too high (systematic). Overshoot endpoint → titre too large → c too high (systematic). Spill analyte from flask → titre too small → c too low. Parallax when reading burette → random variation → reduce with repeats. Incomplete drying of precipitate → mass too high → c too high; fix by drying to constant mass.
Pause — copy the highlighted error scenarios into your book before moving on.
Quick check: An air bubble in the burette tip is expelled during the titration. What is the effect on the calculated c(analyte)?
Fill the blanks: drag each token into the matching blank.
A ___ error shifts every trial in the same direction and reduces ___. A ___ error varies unpredictably and reduces ___.
Worked examples · reveal as you go
A student forgets to check for air bubbles in the burette before starting. During the titration, the air bubble is pushed out. The titre reads 22.6 mL. Identify the error type, the effect on the titre, and the effect on c(analyte).
When filtering the gravimetric precipitate, a small amount falls off the filter paper. Identify the error type and its effect on the calculated c.
Different students read the burette at different angles on different trials. Identify the error type and explain how it affects c(analyte).
Odd one out — three of these are sources of random error. Which one is a systematic error (and so doesn't belong)?
A student performs a titration with three concordant titres of 18.42, 18.45 and 18.48 mL. They want to improve accuracy. Predict: will adding more repeat titrations make the result more accurate, more precise, or both?
How close was your prediction?
Good — precision vs accuracy is a top-of-bank-3 distinction.
Remember: repeats reduce random error (precision); systematic error needs calibration to fix.
Common errors · the 3 traps that cost marks
Writing "human error" in your answer
"Human error" is a meaningless phrase to HSC markers — it does not name a specific source. The marker cannot award marks for it because it gives no information about what actually went wrong or how to fix it.
Fix: Name the specific source (parallax, calibration, air bubble, incomplete drying) and follow the 4-step structure: name → direction → effect on c → fix.
Saying "repeats fix systematic error"
Repeats reduce random error by averaging. They do nothing for systematic error — every trial is shifted by the same amount, so the average is shifted too. Only finding and removing the source of a systematic error restores validity.
Fix: Match the fix to the error type. Random → repeats + averaging. Systematic → identify and eliminate the cause.
Confusing validity with reliability
Precise (tightly clustered) results can still be inaccurate if a systematic error shifts every trial in the same direction. Three concordant titres are reliable but not necessarily valid.
Fix: Validity = free from systematic error (accuracy). Reliability = consistent on repeats (precision). They are independent properties.
Quick-fire practice · 5 reps +2 XP per reveal
A balance is uncalibrated and reads 0.20 g higher than the true mass every time it is used. Classify the error and state its effect on a calculated concentration based on m(primary standard).
The burette is not rinsed with titrant before filling — residual water dilutes the standard. Classify the error and predict its effect on the titre and on c(analyte).
The conical flask is rinsed with distilled water before adding the analyte. Does this introduce an error?
A student weighs a precipitate before it is fully dry. The true dry mass should be 2.30 g but they record 2.45 g. Classify the error and explain its effect on the calculated [Ba²⁺].
Three students report concordant titres of 22.3, 22.4, 22.3 mL. A fourth student then discovers the standard solution had been diluted with distilled water for storage. Are the original three results valid? Reliable?
At the start of this lesson, you thought about why a titration result might come out 15% higher than expected, and whether you could tell if the error was random or systematic.
The key insight is that a consistent overestimate — such as overshooting the endpoint every time or having an air bubble expelled from the burette — is a systematic error that shifts all results in the same direction. Repeating the experiment will not fix it, because the same flaw affects every trial equally. Only identifying and removing the source of error restores validity. Random errors, by contrast, vary unpredictably and can be partially reduced by repeating and averaging.
Pick your answer, then rate your confidence — that tells the system what to drill next.
Q1. A student titrates 25.0 mL of NaOH with 0.100 mol/L HCl. The titres recorded are 18.2, 18.0, and 17.9 mL. (a) Identify the concordant titres and calculate the average titre. (b) The student had an air bubble in the burette that was expelled during the titration. Identify the type of error, explain the effect on the titre, and explain how this affects the calculated concentration of NaOH.
Q2. A student performs a gravimetric analysis to determine [Ba²⁺] in a BaCl₂ solution. They add excess H₂SO₄, filter the BaSO₄ precipitate, and weigh it before it is fully dry. The precipitate mass is recorded as 2.45 g. The true dry mass of BaSO₄ should have been 2.30 g. (a) Identify the type of error and explain its effect on the calculated [Ba²⁺]. (b) Suggest TWO improvements to the procedure to increase both the validity and reliability of the results.
Q3. Two students perform titrations to find the concentration of an NaOH solution. Student A gets titres of 22.3, 22.4, 22.3 mL (concordant). Student B gets titres of 21.8, 22.5, 23.2 mL. (a) Compare the precision of the two students' results. (b) Student A used a burette that was not rinsed with the titrant before filling, so it contained distilled water. Explain how this affects the validity of Student A's results despite high precision.
Q4. A chemistry teacher proposes two different improvements to a titration procedure to reduce error: (i) performing the titration in a temperature-controlled room at exactly 20°C, and (ii) rinsing the burette with the titrant solution before filling. Evaluate which improvement is more likely to have a significant impact on the validity of the results, with reference to specific error types.
📖 Comprehensive answers (click to reveal)
Multiple choice — drill bank
1. B — Consistent direction = systematic. Repeating will not fix it.
2. C — Larger titre → more moles of titrant calculated → more moles of analyte via ratio → higher c.
3. A — A calibrated pipette is designed for exact volume delivery. Measuring cylinders have ±0.5–1 mL precision, unsuitable for titration.
4. D — Undried precipitate has extra mass (water). Recorded mass too high → n too high → c too high.
5. B — Validity = free from systematic error, accurately measuring the intended quantity. Option A describes reliability (reproducibility).
6. C — Systematic errors shift all results in the same direction by the same amount. Averaging five results that all contain the same systematic shift produces the same biased mean. Only removing the error source (replacing the contaminated indicator) restores validity.
7. B — This procedure addresses all sources of systematic and random error: analytical balance maximises mass accuracy; volumetric flask gives accurate volume; rough titre prevents overshoot in concordant titrations; averaging concordant titres reduces random error.
Short answer model answers
Q1 (5 marks):
(a) 18.0 and 17.9 mL are concordant (differ by 0.1 mL). Average = (18.0 + 17.9) ÷ 2 = 17.95 mL. (18.2 is excluded.)
(b) Systematic error. The air bubble was expelled during the titration, meaning part of the recorded volume was air, not solution. The titre is recorded as larger than the true volume of HCl dispensed. This leads to n(HCl) being calculated as too large. Via the 1:1 ratio, n(NaOH) appears too large. Therefore c(NaOH) = n ÷ V is an overestimate.
Q2 (4 marks):
(a) Systematic error. The recorded mass (2.45 g) is larger than the true dry mass (2.30 g) because retained water adds to the mass. This gives n(BaSO₄) too high, leading to an overestimate of [Ba²⁺].
(b) Any two: (i) Dry the precipitate in an oven at 150°C and weigh repeatedly until constant mass is achieved — ensures all moisture is removed (improves validity). (ii) Repeat the experiment multiple times and average results — improves reliability by reducing effect of random variation. (iii) Use an analytical balance (±0.0001 g) — reduces measurement uncertainty. (iv) Ensure all precipitate is transferred to the filter without loss — reduces underestimation from sample loss.
Q3 (4 marks):
(a) Student A's results (22.3, 22.4, 22.3 mL) are precise — they are tightly clustered within 0.1 mL. Student B's results (21.8, 22.5, 23.2 mL) are imprecise — they vary by 1.4 mL, indicating poor reproducibility and significant random error.
(b) If the burette was not rinsed with the titrant before filling, residual water dilutes the HCl. The concentration of HCl in the burette is lower than the stated value. A larger volume of HCl is needed to reach the endpoint, making the titre too large. This is a systematic error — it affects all three of Student A's results equally. Despite high precision (tight cluster), the result lacks validity because it does not accurately measure the intended quantity. Precision and validity are independent properties.
Q4 (4 marks):
Improvement (ii) — rinsing the burette with titrant — is more likely to have a significant impact on validity. If the burette is not rinsed, residual water dilutes the titrant solution, causing a systematic error that increases the titre and overestimates the analyte concentration. This is a direct, consistent source of systematic error that undermines validity for every trial.
Improvement (i) — temperature control — addresses a minor source of variation. While temperature affects solution volume slightly, at room temperatures near 20°C the effect on volume (and thus concentration) is typically less than 0.1%, which is far smaller than the error caused by an undiluted titrant. Temperature variation is better described as a minor source of random rather than systematic error in this context.
Therefore, rinsing the burette (improvement ii) has greater impact on validity.
Five timed questions on working scientifically & experimental error. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaClimb platforms, hit checkpoints, and answer error-analysis questions. Quick recall, lighter than the boss.
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