Two decisions decide how good your data is, and neither of them happens while you are writing up. The first is which instrument you pick. The second is where you put your eye.
📚 What you need to know
Choose apparatus by how precise the measurement needs to be, not by what is nearest.
A measuring cylinder is for approximate volumes, a volumetric pipette for one fixed accurate volume, a burette for an accurate variable volume.
Read a liquid level from the bottom of the meniscus, with your eye level with it, to avoid parallax error.
A burette scale runs downwards from 0.00 cm3 at the top, and each reading carries an uncertainty of ±0.05 cm3.
A gas can be collected in a gas syringe, or over water if it is insoluble, or by downward displacement of air if it is denser than air.
Digital instruments give higher precision and avoid parallax, but must be zeroed and may flicker.
An ammeter goes in series; a voltmeter goes in parallel.
Where you put your eye
A liquid in a narrow glass tube curves. Water climbs the walls slightly, so the surface dips in the middle, and the convention is to read from the bottom of that dip — the bottom of the meniscus — lined up with the graduation.
The second half matters just as much. If your line of sight is not horizontal, the graduation and the liquid surface do not line up in the way you think they do, and you get a parallax error. It is a systematic problem: read from above every time and every reading is wrong in the same direction.
This is why parallax is worth taking seriously: it is a systematic error, so repeating the measurement more times does not help at all.
Measuring mass
A digital balance reading to two decimal places covers most work; preparing a standard solution needs three. Always tare the balance with the empty container on the pan, so that what you read is the mass of the substance alone.
Mass is recorded in grams in chemistry even though the SI unit is the kilogram, and 1 kg = 1000 g.
Measuring the volume of liquids
Three pieces of apparatus, three jobs. Picking the wrong one is one of the easiest marks to lose in a planning question.
Volumes are sometimes marked in ml rather than cm3. They are the same thing, so no conversion is needed.
The burette scale catches people out every year. It starts at 0.00 cm3 at the top and the numbers grow as you go down, because the scale is measuring what has left, not what remains. Read it top-down and the arithmetic looks after itself.
Burette readings and their uncertainty
A burette is marked every 0.10 cm3. It is an analogue instrument, so the uncertainty on a single reading is half the smallest division: ±0.05 cm3. A titre is a difference between two readings, and both carry that uncertainty, so they add.
Writing 24.5 instead of 24.50 throws away information about how precisely the measurement was made, and examiners treat it as an error.
Measuring the volume of gases
Three options, and the gas decides which one you can use.
A gas syringe is the standard choice: the plunger moves out as gas is produced and the volume is read straight off the barrel.
An inverted measuring cylinder or burette over water works only if the gas is insoluble in water. Carbon dioxide is a poor candidate; hydrogen is a good one.
Downward displacement of air into an upright cylinder suits a gas that is denser than air and coloured, so you can see when it is full.
Measuring time, temperature and length
Time comes from a stopwatch reading to one or two decimal places. The limiting factor is not the watch, it is human reaction time — which is why intervals shorter than about a second cannot be measured reliably by hand.
The stopwatch trap: a display of 1.30 means one minute and thirty seconds, which is 90 s or 1.5 min — not 1.30 min. If your intervals are under a minute, record everything in seconds and the problem disappears.
Temperature comes from a liquid-in-glass thermometer, which works by thermal expansion and typically reads to the nearest 1 °C or 0.5 °C, or from a digital probe, which reads to ±0.1 °C or better, responds faster and can log data continuously. Length comes from a ruler, good to the nearest millimetre — useless for anything atomic, since a typical atomic radius is around 1 × 10–10 m.
Measuring pH
Two routes, and they are good at different things. A digital pH meter uses an electrode with a thin glass membrane that hydrogen ions can cross; that changes the voltage, which is converted into a pH value, usually to two decimal places. An indicator changes colour over a pH range and only a few drops are needed, since indicators are intensely coloured.
Natural indicators such as litmus or red cabbage extract cover a broad range and give an approximate value.
Synthetic indicators such as phenolphthalein or methyl orange change sharply over a narrow range, which is what a titration endpoint needs.
Universal indicator is a blend that gives an approximate pH across the whole 1–14 scale, matched against a colour chart supplied by the same manufacturer.
A pH meter is more precise, so it is the better instrument for monitoring pH. But it is not automatically the better choice for a titration: it responds gradually, so the endpoint can be hard to pin down, whereas a sharp indicator changes on a single drop.
Measuring current and potential difference
An ammeter measures current and is connected in series with the component you are interested in. A voltmeter measures potential difference between two points and is connected in parallel across the component.
Both come as analogue or digital. Analogue meters have a needle and are subject to parallax error, so read them perpendicular to the scale. Digital meters avoid parallax and read much smaller values (mA or μA), but the display can flicker between values, and a judgement has to be made about which to record. Whichever you use, check for a zero error first: if the meter does not read zero when it should, subtract that offset from every result.
Quantity
Instrument
Typical precision
Watch out for
Mass
digital balance
±0.01 g, or ±0.001 g
forgetting to tare
Liquid volume
burette or pipette
±0.05 cm3 per burette reading
parallax and the meniscus
Gas volume
gas syringe
±0.5 cm3 or better
gas dissolving or escaping
Time
stopwatch
±0.01 s on the display
reaction time; minutes vs seconds
Temperature
thermometer or probe
±0.5 °C or ±0.1 °C
allowing time to equilibrate
pH
pH meter or indicator
±0.01 with a meter
calibration; the right colour chart
Current
ammeter, in series
depends on the range
zero error and parallax
Potential difference
voltmeter, in parallel
depends on the range
zero error and a flickering display
WORKED EXAMPLE
A student needs to transfer exactly 25.0 cm3 of a solution into a conical flask for a titration. State which piece of apparatus should be used and justify the choice.
What the task needsOne fixed volume, delivered as accurately as possible, and the same volume every time the titration is repeated.a 25 cm³ volumetric pipetteWhy not the othersA measuring cylinder is only good to about ±0.5 cm³, so it would dominate the uncertainty. A burette would work but is designed for variable volumes and is needed for the titrant.Add the technique: fill to the calibration mark with the bottom of the meniscus on the line, at eye level.
WORKED EXAMPLE
A burette reads 0.15 cm3 at the start and 24.65 cm3 at the end. Calculate the titre, state its absolute uncertainty and calculate the percentage uncertainty.
Step 1 — the titre24.65 − 0.15 = 24.50 cm³Step 2 — the absolute uncertaintyEach reading is ±0.05 cm³, and two readings were taken, so the absolute uncertainties add.0.05 + 0.05 = ±0.10 cm³Step 3 — the percentage uncertainty(0.10 ÷ 24.50) × 100 = 0.41%24.50 ± 0.10 cm³, or ±0.41%Keep the trailing zero on 24.50. It is evidence of the precision you worked to.
WORKED EXAMPLE
A student’s table is headed “Time / min” and they write 1.30 for a stopwatch showing one minute and thirty seconds. Explain the error and give the correct entries in minutes and in seconds.
The errorThe stopwatch shows minutes and seconds, but the column heading asks for a decimal number of minutes. Thirty seconds is half a minute, not 0.30 of one.In minutes30 ÷ 60 = 0.5, so 1.5 minIn seconds(1 × 60) + 30 = 90 s1.5 min, or 90 sFor short intervals, head the column in seconds from the start. The conversion is then never needed.
💡 Exam tip
Justify apparatus choices by the precision the task needs, and say what the alternative would cost you.
Quote burette readings to two decimal places, always ending in 0 or 5.
Remember a titre needs two readings, so its uncertainty is ±0.10 cm3.
Use the phrases bottom of the meniscus and at eye level — they are the marking points.
For gas collection, state a property of the gas (soluble? denser than air?) that justifies the method.
Mention zero error and parallax whenever an analogue meter appears in a question.
⚠️ Common mix-up
Reading a burette from the bottom up. The scale runs downwards, so the second reading is the larger number.
Giving a titre an uncertainty of ±0.05 cm3. Two readings were taken, so it is ±0.10.
Recording 1.30 min for one minute thirty. That is 1.5 min or 90 s.
Collecting a soluble gas over water and then wondering why the volume is too low.
Assuming digital always beats analogue. A pH meter is more precise but can give a less obvious titration endpoint than a sharp indicator.
Up next: Applying Practical Techniques — the instruments are chosen and you know how to read them. Now for the procedures they sit inside: making a standard solution, running a titration, separating and purifying what you make.
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