IB Biology SLSkill Set 1 — Experimental TechniquePaper 1 & 2Practical skill~10 min read
Measuring Variables
Every practical ends with numbers, and the numbers are only as good as the thing you measured them with. This page covers the instruments you are expected to know, how to read each one properly, and the idea that ties the whole lot together: knowing how much you can trust a reading before you build a conclusion on it.
📘 What you need to know
Accurate means close to the true value. Precise means the readings sit close to each other. They are not the same thing.
Mass: digital balance, usually to two decimal places, and always tared before use.
Liquid volume: measuring cylinder for rough volumes, volumetric pipette for a fixed volume, burette for a variable volume. Read the bottom of the meniscus at eye level.
Gas volume: a gas syringe, or an inverted measuring cylinder if the gas does not dissolve in water.
Time: stopwatch, but remember human reaction time matters most when the interval is short.
Counting cells: a haemocytometer counts a known volume, so a count becomes a concentration.
Every instrument has an uncertainty. Quote it, and use it to compare instruments.
Accurate, precise, and why you need both words
These two words get swapped all the time, and once you can see the difference you will spot it in every practical you do.
Accurate readings are close to the real answer. Precise readings are close to one another. You can easily have one without the other: a balance that has not been tared gives you five readings that agree beautifully and are all wrong by the same 0.30 g.
This is why repeats alone do not prove your result is right. A fault in the instrument shifts every repeat the same way, and the readings still look neat.
Poor precision shows itself — your repeats disagree and you can see it. Poor accuracy hides, because everything looks tidy. That is why you tare the balance and check the calibration before you start, not after.
Measuring mass
Use a digital balance. Most school balances read to two decimal places, so a mass of 2.34 g.
Tare it first. Taring sets the display to zero with your empty container already on the pan, so the number you read is the substance only.
The standard unit is the kilogram (kg), but in practice you will work in grams (g). 1 kg = 1000 g.
Let the reading settle before you write it down, and keep the balance away from draughts and open windows.
Wet mass or dry mass? In biology this matters more than the balance does. Wet mass includes water, which changes with how thirsty the plant was that morning. Dry mass needs the sample heating to constant mass, which kills it — but it is the fairer comparison.
Measuring the volume of a liquid
Volumes come in cm3, dm3, ml or litres. One handy fact: 1 ml is exactly the same as 1 cm3, so markings in ml on a syringe cause no problem at all.
Which piece of glassware you pick depends on how much accuracy you actually need.
More graduations do not automatically mean more accuracy. A pipette has a single line and beats a cylinder covered in them.
Measuring cylinder — for rough volumes
Graduated, quick, and available from about 10 cm3 up to 1 dm3. Use it when the exact volume is not the point: making up a bath of water for a potometer, or rinsing something out.
Volumetric pipette — for one fixed volume, accurately
Usually 10 cm3 or 25 cm3. It has a single scratch mark on the neck, and you fill it until the bottom of the meniscus sits on that mark. It measures that one volume and nothing else, which is exactly why it is so accurate. Use a pipette filler, never your mouth.
Burette — for a volume you choose
Delivers any volume between 0 and 50 cm3, which is what you want for a titration or for adding substrate a bit at a time. The catch: the scale runs downwards, with 0.00 cm3 at the top. You subtract the first reading from the second to get the volume delivered.
Reading from the wrong angle is a systematic error — it pushes all your readings the same way, so repeats will never reveal it.
WORKED EXAMPLE
A burette reads 3.15 cm3 before a titration and 28.65 cm3 after. Each reading has an uncertainty of ±0.05 cm3. Find the volume delivered and its percentage uncertainty.
Step 1: subtract, because the scale runs downwards28.65 − 3.15 = 25.50 cm3Step 2: uncertainties add when you subtract two readings0.05 + 0.05 = ±0.10 cm3Step 3: turn it into a percentage(0.10 ÷ 25.50) × 100 = 0.39%25.50 ± 0.10 cm3, so 0.39%Two readings, two uncertainties. Forgetting to double it is the classic slip here.
WORKED EXAMPLE
You need 25.0 cm3 of buffer. A measuring cylinder is ±0.5 cm3; a volumetric pipette is ±0.06 cm3. Show which one you should use.
Work out the percentage uncertainty for eachcylinder: (0.5 ÷ 25.0) × 100 = 2.0%pipette: (0.06 ÷ 25.0) × 100 = 0.24%Compare
The pipette is roughly eight times better for the same volume.
Use the 25 cm3 volumetric pipetteThis is how you justify apparatus choice in your IA — with a number, not with “it is more accurate”.
Measuring the volume of a gas
A gas syringe is the usual choice. The gas pushes the plunger out and you read the volume straight off the barrel.
A measuring cylinder or burette inverted over water also works — but only if the gas does not dissolve in water.
If the gas is denser than air and coloured, you can collect it in an upright cylinder and watch it fill.
Check every joint. A leaky bung is the single most common reason a catalase or respiration experiment gives a low result.
Measuring time
Use a stopwatch or stop-clock, usually reading to one or two decimal places.
Seconds and minutes are standard (60 s in a minute), but slow processes may need hours or days — think germination or growth.
Human reaction time is the limit here, not the watch. Your thumb takes a couple of tenths of a second to respond.
So a 3 second reading carries a big percentage uncertainty, while a 300 second reading barely notices the same delay. Design your experiment so the events you time are long ones.
Fixing the reaction-time problem: instead of timing “how long until the reaction finishes”, time to a fixed, obvious end point — a cross disappearing, a colour matching a standard, a set volume of gas collected. Everyone in the class then stops the watch at the same moment.
Measuring temperature
A thermometer or a digital probe. Lab thermometers usually read to the nearest half or whole degree.
Digital probes are more precise, often to 0.1 °C, and they can log readings automatically over a long run.
Liquid thermometers work because the liquid expands and contracts evenly as it warms and cools.
Units are degrees Celsius (°C).
Put the bulb in the liquid, not on the bottom of the beaker, and wait for the reading to stop moving.
Measuring length
A ruler handles a few centimetres and reads to the nearest millimetre. A tape measure covers longer distances, like a transect line.
The standard unit is the metre (m). 1 cm = 10 mm and 100 cm = 1 m.
Anything cellular is far too small for a ruler — a typical cell is measured in micrometres, so you need a microscope with an eyepiece graticule instead.
Always write the unit down with the number. A length of “45” tells the examiner nothing.
Counting things
Biology asks you to count more than the other sciences do, and counting is surprisingly easy to get wrong.
Counting cells
Counting cells in a known volume turns a count into a concentration, which is what you usually want.
A haemocytometer is a slide with an etched grid and a chamber of known depth. Count the cells in the grid, and because you know the volume above it, you know the concentration of the whole sample.
A streak or spread plate is the other route: spread a known volume of culture on agar, incubate, and count the colonies that grow. Each colony started as one cell.
Both methods need no fancy equipment, but both take time.
WORKED EXAMPLE
A haemocytometer square is 1 mm by 1 mm, and the chamber is 0.1 mm deep. A student counts 45 yeast cells in that square. Find the concentration of the culture in cells cm−3.
Step 1: what volume were those cells sitting in?1 × 1 × 0.1 = 0.1 mm3Step 2: convert to cm3 (1 cm3 = 1000 mm3)0.1 ÷ 1000 = 1 × 10−4 cm3Step 3: scale the count up to 1 cm345 ÷ (1 × 10−4) = 450 0004.5 × 105 cells cm−3The whole trick is the depth. Miss the 0.1 mm and your answer is out by a factor of ten.
Counting organisms in the field
Counting every individual is almost never possible, so you take a sample and scale up.
Animals move and hide, which is why counts of them are less reliable than counts of plants.
Errors creep in from miscounting a large group, counting the same individual twice, or identifying it as the wrong species.
More samples means a mean you can trust more — that is the honest way to improve a field count.
Observations that are not numbers
Not every variable gives you a number, and that is fine as long as you record it properly.
Qualitative data describes a quality you cannot measure: fur colour, sex, number of limbs used to group organisms, presence or absence of a feature.
Biological drawings record what you actually saw down the microscope. They are line drawings — clean single lines, no shading, no sketchy strokes — with label lines that do not cross.
Draw what is on the slide, not what the textbook diagram looks like. Examiners can tell.
Always add a title, the magnification, and a scale bar if you have one.
💡 Exam tip
Match the instrument to the volume. If a question asks you to justify a choice, work out the percentage uncertainties and compare them.
Quote readings to the precision the instrument allows: 25.50 cm3 from a burette, not 25.5.
Uncertainties add when you subtract two readings, which is why burette volumes carry double.
Say “read at eye level, at the bottom of the meniscus” — it is a standard mark for method questions.
When timing, design the experiment so the interval is long enough that reaction time hardly matters.
In your IA, name the instrument and its uncertainty in the method table. It looks professional and it is easy marks.
⚠ Common mix-up
Using accurate and precise as synonyms. Precise repeats can all be wrong together.
Reading a burette upwards. The scale starts at 0.00 cm3 at the top.
Reading the top of the meniscus, or reading it from above. Both give a systematic error.
Forgetting to tare the balance, so the container mass ends up in every result.
Thinking more repeats fix accuracy. They only improve your confidence in the mean and expose poor precision.
Dropping the depth of the haemocytometer chamber from the volume calculation.
Writing a number with no unit. It scores nothing, however good the practical was.
Up next: Applying Lab Techniques — chromatography, serial dilutions, graticules, sampling and keys, and the calculations that go with each one.
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