IB Biology HLPractical SkillsPaper 1B & IA~14 min read
Measuring Variables
Every practical comes down to the same question: is this number trustworthy? That depends on picking the right instrument for the job, knowing how finely it can read, and being honest about how far out it might be.
📚 What you need to know
Accurate means close to the true value. Precise means readings are close to each other. They are independent.
Mass: digital balance, usually to two decimal places, tared before use. Standard unit kilograms, though grams are used more often.
Volume of liquid: measuring cylinder for approximate volumes, volumetric pipette for an accurate fixed volume, burette for an accurate variable volume.
1 cm3 = 1 ml, and 1 dm3 = 1 litre = 1000 cm3.
Volume of gas: gas syringe, or an inverted measuring cylinder if the gas is not water-soluble.
Time: stopwatch, limited by human reaction time, which matters most for short intervals.
Temperature: thermometer (0.5–1 °C) or digital probe (often 0.1 °C).
Counts: haemocytometer or plate counts for cells; population sampling for organisms in the field.
Accurate is not the same as precise
These two words get used interchangeably in conversation, and separating them is one of the quickest ways to sound like you know what you are doing in an evaluation.
The distinction
Accuracy = how close a reading is to the true value • Precision = how close repeated readings are to each other
The second target is the dangerous one. Consistent results feel reliable, so a systematic error — an unzeroed balance, a mis-calibrated probe — can hide behind them for a whole investigation.
Measuring mass
Mass is measured with a digital balance, which normally reads to two decimal places.
Balances must be tared (set to zero) before use, so the reading shows the mass of the sample and not the container.
The standard unit is the kilogram (kg), but grams (g) are used far more often in biology. 1 kg = 1000 g.
Taring is worth understanding rather than just doing. If you forget, every single reading is wrong by the same amount — the mass of the weighing boat. That is a systematic error, and repeating the measurement will not reveal it.
Measuring the volume of liquids
Common units are cm3, dm3, ml and l. A millilitre is the same as a cm3, so labels using either can be read the same way.
This is why a burette is more accurate than a measuring cylinder of the same capacity. Its bore is narrow, so the same volume occupies a much longer length of tube and the scale can be split more finely.
The three you need to distinguish
Measuring (graduated) cylinders are used for approximate volumes where high accuracy is not important. They come in sizes from about 10 cm3 up to 1 dm3.
Volumetric pipettes are the most accurate way of measuring a fixed volume, usually 10 cm3 or 25 cm3. They have a scratch mark on the neck, which you line up with the bottom of the meniscus.
Burettes are the most accurate way of measuring a variable volume, typically between 0 and 50 cm3. The awkward part is that the scale runs from top to bottom — 0.00 cm3 is at the top of the column.
Water curves upwards at the edges because it is attracted to glass more strongly than to itself. The convention is always to read the lowest point of that curve, at eye level.
Measuring the volume of gases
A gas syringe is the usual apparatus: gas produced by a reaction pushes the plunger out along a graduated barrel.
A graduated measuring cylinder or burette inverted in water also works, but only if the gas is not water-soluble.
If the gas is heavier than air and coloured, an upright cylinder can be used.
In biology the classic case is oxygen from a catalase or photosynthesis practical. Oxygen has low solubility in water, so collecting it over water is fine. Carbon dioxide is far more soluble, so a gas syringe is the safer choice.
Measuring time
A stopwatch or stop-clock is usually accurate to one or two decimal places.
Units are normally seconds or minutes (1 minute = 60 s), with longer units for very slow processes.
The limiting factor is human reaction time. It has a significant effect when the intervals being measured are short — under a second or so.
This is a useful evaluation point. If a reaction finishes in 3 s, a reaction time of about 0.2 s is roughly 7 % of the measurement. If it takes 300 s, the same 0.2 s is negligible. The fix is to slow the reaction down so the time being measured is longer.
Measuring temperature
Laboratory thermometers usually have a precision of half a degree or one degree.
Digital temperature probes are more precise, often reading to 0.1 °C.
Traditional thermometers rely on the uniform expansion and contraction of a liquid; digital probes can be just as accurate, if not more so.
Units are degrees Celsius (°C).
Measuring length
Rulers measure to the nearest millimetre; the standard unit of length is the metre (m).
Larger distances — a transect on a rocky shore, for instance — need a tape measure.
Many biological structures are far too small for either, which is why microscopy uses micrometres and nanometres.
Watch your units: 1 cm = 10 mm and 100 cm = 1 m.
Variable
Instrument
Typical precision
Main source of error
Mass
Digital balance
0.01 g
Forgetting to tare, draughts
Volume of liquid
Burette or volumetric pipette
0.05 cm3
Parallax, reading the wrong part of the meniscus
Volume of gas
Gas syringe
1 cm3
Gas escaping before the bung is in, or dissolving
Time
Stopwatch
0.01 s
Human reaction time
Temperature
Digital probe
0.1 °C
Probe not fully immersed, water bath not at equilibrium
Length
Ruler
1 mm
Parallax, a worn or bent zero end
Precision is a property of the instrument. Accuracy is a property of the reading you take with it. A digital probe reading to 0.1 °C is precise, but if you leave it dangling in the air above the water it will still be badly inaccurate.
Making counts
Counting cells
Counting the cells in a known volume of a culture lets you work out the concentration of the whole sample.
Direct counting uses a haemocytometer or counting chamber — a slide with a volumetric grid etched into it, divided into squares of known volume. You count the particles in a known volume and scale up.
Plate counts involve plating a known volume of culture onto a petri dish with a growth medium and counting the colonies that grow.
Direct methods do not need highly specialised equipment and are easy to perform, but they are time consuming.
Counting organisms
A great deal of ecology involves counting organisms, and it is far harder than counting cells:
Many organisms move, or are inconspicuous and easily missed.
Accurate total counts are rare, so population sampling is used instead and scaled up.
Counts carry errors from the size of the population, from failing to detect individuals, and from direct counting mistakes such as miscounting large groups or misidentifying individuals.
Recording what you see
Biological drawings
To record observations made under the microscope, or from a photomicrograph, a labelled biological drawing is made. These are line pictures showing the specific features that were actually observed, and they follow a set of conventions: clean continuous lines, no shading or sketchy strokes, label lines that do not cross, and a stated magnification.
Qualitative observations
Not all data is numerical. Classifying organisms is a good example of a qualitative observation — data classified by attributes such as sex, colour of fur or number of limbs. The attribute being observed cannot be measured on a scale, so it counts as qualitative data.
🧠
Quantitative or qualitative?
Quantitative has “quantity” in it — it has a number and a unit. Qualitative has “quality” — it describes what something is like. Counting 14 limpets is quantitative; recording that they are on the shaded side of the rock is qualitative.
Worked examples
WE 1
Justifying a choice of apparatus
A student needs to transfer exactly 25 cm3 of enzyme solution into each of six test tubes. They could use a 50 cm3 measuring cylinder (±0.5 cm3) or a 25 cm3 volumetric pipette (±0.06 cm3). Calculate the percentage uncertainty of each and recommend one. (4 marks)
Step 1: measuring cylinder(0.5 ÷ 25) × 100 = 2.0 %Step 2: volumetric pipette(0.06 ÷ 25) × 100 = 0.24 %Step 3: recommend, with a reason
The volumetric pipette, because its percentage uncertainty is roughly eight times smaller, and it is designed to deliver one fixed volume accurately — which is exactly what is needed here.
Step 4: the extra point
Using the same pipette for all six tubes also makes the volumes consistent, which matters because volume is a controlled variable.
0.24 % versus 2.0 % — use the pipettea burette would be the wrong answer here: it is for variable volumes, and this volume never changes.
WE 2
Accuracy and precision from data
A solution is known to contain 25.0 g dm−3 of glucose. Student A measures 24.9, 25.1 and 25.0 g dm−3. Student B measures 23.0, 23.1 and 23.0 g dm−3. Comment on the accuracy and precision of each set, and suggest a cause of any problem. (4 marks)
Student A
Readings are close to each other (range 0.2) and close to the true value, so they are both precise and accurate.
Student B
Readings are equally close to each other (range 0.1), so they are precise, but they are consistently about 2.0 g dm−3below the true value, so they are not accurate.
Diagnosing student B
A consistent offset in the same direction indicates a systematic error — for example a balance that was not tared, or a colorimeter that was not zeroed with a blank.
Why repeats will not help
Taking more readings would reduce random error, but would not reveal or fix a systematic one.
Same spread, very different quality of datathe giveaway for systematic error is that every reading is out by roughly the same amount, in the same direction.
WE 3
Scaling up a cell count
A student counts 96 yeast cells in a haemocytometer grid with a volume of 0.10 mm3. Calculate the number of cells per cm3 of the culture. (3 marks)
Step 1: cells per mm396 ÷ 0.10 = 960 cells per mm3Step 2: convert the unit of volume1 cm3 = 1000 mm3Step 3: scale up960 × 1000 = 9.6 × 105 cells per cm3A point worth adding
If the culture had been diluted before counting, the answer must be multiplied by the dilution factor.
9.6 × 105 cells cm−3the volume conversion is where nearly all the marks are lost. 1 cm3 is 1000 mm3, not 100.
💡 Exam tips
Justify apparatus by percentage uncertainty where you can — it turns an opinion into evidence.
Match the apparatus to the job: fixed volume means pipette, variable volume means burette, rough volume means measuring cylinder.
Always read the bottom of the meniscus, at eye level.
Remember the burette scale increases downwards.
When evaluating timing, mention human reaction time and say it matters more for short intervals.
Include the unit and a sensible number of decimal places in every answer — matching the precision of the instrument.
⚠ Common mistakes
Using “accurate” when you mean “precise”. They are tested as separate ideas.
Thinking more repeats fix everything. Repeats reduce random error only; a systematic error survives them untouched.
Quoting a mass as 4 g when the balance reads 4.00 g. Dropping the decimal places throws away the instrument’s precision.
Mixing up 1 cm3 = 1000 mm3 with 1 cm = 10 mm. Cubing the length ratio cubes the conversion.
Collecting a soluble gas over water. Some of it dissolves, so the volume is an underestimate.
Calling a description a measurement. “The solution went darker” is qualitative unless you put a number on it.
Up next: Applying Lab Techniques. The last page in this skill set runs through the nine techniques the course expects you to be able to describe the purpose of and explain how to carry out — from chromatography and colorimetry to graticules, sampling and cladograms.
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