IB Chemistry SLTopic 8 — Exploring and DesigningInternal assessmentPractical skill~12 min read
Controlling Variables
Listing your controlled variables is the easy half. The marks are in saying exactly how each one is held constant, why it would have mattered if it were not — and what you do about the ones you cannot control at all.
📚 What you need to know
Controlling variables minimises systematic error — error that pushes every result the same way.
It is what makes the investigation valid: any change in the DV can be attributed to the IV.
Calibration means checking an instrument against a known standard and adjusting it if needed.
A pH meter is calibrated against at least two buffers; a thermometer against ice and boiling water; a balance is tared before every use.
Control the environment too: a water bath for temperature, a draught shield against air currents.
If a variable cannot be controlled, monitor and record it, then discuss its effect in the evaluation.
In calorimetry the dominant error is heat exchange with the surroundings, so insulation is the priority.
Calibration: checking the instrument itself
An uncalibrated instrument produces a systematic error, and systematic errors have a nasty property: repeating the measurement does not reveal them. Every reading is wrong by the same amount and the results look beautifully consistent.
Two points are used for a thermometer because a single check cannot detect a scale that is stretched as well as shifted. The same logic applies to the two buffers for a pH meter.
Controlling the room, not just the flask
Some of the most damaging variables are not in the apparatus at all. Temperature drifts, draughts from a window or an air conditioner cool a reaction vessel, and both effects are invisible unless you plan for them.
Temperature. A thermostatically controlled water bath is the proper answer. Place the reactant solutions in it for a set time — five to ten minutes — before mixing, so they all start from the same point.
Air currents. Close windows and put a draught shield around the apparatus. This matters most in calorimetry, where a draught removes heat you are trying to measure.
The human element. Use the same piece of equipment for all measurements of one type, and have the same person take any judgement-based reading, such as an endpoint or a disappearing cross.
Insulating a calorimeter
In any calorimetry experiment the largest single source of error is unwanted heat exchange with the surroundings. Each insulation technique blocks one route.
Even with all of this, some heat still escapes. That is why temperatures are recorded for a couple of minutes afterwards and the cooling line is extrapolated back to the moment of mixing.
When you cannot control it
Some variables will not stay still. Room temperature drifts through a double lesson; atmospheric pressure is whatever it is. The correct response is not to pretend otherwise, and it is not to give up.
This is why the monitoring route is worth taking seriously. Recorded values give your evaluation something concrete to work with; an unrecorded drift gives it nothing but speculation.
Variable
How it is controlled
Why it matters
Temperature of reactants
water bath at a set temperature for 10 minutes before mixing
rate depends strongly on temperature, so a drift would mimic the effect of the IV
Surface area of a solid
identical lengths from the same roll, lightly polished to remove the oxide layer
more exposed surface gives more collision sites and a faster reaction
Volume of solution
the same volumetric pipette for every trial
volume changes the total number of particles present, not just the concentration
Heat loss
lidded polystyrene cup inside a beaker, with a draught shield
escaping heat makes every enthalpy value less exothermic than it should be
Instrument accuracy
calibrate before use; tare the balance every time
an offset shifts every reading in the same direction
Prioritise. You cannot write a paragraph on every controlled variable, and you should not try. Spend the space on the one or two that would do the most damage — heat loss in a calorimetry investigation, temperature in a kinetics one — and deal with the rest briefly. Knowing which matters most is itself a sign of insight.
WORKED EXAMPLE
For the investigation “What is the effect of hydrochloric acid concentration on the rate of reaction with magnesium ribbon?”, state a method of control and a justification for two controlled variables.
Temperature of the acidMethod: place all the acid solutions in a water bath at 25 °C for 10 minutes before adding the magnesium.so any change in rate is due to concentration aloneJustification: rate is highly sensitive to temperature, so a drift between trials would be indistinguishable from the effect being investigated.Surface area of the magnesiumMethod: use 2.0 cm lengths cut from the same roll, each polished lightly with sandpaper to remove the oxide layer.so the exposed surface is the same every timeJustification: surface area determines how many magnesium atoms are available for collision, so an unpolished or longer strip would react at a different rate for the wrong reason.
WORKED EXAMPLE
Describe how you would check a digital thermometer before a calorimetry investigation, and state which type of error this addresses.
The two fixed pointsPlace the probe in a beaker of crushed melting ice, which should read 0.0 °C, and then in boiling distilled water, which should read 100.0 °C at standard pressure.What you do with the resultIf either reading is offset, either adjust the instrument or subtract the offset from every recorded value.this addresses systematic errorSay systematic explicitly. Repeating the experiment cannot reveal an offset like this, because it shifts every trial by the same amount.
WORKED EXAMPLE
A student notes that the temperature of the laboratory rises by about 3 °C over the course of an afternoon’s data collection. Explain what they should do.
Can it be controlled?Not realistically, in a school laboratory over several hours.So monitor it insteadRecord the ambient temperature alongside every trial, so that the drift is documented rather than hidden.monitor, record, and discuss in the evaluationWhat to say laterTrials run later in the afternoon started slightly warmer, which would tend to increase the measured rate. If the trials were also run in order of increasing concentration, this becomes a possible systematic bias worth naming.A neat improvement: randomise the order in which the concentrations are tested, so any drift spreads across the whole data set rather than pushing one end of it.
💡 Exam tip
For each controlled variable give how and why, not just what.
Be concrete: “a water bath set to 25 °C”, not “the temperature will be kept constant”.
Name calibration for any instrument that has a reference standard.
Use the word systematic when explaining what controlling a variable prevents.
If a variable cannot be controlled, say you will monitor and record it.
Prioritise the variable with the largest effect, and say why it is the largest.
⚠️ Common mix-up
Writing “keep the temperature constant” with no method attached.
Assuming repeats fix everything. Repeating reduces random error and does nothing to a systematic one.
Forgetting to tare the balance, then measuring the container along with the chemical.
Ignoring a variable because it cannot be controlled instead of monitoring it.
Treating every controlled variable as equally important rather than prioritising the biggest.
That completes Inquiry 1. The question is focused, the method is replicable, and the variables are pinned down — which means the numbers you are about to collect will actually be worth something.
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