IB Chemistry SLTopic 8 — Exploring and DesigningInternal assessmentPractical skill~13 min read
Designing Investigations
There is one test for a method, and it is brutal: could a chemist who has never met you follow your instructions and get your numbers back? Everything in this section exists to make the answer yes.
📚 What you need to know
Name and justify three kinds of variable: the independent (IV), the dependent (DV) and the controlled (CVs).
Collect at least five values of the independent variable, and justify the range at both ends.
Repeat each value at least three times, so a mean can be taken and anomalies spotted.
A method must be replicable: precise apparatus, with sizes, precision and quantities stated.
Structure the write-up as materials and apparatus, then safety, ethical and environmental, then procedure.
Investigations can also be built on databases or simulations, not only on hands-on lab work.
A pilot study is a small trial run that tells you whether the plan works before you commit to it.
The three kinds of variable
A useful sanity check: if you cannot decide whether something is a controlled variable, ask whether changing it would change your dependent variable. If it would, it needs controlling.
Range and repeats
Two separate decisions, and both need justifying rather than asserting. The range is how far apart your extreme values are and how many you take in between; the repeats are how many times each of those values is measured.
Fifteen measurements sounds like a lot until you realise it is the minimum. Anything less and you are relying on a line drawn through three points, one of which might be wrong.
Justifying the range means explaining both ends. A good justification sounds like this: a range of 20 °C to 60 °C was chosen because below 20 °C the reaction is too slow to time reliably, while above 60 °C significant evaporation of the solution begins to change the concentration during the run.
Notice the shape of that sentence. It gives a reason for the lower limit and a separate reason for the upper limit, and both reasons are chemical rather than practical. “Because that is what the school has” is not a justification; “because below this the reaction is impractically slow” is.
Writing a method someone else could follow
Naming the apparatus is not fussiness. A 25 cm3 measuring cylinder and a 25.00 cm3 pipette deliver the same nominal volume with about ten times the difference in uncertainty.
Section
What goes in it
Materials and apparatus
every chemical with its concentration, and every piece of equipment with its size and precision
Safety, ethical and environmental
a brief risk assessment naming the hazards, the specific precautions, and how waste will be disposed of
Procedure
numbered steps, in order, precise enough to be followed without you in the room
Creativity in a method usually shows up as a better way of measuring, not a more exotic reaction. Following a reaction that produces a coloured product with a colorimeter, rather than timing a colour change by eye, is more reliable and more objective — and it is exactly the kind of decision an assessor notices.
Not every investigation happens at a bench
Hands-on laboratory work is the usual route, but it is not the only one. Two alternatives are explicitly available.
Database investigations. Extract published data on a property — the boiling points across a homologous series, say — then process and analyse it to find and explain a trend. The chemistry is in the analysis, not in the collecting.
Simulations. Collect data on a process that would be too dangerous, too slow or too expensive to run in a school laboratory, such as the behaviour of gases over a wide range of pressures.
Both still need a focused research question, controlled variables and a justified range. What changes is where the numbers come from, not how rigorous the design has to be.
Pilot studies
A pilot is a small-scale trial run, and it is the cheapest insurance available. It tells you whether your quantities give a measurable result, whether your timing method works, and whether your range is sensible — before you have spent three lessons collecting data you cannot use.
The familiar example is the rough titration. It is a pilot: it finds the approximate endpoint so that in the recorded runs you know when to slow to dropwise addition.
WORKED EXAMPLE
For the research question “What is the effect of temperature on the rate of reaction between aqueous sodium thiosulfate and hydrochloric acid?”, identify the independent and dependent variables and three controlled variables.
Independent variablethe temperature of the reaction mixture, in °CDependent variableThe rate, found from the time taken for enough sulfur to form to obscure a cross drawn beneath the flask.rate, calculated as 1 ÷ timeControlled variablesConcentration of the sodium thiosulfate, concentration of the hydrochloric acid, and the total volume of the mixture.The cross itself is worth adding: the same cross, drawn with the same pen, viewed by the same person. Otherwise “obscured” means something different each time.
WORKED EXAMPLE
Justify a range of five temperatures from 20 °C to 60 °C for the investigation above, and state how many measurements will be taken in total.
The lower limitBelow 20 °C the reaction becomes so slow that the timing is dominated by how long the cross takes to fade, which is a judgement rather than a measurement.The upper limitAbove 60 °C evaporation becomes significant, which changes the concentrations during the run and would introduce a second variable.Number of valuesFive evenly spaced values at 20, 30, 40, 50 and 60 °C give enough points to establish a clear trend.Total measurements5 values × 3 trials = 1515 measurements, giving 5 means to plot
WORKED EXAMPLE
A student writes the step “Put the acid in a cup and record the temperature change.” Rewrite it so that it could be replicated, and state what you added.
What is missingThe volume, the concentration, the apparatus used to measure it, the container, and how and how often the temperature is recorded.A replicable version“Use a 25.00 cm³ volumetric pipette to transfer 25.00 cm³ of 1.0 mol dm⁻³ hydrochloric acid into a polystyrene cup fitted with a lid, and record the temperature every 15 s for 2 minutes using a digital thermometer reading to ±0.1 °C.”Every number added is one fewer thing a reader has to guess. That is the whole standard.
💡 Exam tip
State the IV, the DV and the CVs explicitly under their own headings.
Justify the range at both ends with a chemical reason, not a practical one.
Say five values and three trials, and say what the repeats are for.
Give apparatus with size and precision: “a 50.0 cm3 burette”, not “a burette”.
Include a real safety section, naming hazards and disposal.
Mention a pilot study; it shows you tested the plan before trusting it.
⚠️ Common mix-up
Listing controlled variables without saying how they will be kept constant.
Choosing a range with no justification, or one so narrow that no trend appears.
Confusing repeats with values. Three trials at three temperatures is not the same as three trials at five.
Writing “measure the solution” without naming the apparatus or the volume.
Treating safety as a closing sentence rather than a section of the design.
Up next: Controlling Variables — you have now listed what must be held constant. The next page is about actually holding it there, and about what to do with the variables that refuse to cooperate.
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