Planning¶
Syllabus mapping
Working Scientifically outcome — Planning. Covers identifying and managing variables, designing a fair and valid method, assessing risk, and selecting appropriate equipment. Like every Working Scientifically outcome, it's assessed through application inside a focus area's practicals and depth study, not as standalone content.
Outcome codes: CH-11WS-02 (Year 11) and CH-12WS-02 (Year 12) — Planning scientific investigations. Same outcome wording in both years, with a separate code for each. Wording on this page is paraphrased for teaching use — check the official syllabus before treating any line as verbatim NESA text.
You need to know: how to name your three types of variables correctly, how to design a method that actually isolates the relationship you're testing, how to choose glassware that's precise enough for the job, and how to fill out a risk assessment properly — not just as a formality.
Core¶
The three types of variable¶
Every fair investigation in this course has (at minimum) these three:
- Independent variable (IV) — the one thing you deliberately change between trials.
- Dependent variable (DV) — the thing you measure, which you expect to respond to the IV.
- Controlled variables — everything else that could affect the DV, which you deliberately hold constant so the only thing changing is the IV.
Worked example
Investigation: how temperature affects the rate of the sodium thiosulfate + hydrochloric acid reaction (see Questioning and Predicting).
| Variable | Role | This investigation |
|---|---|---|
| Temperature of the thiosulfate solution | Independent | Set to 20, 30, 40, 50 °C using a water bath |
| Time for the cross to disappear | Dependent | Measured with a stopwatch |
| Volume and concentration of thiosulfate | Controlled | Same 50 mL of the same solution every trial |
| Volume and concentration of acid | Controlled | Same 5 mL of the same acid every trial |
| Size of the flask and the cross | Controlled | Same flask, same printed cross, same depth of liquid |
| Observer | Controlled | Same person judges "disappeared" every time |
A method that changes the temperature and swaps to a different bottle of acid partway through hasn't isolated anything — if the time changes, you can't tell whether it was the temperature or the acid. This is the whole point of "fair testing": change one thing, measure one thing, hold everything else still.
Fair testing vs a control¶
Two related but different ideas get called "control" in science:
- Controlled variables (above) — the things you deliberately keep the same across every trial of a single experiment.
- A control experiment (or control group) — a separate, baseline trial run with no treatment applied, used as a comparison point. In chemistry this often shows up as a blank: for example, when testing whether manganese dioxide catalyses the decomposition of hydrogen peroxide, you also run the same hydrogen peroxide with no catalyst, so you can see how much of the effect is really due to the catalyst. It's worth knowing the distinction, since "control" gets used loosely and command-verb questions sometimes expect you to be precise about which one you mean.
Designing the method itself¶
A few things worth deciding before you're standing at the bench, not while you're there:
- Range of the independent variable — spread values across a sensible range (not clustered at one end), so a resulting graph actually shows a trend rather than a cluster of nearby points.
- Number of data points — enough IV values to draw a convincing line or curve; five or six is a reasonable minimum for a Year 11 prac unless told otherwise.
- Repeats — plan to repeat each trial (typically 3+) so you can calculate a mean and get a sense of how consistent your results are. This matters for Conducting and becomes essential once you get to processing uncertainty.
- Equipment precision — pick equipment whose precision actually matches what you're trying to measure:
| Equipment | Typical uncertainty | Use it for |
|---|---|---|
| Beaker (graduations) | Roughly ±5% — only a guide | Holding and mixing, not measuring |
| Measuring cylinder (50 mL) | About ±0.5 mL | Volumes where a few % doesn't matter (e.g. adding excess acid) |
| Pipette (25.00 mL) | About ±0.03 mL | Transferring an exact volume |
| Volumetric flask (250.0 mL) | About ±0.2 mL | Making up a solution to an exact volume |
| Electronic balance (0.01 g) | ±0.01 g | Weighing — weigh a larger mass where possible to keep the percentage uncertainty small |
A standard solution made up in a beaker, or measured with a measuring cylinder, can't be "standard" — the volume is only known to a few percent.
Risk assessment¶
A risk assessment isn't a box-ticking formality — it's how you show you thought about what could go wrong before it does. NESA (and every school's WHS policy) expects one to accompany any practical involving chemicals, heat, glassware or electricity. Check the safety data sheet (SDS) for each chemical to find its hazards.
The standard structure:
| Hazard | Risk | Likelihood | Consequence | Control |
|---|---|---|---|---|
| What could cause harm | What could actually happen | How likely, given your setup | How bad if it happens | What you'll do to reduce it |
Worked example — marble chips and hydrochloric acid
From the gas-production prac on Conservation of Mass:
| Hazard | Risk | Likelihood | Consequence | Control |
|---|---|---|---|---|
| 2 mol L⁻¹ hydrochloric acid (corrosive/irritant) | Splashing into eyes or onto skin | Medium | Eye damage; skin irritation | Wear safety glasses; use the lowest concentration that works; rinse any skin contact immediately with plenty of water |
| Glassware (conical flask, gas syringe) | Breakage, cuts | Low | Minor cut | Check for chips and cracks first; clamp the flask; report breakages and use a dustpan, not hands |
| Carbon dioxide gas produced | Pressure build-up in a sealed flask | Low | Stopper or syringe forced out | Use a gas syringe or balloon that can expand; never seal a gas-producing reaction completely |
Note the pattern: every hazard gets a specific, actionable control — not "be careful," which controls nothing.
Advanced
Pilot studies. Before committing to a full run of trials, a short pilot (one or two quick trials across the planned IV range) can reveal problems a plan alone won't — a reaction that's too fast to time at the highest concentration, a cross that never fully disappears at the lowest temperature, or a temperature that drops before the reaction finishes. Adjusting the method after a pilot (e.g. diluting all the solutions, or changing the IV range), and explicitly saying so in a report, is good practice, not a sign the original plan was bad — it's evidence of exactly the kind of iterative refinement Working Scientifically as a whole is trying to develop.
Extension
Beyond the syllabus for this course — won't appear in the HSC, included for interest / depth study inspiration.
📎 Depth study idea
Clinical and pharmaceutical trials take the "control" idea much further than a typical chemistry prac does, using a control group that receives no treatment (or a placebo) and comparing it against a treatment group — often blinded (the participant doesn't know which group they're in) or double-blinded (neither the participant nor the person giving the treatment knows, to remove bias from expectation). There's a genuine chemistry parallel: any measurement that relies on human judgement — deciding when a cross has "disappeared", when a colour change has "happened", or matching a colour by eye — inherits the same expectation bias. A depth study could compare a human-judged endpoint with an instrument (a light sensor or colorimeter) for the same reaction.
Video/visual resources¶
- 🎥 Khan Academy — TODO: source a variables / experimental design explainer. Must define IV/DV/controlled clearly with a worked example.
- 🎥 TODO: source a video on choosing and using volumetric glassware (pipette, volumetric flask, measuring cylinder) — the precision table above is much more convincing when you can see the difference.
Check yourself¶
-
For an investigation into how the concentration of hydrochloric acid affects the rate at which it reacts with magnesium ribbon, identify the independent variable, the dependent variable, and three controlled variables.
Answer
Independent: concentration of the acid. Dependent: rate of reaction (e.g. time for the magnesium to disappear, or volume of hydrogen collected in 30 s). Controlled (any three): volume of acid, length/mass of magnesium ribbon, temperature, surface condition of the magnesium (cleaned the same way), stirring.
-
Explain why a measuring cylinder is a poor choice for making up a standard solution, even though it has volume markings.
Answer
A measuring cylinder is only accurate to about ±0.5 mL — several times less precise than a volumetric flask. A standard solution needs its concentration known exactly, so the volume must be known exactly too. The uncertainty in the volume would carry straight through into the concentration, so the solution wouldn't really be "standard".
-
Write a two-row risk assessment (Hazard, Risk, Likelihood, Consequence, Control) for a flame test prac (see Atomic Structure).
Answer
Example (answers will reasonably vary):
Hazard Risk Likelihood Consequence Control Bunsen burner flame Burns; hair or sleeves catching fire Low Burn Tie hair back; use the safety flame when not testing; keep the bench clear of paper Metal salt solutions (e.g. barium and copper compounds are toxic) Swallowing or skin contact Low Poisoning; irritation Don't eat or drink in the lab; wash hands after the prac; use small volumes