1.1.1 - Planning practical investigations

From Question to Method

A practical investigation is designed to answer a scientific question. In chemistry, that question is usually about a measurable change: a colour becomes less intense, a gas is produced, a temperature changes, a precipitate forms, a pH changes, or the mass of a substance changes.

Planning begins by translating the question into evidence.

A useful planning chain is:

1. State the practical question.
2. Predict the expected chemical outcome.
3. Identify what you will change.
4. Identify what you will measure or observe.
5. Choose apparatus and technique that measure that evidence.
6. Decide which variables must be controlled.
7. Check whether the method is appropriate for the expected outcome.

For example, suppose the question is:

"How does the concentration of copper(II) sulfate solution affect the rate of reaction with zinc?"

The chemical context matters. Copper(II) sulfate solution is blue because it contains aqueous Cu²⁺ ions. Zinc reacts with Cu²⁺ ions, so the blue colour becomes less intense as Cu²⁺ ions are used up and copper forms. A method that follows the blue colour over time can therefore provide evidence for the rate of reaction.

A good plan would not begin with a random list of apparatus. It would reason like this:

• Change: concentration of CuSO₄(aq).
• Measure: how quickly the blue colour decreases.
• Suitable technique: colorimetry, because absorbance can track colour intensity.
• Important controls: temperature, volume of solution, mass and surface area of zinc, total volume, mixing method and timing point.
• Expected outcome: a higher concentration should produce a faster observable decrease in blue colour, if the method is sensitive enough.

This is the core habit for practical planning: every apparatus choice should link to the evidence needed.

This colorimetry plan also needs a limitation check. Suspended zinc or copper particles can scatter light, so absorbance must be measured on consistently cleared liquid samples, with the sampling times defined. A suitable blank and calibration help distinguish the solution's colour from background absorbance. Without controlling scattering, a changing reading would not reliably measure copper(II) ion concentration.

Choosing Apparatus and Techniques

An apparatus choice is strong only if it measures the right quantity with enough detail for the practical question. The most familiar piece of equipment is not always the most suitable one. Resolution is the smallest change an instrument can distinguish or display; fine resolution helps with small changes, but calibration and technique still matter.

Use this decision route:

The word "suitable" is doing a lot of work. Suitable means the apparatus measures the intended quantity, has an appropriate range, has enough precision for the expected change and fits the chemistry of the reaction.

ICT is part of suitable methodology when it genuinely improves the investigation. A pH probe, temperature probe, colorimeter or data logger is useful when it records the needed evidence more consistently or at better time intervals. It is not automatically better if it does not measure the dependent variable.

Do not write "use more accurate apparatus" as a complete answer. Name the apparatus and the quantity it improves. For example:

• Weak: "Use better apparatus."
• Strong: "Use a 25.0 cm³ volumetric pipette instead of a measuring cylinder to transfer the acid, because the fixed volume needs to be measured accurately."

Worked example:

A student wants to compare the effect of acid concentration on the rate of reaction between magnesium ribbon and hydrochloric acid:

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

The evidence of reaction rate could be the volume of H₂(g) produced per unit time. A gas syringe is suitable because it collects and measures gas volume directly. A pH meter would not be the best choice for this question because the expected evidence is gas production, not pH change.

Identifying Variables

Planning questions often test whether you can identify variables in a practical context.

A controlled variable is not complete unless you know how it will be controlled. In a method, it is usually best to state:

1. the variable,
2. why it matters,
3. how it will be controlled.

For example:

• Weak: "Keep temperature the same."
• Strong: "Keep temperature at 25 °C using a water bath, because rate changes with temperature."

The second version is better because it connects the variable to the chemistry and gives a practical control method.

Worked example:

A student investigates how sodium thiosulfate concentration affects the rate of its reaction with dilute hydrochloric acid. The dependent variable is the time taken for a cross under the flask to disappear as sulfur forms and the mixture becomes cloudy.

A good variable plan could be:

This does not guarantee perfect results, but it makes the comparison fairer. If temperature, acid volume and total mixture depth changed as well as sodium thiosulfate concentration, the result could not confidently be linked to the independent variable.

Building a Workable Method

A method is more than a list of equipment. It is a sequence that shows how the evidence will be collected while the variables are controlled.

A strong method usually includes:

• the range of independent variable values,
• how each value will be prepared or selected,
• the fixed quantities and conditions,
• the measurement or observation made,
• when timing or recording begins and ends,
• repeats if they are needed to make the evidence more dependable,
• a plan for comparing results with the expected outcome.

You do not need to write a full risk assessment for a planning answer unless the context makes safety part of whether the method is appropriate. You do need enough detail for another competent chemist to understand how the investigation answers the question.

Worked example:

Question: Plan an investigation to test how hydrochloric acid concentration affects the rate of reaction with magnesium.

Good planning response:

1. Prepare several concentrations of HCl(aq), for example by diluting a stock acid to known concentrations.
2. Add the same volume of acid to a conical flask each time.
3. Use the same length or mass of magnesium ribbon each time, cleaned in the same way if needed.
4. Keep temperature constant, for example at 25 °C.
5. Connect the flask to a gas syringe.
6. Start the reaction and timer together using a closed addition method where possible; fitting the bung only after mixing can lose early gas.
7. Record the volume of H₂(g) at fixed time intervals, or record the time to collect a fixed volume of gas.
8. Repeat each concentration and compare the rate measure for each concentration.

This method is appropriate because the independent variable is acid concentration, the dependent variable is linked to hydrogen gas production, and major alternative variables are controlled.

The method would be weaker if it simply said:

"Put magnesium in acid and see what happens."

That version does not define concentrations, quantities, apparatus, timing, controls or how the result will be judged.

Choose a range and test it in a pilot

A short pilot establishes whether the reaction is too fast to record, too slow to distinguish from background change, or likely to exceed the gas syringe capacity. Choose at least several concentrations spread across a safe, useful range, with closer spacing where a change in behaviour needs resolving. A range is a justified design choice, not a rule that every investigation needs exactly five trials.

For example, to prepare 50.0 cm³ of 0.400 mol dm⁻³ acid from a 2.00 mol dm⁻³ stock, use $c_1V_1=c_2V_2$: $V_1=0.400\times50.0/2.00=10.0\,\mathrm{cm^3}$. Transfer that stock volume into suitable volumetric glassware and dilute to a final volume of 50.0 cm³. The final volume, rather than a guessed amount of added water, defines the concentration. Repeat the same final-volume method for the other concentrations.

Keep gas collection connected before the reaction starts: hold the solid separately inside the flask, then tip it into the acid, or inject one reagent through a suitable sealed inlet. The gas syringe plunger must remain free to move. Compare initial gradients of gas-volume–time curves, or average rates over the same short interval if tangents cannot be resolved. These are different rate measures; label the chosen measure and use it consistently. Repeat complete runs and compare their calculated rates.

Evaluating Whether a Method Fits

Evaluation in this lesson is about whether the method is appropriate to meet the expected outcome. It is not yet a full uncertainty calculation or graph-analysis task.

Use these questions when judging a method:

A high-quality evaluation does not just say "the method is good" or "the method is bad". It gives a reason linked to the expected outcome.

Worked example:

A student wants to compare two weak acids by measuring their pH. The expected pH difference is small. The student proposes using universal indicator paper.

Evaluation:

Universal indicator paper may not be appropriate if the expected pH difference is small, because it gives only an approximate pH from a colour comparison. A pH meter or pH probe would be more suitable because it can measure smaller differences in pH. The method should also use the same acid concentration and temperature for both acids, otherwise the pH comparison may not be due to acid strength alone.

Notice the structure:

• identify the weakness,
• explain why it affects the expected outcome,
• suggest a method improvement.

Common weak evaluations:

• "Repeat to make it accurate." Repeats mainly improve reliability/precision by showing whether results are consistent. They do not automatically remove a systematic error.
• "Use accurate apparatus." This is too vague. Name the apparatus and the measurement it improves.
• "Control everything." This is not practical. Name the variables that matter for this chemical question.
• "The method worked." A method can produce a result and still be inappropriate if it does not answer the question.

Quick Recap

• Start with the chemical question and choose a measurement that supplies relevant evidence.
• Match apparatus range and resolution to the expected change; explain the choice.
• Define the independent and dependent variables, and state how relevant controls are maintained.
• Use a pilot to choose a useful range, and specify quantities, timing, repeats and processing.
• Evaluate whether the complete method can distinguish the expected outcome; precision alone does not guarantee accuracy.