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.

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.

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 degrees 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 this specification row 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 degrees C.
5. Connect the flask to a gas syringe.
6. Add magnesium, fit the bung quickly and start the timer.
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.

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.

Planning Checklist

When faced with an unfamiliar practical-planning question, do not panic and search your memory for a whole required practical. Build the plan from the chemistry.

Use this checklist:

1. What is the practical question?
2. What chemical change or expected outcome should happen?
3. What is the independent variable?
4. What is the dependent variable?
5. Which apparatus or technique measures the dependent variable?
6. Which controlled variables are needed, and how will each be controlled?
7. What range, repeats or timing method will give enough evidence?
8. Does the method actually test the question, or could another factor explain the result?

Here is a complete worked planning paragraph:

"To investigate the effect of acid concentration on the rate of reaction with magnesium, use hydrochloric acid solutions of known concentrations and the same measured volume of acid each time. Add the same mass and surface area of magnesium ribbon to a conical flask connected to a gas syringe, start timing when the magnesium is added, and record the volume of H2(g) produced every 10 seconds. Keep temperature constant using a water bath and use the same total volume of solution in each trial. Repeat each concentration and compare the initial rate or the time taken to collect a fixed volume of hydrogen. This method is appropriate because the gas syringe measures the expected product, while controlling magnesium amount, temperature and acid volume helps ensure that acid concentration is the main factor affecting the result."

This paragraph covers the planning skill because it links apparatus, variables and evaluation to the expected chemical outcome. It is not just a method; it is a justified method.