Paper 6 Planning Investigations: A Full-Mark Method

A Paper 6 planning question rewards a method that could actually be carried out and produce interpretable results. The marker needs to see what changes, what is measured, what is kept constant, how the measurements are made, and how reliability and safety are addressed.

The plan should be specific enough for another student to follow without guessing.

The planning framework

Use V-A-R-I-A-B-L-E-S as a checking framework:

• Variable changed
• Apparatus and quantities
• Range of values
• Instructions in a logical sequence
• Accurate measurement method
• Baseline and controlled variables
• Limitations, safety and leakage/heat-loss controls where relevant
• Each value repeated
• Summary of processing: mean, graph or comparison

The acronym is not an official Cambridge requirement. It is a way to ensure the required experimental decisions are visible.

1. Identify the independent variable

The independent variable is the factor deliberately changed.

Examples:

• concentration of acid
• temperature
• mass of catalyst
• particle size of a solid
• length of magnesium ribbon
• voltage across an electrolytic cell

State how it will be changed. “Change concentration” is incomplete if the method does not explain how the different concentrations are prepared or selected.

2. Identify the dependent variable

The dependent variable is measured to determine the effect.

Examples:

• time for a cross to disappear
• volume of gas after a fixed time
• time to collect a fixed gas volume
• maximum temperature reached
• mass lost after a fixed interval
• current in a circuit

State the apparatus and unit.

Weak:

Measure the reaction.

Better:

Use a gas syringe to measure the volume of carbon dioxide produced every 20 seconds, in cm³.

3. Choose controlled variables

Controlled variables are conditions that could affect the dependent variable and must remain constant.

For an acid-solid rate experiment, possible controls include:

• volume of acid
• mass of solid
• particle size or surface area of solid
• temperature
• same apparatus
• same mixing method

Do not list variables that are irrelevant or impossible to control. Explain how important controls are maintained.

Weak:

Keep temperature the same.

Better:

Carry out every trial in the same water bath and allow the reactants to reach the selected temperature before mixing.

4. Select suitable apparatus

Name apparatus according to the required measurement.

Measurement	Suitable apparatus
Fixed accurate liquid volume	volumetric pipette
Approximate or variable liquid volume	measuring cylinder or burette, depending on precision needed
Delivered titrant volume	burette
Gas volume	gas syringe or suitable collection over water where the gas is not too soluble/reactive
Mass	balance
Temperature	thermometer or temperature probe
Time	stopwatch
pH	pH meter/probe or suitable indicator for a less precise comparison

The best apparatus depends on the question. A measuring cylinder may be sufficient for preparing a broad concentration series; a pipette is better for a fixed precise volume.

5. Use a useful range

A trend needs more than two points. Select several values across a safe, measurable range.

For concentration, a plan could use five concentrations prepared from a stock solution. For temperature, choose values that can be maintained safely with water baths. For mass of catalyst, choose values large enough for the balance to distinguish.

Good intervals are:

• clearly stated
• achievable with the available apparatus
• spread enough to reveal a trend
• not so extreme that the reaction is instantaneous, negligible or unsafe

6. Write the method in operational steps

Use numbered steps and include quantities.

A useful sequence is:

1. prepare or measure the first value of the independent variable
2. measure the controlled quantities
3. assemble the apparatus and check it is suitable or airtight where relevant
4. mix the reactants and start the timer consistently
5. measure the dependent variable at stated intervals or until a stated endpoint
6. record the result with units
7. clean, reset or use fresh apparatus as necessary
8. repeat for the remaining values
9. repeat each value and calculate a mean
10. plot or compare the processed results

Avoid vague instructions such as “do the reaction” or “record what happens”.

7. Make the starting point consistent

Timing errors often arise because the reaction starts while the apparatus is still being assembled.

State a consistent trigger:

Add the acid, immediately fit the bung and start the stopwatch.

For a disappearing-cross experiment:

Place the flask over the cross, add the final reactant, swirl once and start the stopwatch immediately.

Acknowledge that the short delay can be a limitation and propose an improvement where appropriate.

8. Repeat and process results

A complete reliability statement can be:

Repeat each concentration at least twice more, identify any anomalous result, and calculate the mean of the concordant values.

Then state how the mean is used:

• plot mean rate against concentration
• plot gas volume against time
• compare mean temperature change
• calculate rate as 1/time when the endpoint is fixed

Do not calculate a mean that includes an obvious anomaly without comment.

9. Include safety that matches the chemicals

Generic “wear goggles” wording may earn limited credit if the question expects a specific risk.

Link hazard to control:

• corrosive acid → wear eye protection; rinse spills with plenty of water
• flammable ethanol → keep away from naked flames; use a water bath
• toxic gas → use small quantities in a fume cupboard
• hot apparatus → use tongs and allow it to cool
• pressurised gas setup → ensure the syringe moves freely and avoid excessive reagent quantities

Do not invent a hazard unsupported by the materials.

Full original planning example

Question: Plan an investigation to determine how the concentration of hydrochloric acid affects the initial rate of reaction with calcium carbonate. You have marble chips, hydrochloric acid, water, a conical flask, a bung, delivery tube, gas syringe, measuring cylinders, a balance and a stopwatch.

Variables

• independent variable: concentration of hydrochloric acid
• dependent variable: initial rate, determined from carbon dioxide volume produced during the first 40 seconds or from the initial gradient of a volume-time graph
• controls: mass and particle size of marble chips, total acid volume, temperature, apparatus and mixing/start procedure

Method

1. Prepare at least five hydrochloric acid concentrations by diluting the stock acid with water while keeping the total liquid volume constant, for example 50 cm³ per trial.
2. Select marble chips of similar size and measure the same mass for every trial using a balance.
3. Place the chips in a conical flask connected by a tight bung and delivery tube to a gas syringe. Check that the syringe moves freely and the apparatus is airtight.
4. Measure 50 cm³ of the first acid concentration.
5. Add the acid to the flask, immediately fit the bung and start the stopwatch.
6. Record the gas-syringe volume every 10 seconds for 40 seconds.
7. Rinse and reset the apparatus. Use fresh marble chips of the same mass and size for the next concentration.
8. Repeat each concentration at least twice more and calculate mean gas volumes after checking for anomalous readings.
9. Plot mean carbon dioxide volume against time for each concentration and compare the initial gradients, or calculate the initial rate from the early straight section.
10. Wear eye protection because hydrochloric acid is irritant/corrosive depending on concentration, and clean spills promptly with plenty of water.

Why this earns marks

The plan contains:

• a measurable dependent variable
• several concentration values
• fixed total acid volume
• controlled solid mass and surface area
• consistent timing
• suitable gas-measuring apparatus
• repeats and means
• a graph-based method for determining initial rate
• relevant safety

Alternative endpoint method

A different valid design could measure the time required to collect a fixed volume of gas, such as 20 cm³.

Then:

rate indicator = 1 ÷ time

The fixed volume must be reachable for all concentrations. If the slowest reaction cannot produce that volume within a practical time, the endpoint is unsuitable.

Common planning mistakes

1. Changing more than one factor. Changing concentration and acid volume together prevents a fair comparison.
2. Using only two values. This shows a difference but not a reliable trend.
3. Not naming the measurement. “Measure the rate” is not an operational method.
4. No units or intervals. State cm³, seconds, °C and how often readings are taken.
5. Reusing a reactant that has already reacted. Use fresh materials for each trial.
6. Ignoring surface area. Equal mass does not mean equal rate if chip sizes differ.
7. Starting the timer inconsistently. State exactly when timing begins.
8. Repeating without calculating a mean. Explain how repeat results are processed.
9. A generic safety sentence. Connect the actual hazard to the precaution.
10. A graph without named axes. State what is plotted against what.

Planning checklist

Before finishing, ask:

• Is one independent variable clearly changed?
• Is the dependent variable measurable with named apparatus?
• Are the important controls stated and maintained?
• Are there enough values to reveal a trend?
• Are quantities, units and intervals given?
• Can another student follow the order?
• Are repeats and anomalies handled?
• Is the processing method stated?
• Is safety relevant and specific?

Continue with Paper 6 tables and graphs and Paper 6 errors and improvements to complete the experimental-skills sequence.