Required practicals: method, safety and evaluation — AQA GCSE Chemistry revision

General practical skills

• Always identify the independent variable (what you change), dependent variable (what you measure) and control variables (what you keep the same) before starting a practical.
• Repeat readings at least three times and calculate a mean, ignoring any anomalous results, to improve the reliability of your data.
• Choose apparatus with appropriate resolution — for example, a measuring cylinder with smaller graduations gives a more precise volume reading than one with large graduations.
• Record results in a clearly labelled table with units in the column headings, not repeated in every cell.

Titration technique

• In an acid-base titration, a burette is used to add measured volumes of one solution to a known volume of the other solution in a conical flask, using an indicator to show the end point.
• A rough titration is carried out first to find an approximate end point, then accurate titrations are repeated until at least two 'concordant' results (within 0.10 cm³ of each other) are obtained.
• The mean titre should be calculated using only the concordant results, not all repeats.
• Reading the burette at eye level from the bottom of the meniscus avoids parallax error in volume readings.

Key terms

Independent variable

The factor that is deliberately changed in an experiment.

Dependent variable

The factor that is measured as it may change in response to the independent variable.

Control variable

A factor kept constant so it does not affect the results of the experiment.

Concordant results

Repeat titration results that are within 0.10 cm³ of each other, considered reliable enough to average.

Anomalous result

A data point that does not fit the general pattern of the rest of the results.

Rf value

In chromatography, the distance moved by a substance divided by the distance moved by the solvent.

Common exam traps

• In titrations, don't average all your repeats — only average the concordant results (within 0.10 cm³).
• The disappearing cross method gives a subjective, less precise result compared with gas syringe methods; don't call it fully objective.
• Chromatography baselines must be drawn in pencil, not pen, because pen ink would dissolve and move with the solvent, ruining the results.

Practice questions with answers

1. 1. In an experiment testing how temperature affects reaction rate, what is the independent variable?

   • • Reaction rate
   • • Temperature
   • • Volume of gas produced
   • • Time

   Answer: Temperature

   Temperature is what the experimenter deliberately changes; reaction rate is measured as the dependent variable.

2. 2. Why should you repeat readings at least three times in an experiment?

   Answer: To identify anomalies and calculate a reliable mean.

   Repeats improve reliability by allowing anomalous results to be spotted and excluded before averaging.

3. 3. What is a concordant pair of titration results?

   • • Any two results from the same day
   • • Results within 0.10 cm³ of each other
   • • The first and last results only
   • • Results using different indicators

   Answer: Results within 0.10 cm³ of each other

   Concordant results are close enough (within 0.10 cm³) to be considered reliable for averaging.

4. 4. Why is a rough titration carried out before accurate repeats?

   Answer: To find an approximate end point so accurate titrations can be added dropwise near that volume.

   This saves time and improves accuracy in later, more careful titrations.

5. 5. How should you read the volume on a burette to avoid parallax error?

   • • From above, looking down
   • • At eye level, at the bottom of the meniscus
   • • From the side at an angle
   • • Using a torch only

   Answer: At eye level, at the bottom of the meniscus

   Reading at eye level ensures an accurate reading without distortion from viewing angle.

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