Welcome to Experiment Planning!

In Chemistry, we don't just "mix things and see what happens." To get results that actually mean something, we need a solid plan. Think of experiment planning as the blueprint for a building. If the blueprint is wrong, the whole building falls down—no matter how good the bricks are! In your Oxford AQA International AS Chemistry exams (Units 1 and 2), you will be asked to describe and evaluate these procedures, so mastering the "how" and "why" of planning is essential.

1. Identifying the Variables

When you plan an experiment, you need to decide what you are changing, what you are measuring, and what you are keeping the same. We call these variables.

The Independent Variable

This is the variable you decide to change. For example, if you are investigating how temperature affects the rate of a reaction, the temperature is your independent variable because you are choosing which temperatures to test.

The Dependent Variable

This is the variable that changes because of what you did. It is the data you measure. In the rate experiment mentioned above, the dependent variable might be the time it takes for a precipitate to form or the volume of gas produced.

Control Variables

These are the factors you must keep exactly the same to ensure a fair test. If you are changing the temperature but you also accidentally change the concentration of your reactants, you won't know which one caused the change in rate!
Common control variables include:

  • Concentration of solutions (\( mol \cdot dm^{-3} \))
  • Volume of liquids (\( cm^3 \))
  • Surface area of solids (e.g., using the same size of marble chips)
  • Pressure of gases (especially relevant for the ideal gas equation \( pV = nRT \))

Quick Tip: If a question asks you how to make a result more reliable, often the answer involves controlling a specific variable more strictly!

2. Choosing the Right Equipment

Part of your AO4 assessment objective is selecting the correct tools for the job. You need to choose equipment that is precise enough for your experiment.

Measuring Volume

If you need to measure exactly \( 25.0 \text{ } cm^3 \) for a titration (Required Practical 1), a measuring cylinder is usually too "clumsy." Instead, you would use:

  • A pipette: For one specific, highly accurate volume (usually \( 10.0 \text{ } cm^3 \) or \( 25.0 \text{ } cm^3 \)).
  • A burette: If you need to add varying amounts of liquid accurately, as it allows you to measure to the nearest \( 0.05 \text{ } cm^3 \).
  • A volumetric flask: Used specifically for making a standard solution of a known concentration.

Measuring Temperature and Mass

  • Thermometers: Use a thermometer with graduations of \( 0.1^{\circ}C \) if you are measuring small enthalpy changes (\( \Delta H \)).
  • Balances: A "two-decimal place" balance (measuring to \( 0.01 \text{ } g \)) is standard for most AS experiments, like weighing out a solid for a volumetric solution.

Key Takeaway: Always choose the equipment with the smallest uncertainty that is practical for the task. (See the "Data handling and uncertainties" chapter for more on this!)

3. Designing a Step-by-Step Method

When the exam asks you to "describe a method," they want a logical, "recipe-style" list. Don't worry if it feels long; just be clear!

The Checklist for a Great Method:

1. Quantities: Use specific amounts (e.g., "Measure \( 50.0 \text{ } cm^3 \) of \( 1.0 \text{ } mol \cdot dm^{-3} \text{ } HCl \)").
2. Apparatus: Name the equipment (e.g., "Use a polystyrene cup for calorimetry to minimize heat loss").
3. The "Action": What do you actually do? (e.g., "Add the zinc powder and stir immediately").
4. Measurement: What are you recording? (e.g., "Record the maximum temperature reached").
5. Repetition: Mention that you should repeat the experiment to identify anomalies and calculate a mean.

4. Risks and Safety

No chemistry plan is complete without considering safety. You don't need to be an expert on every chemical, but you should know the basics for the AS required practicals:

  • Corrosive acids/alkalis: Wear safety goggles and gloves.
  • Flammable liquids (like alcohols): Keep away from naked flames; use a water bath for heating instead of a Bunsen burner.
  • Toxic gases: Use a fume cupboard (especially important in some organic tests or Group 7 reactions).

5. Recording Data and Units

Once you have a plan, you need a way to record the results. This is almost always a results table.
Rules for tables:

  • The Independent Variable usually goes in the first column.
  • The Dependent Variable goes in the next columns.
  • Units should be in the column headings only, using a solidus (e.g., \( Time / s \) or \( Temperature / ^{\circ}C \)).
  • Consistency: If you record one mass as \( 5.00 \text{ } g \), record the next as \( 5.10 \text{ } g \), not \( 5.1 \text{ } g \). Keep the decimal places the same!

Summary: The "Golden Rules" of Planning

1. Be Specific: Don't say "measure some water," say "measure \( 25.0 \text{ } cm^3 \) of distilled water using a measuring cylinder."
2. Change One, Keep the Rest: Only change the independent variable. Keep everything else (the control variables) the same.
3. Safety First: Always mention goggles or a specific precaution if heating flammable substances.
4. Check the Syllabus: This planning skill applies directly to your Required Practicals (RP1-5), such as making a volumetric solution or measuring an enthalpy change.

Don't worry if this seems like a lot to remember! Experiment planning is a skill that grows with practice. Every time you look at a new practical, ask yourself: "What am I changing, what am I measuring, and how can I make this a fair test?"