Introduction to Planning and Experimental Design

Welcome to one of the most important parts of your A Level Chemistry course! Planning isn't just about following a recipe; it's about being the architect of an experiment. In your OCR Chemistry B (Salters) exams—especially Component 03 (Practical skills in chemistry)—you will often be asked to design an experiment from scratch or suggest improvements to an existing one. Don't worry if this feels overwhelming at first; once you learn the logical steps of experimental design, it becomes much easier to tackle even the most unfamiliar chemical contexts.

1. Identifying the Aim and Scientific Context

Every good plan starts with a clear goal. In the Salters course, this is often linked to a Storyline module. For example, you might be asked to plan an experiment to find the enthalpy change of a fuel (like in Developing Fuels) or determine the concentration of a household bleach (like in Elements from the Sea).

Step 1: Identify what you are trying to find (the quantity).
Step 2: Identify the chemical reaction involved (write a balanced equation if possible).

2. Managing Variables: The "Fair Test" Logic

To get valid results, you must understand your variables. Think of this like adjusting the settings on a phone—you only change one thing at a time to see what happens.

  • Independent Variable: The factor you intentionally change (e.g., the concentration of \(HCl\)).
  • Dependent Variable: The factor you measure to see the effect (e.g., the volume of \(H_2\) gas produced).
  • Control Variables: Factors you must keep constant so they don't interfere with your results (e.g., temperature, total volume of solution, or the surface area of a solid catalyst).

Quick Tip: When asked to "identify variables," always specify how you will control them. Don't just say "temperature"; say "keep the temperature constant using a thermostatically controlled water bath."

3. Selecting the Right Apparatus and Techniques

Choosing the right tool for the job is vital for accuracy. The syllabus requires you to select equipment based on the quantities you are dealing with.

Measuring Mass and Volume

  • Mass: Use a balance, usually to \(2\) or \(3\) decimal places. Always record the mass "by difference" to improve accuracy.
  • Liquids:
    • For fixed, highly accurate volumes (e.g., \(25.0\text{ cm}^3\)), use a pipette.
    • For variable accurate volumes (e.g., adding an acid until a reaction stops), use a burette.
    • For approximate volumes where high precision isn't required, a measuring cylinder is fine.
  • Gases: Use a gas syringe for high precision, or an inverted measuring cylinder over water if the gas is not soluble in water.

Heating and Temperature

  • Direct Heating: Use a Bunsen burner for stable, non-flammable substances.
  • Safe Heating: If you are using flammable organic liquids (like in What's in a Medicine?), use an electric heater or a water bath to avoid naked flames.
  • Reflux and Distillation: Use these for organic synthesis to prevent volatile reactants or products from escaping.

Note: For more details on using this equipment, see the "Apparatus, techniques and implementing" chapter.

4. Identifying Hazards and Minimising Risk

Safety is a huge part of experimental design. You must distinguish between a hazard and a risk.

  • Hazard: An intrinsic property of a substance (e.g., \(NaOH\) is corrosive; ethanol is flammable).
  • Risk: How the hazard could cause harm in your specific experiment (e.g., "ethanol could catch fire if heated with a Bunsen burner").
  • Control Measure: How you reduce the risk (e.g., "wear gloves and goggles," "use a fume cupboard for toxic gases," or "use a water bath instead of a flame").

5. Designing a Robust Procedure

When writing a method, imagine you are writing instructions for someone who has never done chemistry before. It needs to be logical and chronological.

Key elements to include:
  1. Quantities: Use specific amounts (e.g., "measure \(50.0\text{ cm}^3\) of \(1.0\text{ mol dm}^{-3}\text{ HCl}\)").
  2. Range and Intervals: If you are investigating the effect of concentration on rate, state the range you will test (e.g., \(0.2\) to \(1.0\text{ mol dm}^{-3}\)) and the intervals (e.g., "every \(0.2\text{ mol dm}^{-3}\)").
  3. Repeatability: Always state that you will repeat the experiment (usually until you have concordant results in titrations or at least three times to identify anomalies and calculate a mean).

6. Evaluating the Method

Before starting, you must ask: Is this method appropriate to meet the expected outcomes?

Precision: Are the instruments sensitive enough? (e.g., using a thermometer that reads to \(0.1 \text{ } ^\circ\text{C}\) rather than \(1 \text{ } ^\circ\text{C}\)).
Accuracy: Does the method minimize systematic errors? (e.g., adding a lid to a calorimeter to prevent heat loss in Developing Fuels experiments).

Summary Key Takeaway: A perfect experimental plan identifies the variables, selects precise apparatus, accounts for safety with specific control measures, and includes repeats to ensure the data is reliable.

For information on how to process the data you collect, refer to the chapter on "Analysis, graphs and significant figures." To understand how to calculate the errors in your chosen apparatus, see "Evaluation, errors and uncertainties."