Unit 3: Planning an Experiment and Identifying Variables
Welcome to the "blueprint" stage of Biology! Before a scientist ever picks up a test tube, they must have a crystal-clear plan. In your Unit 3 (WBI13) exam, you won't just be asked what happened in an experiment; you will be asked how to design one from scratch. Think of this chapter as learning how to be the architect of a scientific investigation.
Planning is all about making sure your results are valid (meaning you are actually measuring what you think you are) and reliable (meaning someone else could do the same thing and get the same results). Let's break down the essential steps for a perfect experimental plan.
1. Identifying Your Variables
In every experiment, there are three types of variables you must identify. If you get these mixed up, your whole experiment might fail to show a clear relationship!
The Independent Variable (IV)
This is the factor that you change or select. It is the "cause." For example, if you are investigating the effect of temperature on enzyme activity (Core Practical 4), the temperature is your IV.
- Pro Tip: When planning, always state the range of the IV. For temperature, you might say: "I will use five different temperatures: \(20^\circ\text{C}\), \(30^\circ\text{C}\), \(40^\circ\text{C}\), \(50^\circ\text{C}\), and \(60^\circ\text{C}\)."
The Dependent Variable (DV)
This is what you measure as a result of changing the IV. It is the "effect" or the "data." In the enzyme experiment, the DV might be the volume of oxygen gas produced per minute.
- Memory Aid: Dependent variable = Data collected.
Controlled (Standardised) Variables
These are all the other factors that could affect your DV. To make it a fair test, you must keep these constant (the same). If you were testing temperature but accidentally changed the enzyme concentration halfway through, you wouldn't know which one caused the change in your results!
- Common Controlled Variables: pH (controlled using a buffer solution), volume of reagents, concentration of reactants, and time.
Key Takeaway: Change only one thing (IV), measure one thing (DV), and keep everything else the same (CVs).
2. Choosing the Right Apparatus
Choosing the correct equipment is vital for accuracy. You need to select tools that have the right resolution (the smallest change an instrument can detect).
- For Volume: Don't just say "a beaker." Beakers are for holding liquids, not measuring them accurately. Use a measuring cylinder, or better yet, a graduated pipette or a syringe for small, precise volumes like \(5.0 \text{ cm}^3\).
- For Temperature: Use a thermometer, but to keep the temperature constant, use a thermostatically controlled water bath.
- For Time: A stopwatch measuring in seconds (\(\text{s}\)).
- For Mass: A digital balance measuring in grams (\(\text{g}\)) or milligrams (\(\text{mg}\)).
Quick Review: If you need to measure \(0.5 \text{ cm}^3\) of a liquid, a \(10 \text{ cm}^3\) measuring cylinder is a poor choice because the scale is too large. A \(1 \text{ cm}^3\) syringe would be much more accurate!
3. Measuring Techniques and Accuracy
It’s not just about what you use, but how you use it. To reduce errors:
Avoiding Parallax Error: When reading a volume in a pipette or cylinder, always look at the meniscus (the curve of the liquid) at eye level. Looking from above or below will give you a false reading.
Using a Microscope: In Core Practical 5 and 7, you use a graticule (a tiny ruler inside the eyepiece) to measure the size of cells. You must calibrate this using a stage micrometer to ensure your measurements are real units (like micrometres, \(\mu\text{m}\)) rather than just "arbitrary units."
4. Repeats and Reliability
Why do we never do an experiment just once? Because anomalies happen! An anomaly is a result that doesn't fit the trend, often caused by a random error.
The Plan: You should always state that you will repeat the experiment at least three times (or more) for each value of your Independent Variable.
The Benefit:
1. It allows you to identify and ignore anomalous results.
2. It allows you to calculate a mean (average).
3. It makes your results more reliable.
Note: For more on calculating means and identifying errors, see the chapter "Processing Data" and "Evaluating Methods."
5. Health and Safety (Risk Assessment)
In Biology, "Safety" isn't just a box-ticking exercise; it's about identifying Hazards and managing Risks.
- Hazard: Something with the potential to cause harm (e.g., hydrochloric acid is corrosive).
- Risk: The likelihood of that harm happening (e.g., acid splashing into eyes during pouring).
- Control Measure: How you stop the harm (e.g., wear safety goggles).
Common Biological Safety Examples:
Aseptic Technique (Core Practical 9): When working with bacteria, you must work near a Bunsen burner flame to create an upward current of sterile air, preventing contamination. You also disinfect surfaces and flame the mouths of test tubes.
Ethical Safety: While Unit 6 covers complex ethics, for Unit 3, remember to handle living organisms (like plant fibres in Core Practical 8) with respect and dispose of biological waste correctly.
6. Dealing with Uncertainty and Error
No measurement is perfect. You need to identify where "wobbles" in your data might come from.
Systematic Errors: These are errors that are the same every time you take a measurement. For example, if your digital balance isn't "zeroed" correctly, every mass you record will be exactly \(0.1 \text{ g}\) too heavy.
Solution: Check and calibrate your equipment before starting.
Random Errors: These are unpredictable. Maybe you reacted a split-second slower when hitting the stopwatch.
Solution: Take repeat readings and calculate a mean to "cancel out" the effect of these errors.
Did you know? Using a larger range of the Independent Variable (e.g., \(0^\circ\text{C}\) to \(100^\circ\text{C}\) instead of just \(20^\circ\text{C}\) to \(40^\circ\text{C}\)) helps you see the overall trend more clearly and reduces the impact of small measurement uncertainties!
Summary Checklist for an Exam "Plan" Question
If you are asked to "Devise an investigation" or "Plan an experiment," ensure your answer includes:
- The Independent Variable and the range you will use.
- The Dependent Variable and exactly how you will measure it (instrument + units).
- At least three Controlled Variables and how you will keep them constant (e.g., "Use a buffer to keep pH at 7").
- A mention of repeats to calculate a mean.
- Specific Health and Safety precautions related to the chemicals or organisms used.
- A brief mention of a Control Experiment (a setup where the IV is removed to prove that the IV is what's causing the change).