Planning an Experiment (Unit 3)

Welcome to Unit 3: Becoming the Scientist

In your Biology IAS journey, Unit 3 (Practical Skills in Biology I) is where you move from just learning facts to understanding how those facts are discovered. This unit isn't just about memorizing experiments; it’s about learning the logic behind them. Whether you are looking at how temperature affects enzymes or testing the vitamin C in juice, the way you plan the experiment follows the same "scientific blueprint."

Don’t worry if planning seems a bit overwhelming at first! Think of it like writing a recipe: you need the right ingredients (apparatus), the right steps (method), and a way to make sure it tastes the same every time (controlling variables).

1. The Three Pillars: Independent, Dependent, and Controlled Variables

To plan a successful experiment, you must be very clear about your variables. Think of these as the "moving parts" of your investigation.

The Independent Variable (IV) — "The One YOU Change"

This is the factor you are investigating. For example, if you are testing how temperature affects enzyme activity, temperature is your IV.
Top Tip for Planning: You should always aim for at least five different values (or levels) for your IV. If you only test two temperatures, you can't really see a trend! For example, test at \(10^{\circ}C\), \(20^{\circ}C\), \(30^{\circ}C\), \(40^{\circ}C\), and \(50^{\circ}C\).

The Dependent Variable (DV) — "The Data You Collect"

This is what changes because of your IV. It is what you measure. In the enzyme experiment, the DV might be the volume of oxygen gas produced in \(cm^{3}\).
Top Tip for Planning: Always state how you will measure it and what equipment you will use. Don't just say "measure the gas"; say "measure the volume of gas using a gas syringe over a period of 5 minutes."

Controlled Variables (CV) — "The Keep-it-Fair Factors"

These are all the other things that could affect your results. If you don't keep these the same, you won't know if your IV caused the change or if something else did. This makes your experiment valid.
Example: If testing enzymes, you must keep the concentration of the enzyme, the concentration of the substrate, and the pH the same in every single test.
Key Takeaway: For every CV you mention, explain how you will keep it the same (e.g., "Use a buffer solution to maintain a constant pH of \(7.0\)").

2. Choosing Your Tools (Apparatus and Techniques)

In Unit 3, you need to identify the correct equipment for the job. Using the right tool reduces uncertainty (the doubt about a measurement).

• For Volume: Use a measuring cylinder or, for better precision, a graduated pipette or a syringe.
• For Temperature: Use a thermostatically controlled water bath rather than just a beaker of hot water, as it keeps the temperature steady.
• For Time: Use a stopwatch with a resolution of at least \(0.01\) seconds.
• For Color: Instead of just "looking" at a color change (which is subjective), suggest using color standards (like in Core Practical 1) or a colorimeter to get quantitative data.

3. Writing the Method: Step-by-Step

When you describe your measurement technique, imagine you are writing instructions for someone who has never done biology before. Be logical and sequential.

Step 1: Set up. Describe how to prepare your materials (e.g., "Cut five \(1cm^{3}\) cubes of beetroot using a cork borer").
Step 2: Apply the IV. Explain how you change the independent variable.
Step 3: Control the CVs. State how you ensure everything else stays constant.
Step 4: Measure the DV. Describe exactly how and when you take your reading.
Step 5: Repeats. Always state that you will repeat the experiment at least three times at each level of the IV. This allows you to calculate a mean and identify anomalous results (results that don't fit the pattern).

4. Safety First (Health and Safety)

Every plan needs a risk assessment. You should identify a hazard, the risk it poses, and a control measure to stay safe.

Example 1 (Chemical): Ethanol is flammable (hazard). It could catch fire if exposed to a naked flame (risk). Control: Use a water bath to heat it instead of a Bunsen burner.
Example 2 (Biological): Bacteria (hazard) can cause infection (risk). Control: Use aseptic techniques, such as flaming the neck of the bottle and working near a blue Bunsen flame (as seen in Core Practical 9).

5. Dealing with Data and Errors

Part of your plan should involve how you will process the numbers you collect.

Mean, Median, and Mode: Know how to calculate these to find the "average" of your repeats.
Range and Standard Deviation: These help you see how "spread out" your data is. A small range usually means your results are precise.
Sources of Error:
• Systematic Error: This is when your equipment is consistently "off" (e.g., a balance that isn't zeroed).
• Random Error: These are unpredictable differences caused by things like human reaction time or slight fluctuations in room temperature.

6. Context: The Core Practicals

Your Unit 3 exam will ask you to apply these planning skills to the 9 IAS Core Practicals you have studied. Whether you are asked about Vitamin C content (Practical 2) or tensile strength of plant fibers (Practical 8), the planning principles remain the same: What am I changing? What am I measuring? What am I keeping the same? How do I stay safe?

Note: For more details on the specific steps of these experiments, please see the chapter "IAS Core Practicals 1-9". For details on how to draw graphs and calculate percentage error, see "Processing Results, Graphs and Uncertainties".

Quick Review: The Planning Checklist

Before you finish an exam answer on "designing an experiment," check if you have included:
1. A clear Independent Variable with at least 5 levels.
2. A clear Dependent Variable with specific units (e.g., \(mg\), \(cm^{3}\), \(s\)).
3. At least two Controlled Variables and how to control them.
4. A mention of repeats (to calculate a mean).
5. Specific apparatus (e.g., "water bath" instead of "heat").
6. A Safety precaution relevant to the experiment.

Key Takeaway: A good plan is valid (it tests what it's supposed to) and reliable (it can be repeated to get the same results).