Designing Experiments (Copy)

Designing Experiments – Expanded Section (O Level & IGCSE Biology Alternate to Practical)

Key Components of an Experimental Design Question

1. Aim / Hypothesis
   • Clearly state what you are investigating.
   • Should include both variables.
   • Example: “To investigate the effect of temperature on the rate of photosynthesis in pondweed.”
2. Independent Variable (what you change)
   • Only one variable should be changed at a time.
   • Examples: Temperature, pH, light intensity, sugar concentration, etc.
3. Dependent Variable (what you measure)
   • Must be measurable and observable.
   • Examples: Height of seedlings, number of bubbles produced, time taken to decolorize solution.
4. Controlled Variables (what you keep constant)
   • All other variables must be kept the same to ensure a fair test.
   • Examples: Volume of solution, same type of organism, same duration of experiment.
5. Control Experiment
   • A parallel setup without the independent variable.
   • Shows that any change observed is due to the variable tested.
   • Example: Boiled enzyme or dead seeds as control.
6. Apparatus and Method
   • List exact quantities, temperatures, time intervals.
   • Write steps in logical, numbered sequence.
   • Don’t skip key actions like mixing, rinsing, waiting, or recording time.
7. Repetition
   • Repeat at least 3 times for each condition to calculate average results and increase reliability.
8. Safety Precautions
   • Include relevant safety steps.
   • Examples:
     • Wear goggles when using Benedict’s solution.
     • Use tongs to handle hot apparatus.
     • Avoid direct contact with biological material.

Common Experiment Topics & How to Set Them Up

How to Answer a Design Question Step-by-Step

Example Question: Describe how you would investigate the effect of temperature on enzyme activity.

1. State the Aim:

• To investigate the effect of temperature on the time taken for amylase to break down starch.

2. List Apparatus:

• Test tubes, thermometer, water bath, iodine, starch solution, amylase, stopwatch, pipette.

3. Method Outline:

1. Label five test tubes with temperatures: 20°C, 30°C, 40°C, 50°C, 60°C.
2. Add 5 cm³ starch solution and 1 cm³ amylase to each.
3. Place each in a water bath at the correct temperature for 5 minutes.
4. At every 30 seconds, take a drop and place it on a tile with iodine.
5. Stop when iodine no longer turns blue-black (starch fully broken down).
6. Record time for each temperature.
7. Repeat each test 3 times.

4. Variables:

• Independent: Temperature
• Dependent: Time for starch to disappear
• Controlled: Same volume of enzyme/starch, same pH, same time intervals, same equipment.

5. Control Setup:

• Use boiled amylase at one temperature to ensure no breakdown occurs (proves enzyme is responsible).

6. Safety:

• Handle hot water carefully.
• Use goggles when using iodine.

Marking Scheme Expectations

• 1–2 marks: Clear aim and variables identified
• 1–2 marks: Logical method with apparatus and procedure
• 1 mark: Mention of control
• 1 mark: Safety and repeatability
• 1 mark: Measurement and data collection

Common Errors to Avoid

• Changing multiple variables at once.
• Not including a control experiment.
• Skipping units (e.g., °C, cm³, minutes).
• No mention of repeat trials or data recording.
• Describing an impractical or vague method (e.g., “heat it for some time”).

Tips for Fast and Accurate Answers

• Memorize 2–3 standard setups (enzyme test, osmosis, germination).
• Use keywords: repeat, control, accurate, same volume, reliable, fair test.
• Practice converting any variable → design method in 5 mins or less.