Independent Variable (IV) — the variable you deliberately change in an experiment. There is only one IV per experiment. It is what you are testing the effect of.
Dependent Variable (DV) — the variable you measure or observe as a result of changing the IV. It depends on what you do to the IV.
Control Variables (CV) — all other variables that could affect the DV, which you keep the same throughout the experiment to make it a fair test.
Memory tip: Ask yourself three questions:
→ What am I changing? = Independent Variable
→ What am I measuring? = Dependent Variable
→ What must I keep the same? = Control Variables
Section A: Identify the Variables
Read each experiment description carefully. Identify the independent variable, dependent variable, and at least two control variables.
1. A student investigates how light intensity affects the rate of photosynthesis in pondweed. They place the pondweed at five different distances from a lamp and count the number of oxygen bubbles produced per minute at each distance. The experiment is carried out in a beaker of water at room temperature. [4]
| Variable type |
Your answer |
| Independent variable |
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| Dependent variable |
|
| Control variable 1 |
|
| Control variable 2 |
|
2. A student tests the effect of temperature on the activity of the enzyme amylase. They mix amylase with starch solution in test tubes placed in water baths at 10 °C, 20 °C, 30 °C, 40 °C and 50 °C. Every 30 seconds they take a drop of the mixture and add it to iodine solution to test for starch. They record the time taken for the starch to completely disappear. [4]
| Variable type |
Your answer |
| Independent variable |
|
| Dependent variable |
|
| Control variable 1 |
|
| Control variable 2 |
|
3. A class investigates whether the concentration of salt solution affects the mass of potato cylinders. They cut potato cylinders of equal size and place each one in a different salt solution (0%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0%). After 30 minutes, they remove and dry the cylinders and record their new mass. [4]
| Variable type |
Your answer |
| Independent variable |
|
| Dependent variable |
|
| Control variable 1 |
|
| Control variable 2 |
|
4. A scientist tests whether a new fertiliser increases the height of tomato plants. She grows 20 plants with the fertiliser and 20 without, keeping all other conditions identical. After 6 weeks she measures the height of each plant and calculates the mean height for each group. [4]
| Variable type |
Your answer |
| Independent variable |
|
| Dependent variable |
|
| Control variable 1 |
|
| Control variable 2 |
|
5. A student wants to know whether exercise intensity affects breathing rate. Volunteers walk, jog, and sprint on a treadmill for 3 minutes each. After each activity, the student counts the number of breaths per minute. [4]
| Variable type |
Your answer |
| Independent variable |
|
| Dependent variable |
|
| Control variable 1 |
|
| Control variable 2 |
|
Section B: Design the Experiment
You are given the variables. Describe a suitable experiment to investigate them. Your description should include what you would do, what you would measure, and how you would make it a fair test.
6. Independent variable: pH of the solution Dependent variable: rate of enzyme activity (time for colour change in a pH indicator)
Describe an experiment to investigate this. Include how you would control variables to make it fair. [5]
7. Independent variable: carbon dioxide concentration in the air Dependent variable: rate of photosynthesis
Describe an experiment to investigate this. Include the equipment you would use and two variables you must control. [5]
8. Independent variable: type of surface (rough/smooth)
Dependent variable: number of woodlice found on each surface after 10 minutes
Describe an experiment to investigate this. Suggest a suitable piece of equipment and explain why the number of woodlice used matters. [5]
Section C: Spot the Mistake
Each experiment below has a problem with variable control. Identify the mistake and explain why it makes the results unreliable.
9. A student investigates the effect of light colour on plant growth. She grows one plant under red light and another under blue light. After two weeks, the plant under blue light is taller. She concludes that blue light causes faster growth. State one problem with this experiment. [2]
10. A student tests whether adding sugar to water affects how quickly yeast produces CO₂. He uses 1 g of yeast with 5 g of sugar in 100 cm³ of water at 30 °C, and 2 g of yeast with no sugar in 100 cm³ of water at 25 °C. He measures the volume of CO₂ produced in 5 minutes.
(a) Identify two variables that were not controlled. [2]
(b) Explain why having two uncontrolled variables makes it impossible to draw a valid conclusion. [2]
Section D: Stretch & Challenge
11. A student claims: "I changed the temperature and the pH at the same time to save time. My results still show enzyme activity went down, so my conclusion is valid."
Evaluate this claim. Explain why changing two variables at once undermines the validity of a conclusion, even if the result seems clear. [3]
12. A student designs an experiment to test whether music helps plants grow faster. Suggest a suitable independent variable, dependent variable, and three control variables they should include. Then suggest one way to make their results more reliable. [5]
| Variable type |
Your answer |
| Independent variable |
|
| Dependent variable |
|
| Control variable 1 |
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| Control variable 2 |
|
| Control variable 3 |
|
How would you make the results more reliable?
Total marks: 49
Mark Scheme
1.
IV: distance from the lamp (light intensity) [1]
DV: number of oxygen bubbles produced per minute [1]
CV (any two): temperature of water; volume/concentration of CO₂ in the water; species/size of pondweed; duration of counting [1 each, max 2]
2.
IV: temperature (°C) [1]
DV: time taken for starch to completely disappear / time for iodine to stop turning blue-black [1]
CV (any two): pH; concentration of amylase; concentration of starch solution; volume of solutions used [1 each, max 2]
3.
IV: concentration of salt solution [1]
DV: change in mass of potato cylinder (or percentage change in mass) [1]
CV (any two): size/mass of potato cylinders at the start; volume of solution; temperature; time in solution; species/type of potato [1 each, max 2]
4.
IV: presence or absence of the new fertiliser [1]
DV: height of tomato plants after 6 weeks [1]
CV (any two): amount of water given; light intensity/hours of light; temperature; type of soil/compost; age/size of seedlings at the start [1 each, max 2]
5.
IV: exercise intensity (walking / jogging / sprinting) [1]
DV: breathing rate (breaths per minute) [1]
CV (any two): duration of exercise; same treadmill speed for each intensity category; age/fitness of volunteers; rest period between activities; time of day [1 each, max 2]
6. (5 marks — award for each of the following points up to 5):
• Use the same enzyme and substrate (e.g. protease and casein, or amylase and starch) at the same concentration [1]
• Prepare a range of pH values (e.g. pH 4, 5, 6, 7, 8) using buffer solutions [1]
• Mix equal volumes of enzyme and substrate at each pH in a test tube [1]
• Measure the time for the reaction to complete (e.g. colour change, or clearing of a cloudy solution) [1]
• Keep temperature, enzyme concentration, and substrate concentration the same in all tubes [1]
7. (5 marks — award for each of the following points up to 5):
• Use pondweed (e.g. Elodea) in a beaker of water and count oxygen bubbles per minute as a measure of photosynthesis rate [1]
• Vary CO₂ concentration by adding different amounts of sodium hydrogen carbonate to the water [1]
• Measure bubbles produced per minute (or use a gas syringe to collect gas volume) [1]
• Control variable 1: light intensity — keep lamp at a fixed distance [1]
• Control variable 2: temperature — keep water temperature constant (e.g. using a water bath) [1]
8. (5 marks — award for each of the following points up to 5):
• Use a choice chamber divided into rough and smooth halves [1]
• Place equal numbers of woodlice on each side (e.g. 10 per side / 20 total) [1]
• Count the number of woodlice on each surface after 10 minutes [1]
• The number of woodlice used matters because a larger number gives more reliable results / reduces the effect of chance movements [1]
• Keep light, humidity, and temperature the same on both sides [1]
9.
Only one plant was used per condition / there is no repetition [1]; one plant could be taller due to chance/natural variation so the results are unreliable and the conclusion is not valid [1]
10a. Any two of: mass of yeast used (1 g vs 2 g); temperature of the water (30 °C vs 25 °C) [1 each, max 2]
10b. It is impossible to tell which variable caused the difference in CO₂ production [1]; both variables could have affected the result, so no valid conclusion can be drawn about the effect of sugar [1]
11.
Claim is invalid [1]; because it is impossible to know whether the drop in enzyme activity was caused by the temperature change, the pH change, or both acting together [1]; to draw a valid conclusion you must only change one variable at a time (fair test) [1]
12. (5 marks)
IV: whether music is playing (music vs no music) / type of music / volume of music [1]
DV: height of plant / mass of plant / rate of growth after a set time [1]
CV (any three): light intensity; temperature; volume of water given; type of soil; species/age of plant [1 each, max 3]
Reliability: repeat the experiment with more plants / take repeat readings and calculate a mean [1]