The three types of variable
Every biology experiment involves three types of variable. You must identify and control them clearly:
Independent variable (IV)
The variable you deliberately change. There should only be one. Example: temperature of water bath (10°C, 20°C, 37°C, 45°C, 60°C).
Dependent variable (DV)
The variable you measure or observe as a result of changing the IV. Example: time for starch to be fully digested (iodine stays orange).
Control variables (CV)
All other variables that must be kept constant to make it a fair test. Example: pH, concentration of enzyme, concentration of starch, volume of each solution.
State the expected relationship between IV and DV with a reason. Example: "As temperature increases from 10°C to 37°C, amylase will digest starch faster because enzyme and substrate molecules collide more frequently. Above 37°C, activity will decrease as the enzyme becomes denatured."
Students often say "keep all variables the same" — but the IV must change. Only control variables are kept constant. Be specific: name the control variables, don't just say "keep everything the same."
Writing a method — the 5 essentials
- 1Set up the apparatus: state the volumes, concentrations, and equipment you will use. Be specific — "5 cm³ of 1% starch solution" not just "starch."
- 2State how you vary the IV: list the exact values and how you will control the independent variable (e.g. using a water bath at specific temperatures).
- 3State how you measure the DV: describe precisely how you will record results (e.g. "test a drop with iodine solution on a spotting tile every 30 seconds until the colour no longer turns blue-black").
- 4List control variables: name each one and state how it will be kept constant (e.g. "pH kept at 7 using a phosphate buffer solution").
- 5Repeats: state that you will repeat each temperature at least three times and take a mean to improve reliability.
- IV: distance of lamp from pondweed (or light intensity in lux) [1]
- DV: rate of photosynthesis — measured by counting oxygen bubbles produced per minute, or volume of O₂ collected in a set time [1]
- Control variable 1: temperature (keep constant using a water bath / large water volume to act as heat buffer) [1]
- Control variable 2: CO₂ concentration (add a fixed amount of sodium hydrogen carbonate to water) [1]
- Control variable 3: type/size/health of pondweed used; colour/wavelength of light source [1]
- Cut equal-sized pieces of potato (same surface area/mass) for each temperature — same amount of catalase [1]
- Set up two water baths: one at 20°C and one at 37°C; place equal volumes of the same concentration H₂O₂ in each [1]
- Add potato pieces; collect oxygen gas produced in an inverted measuring cylinder over water (or count bubbles per minute) [1]
- Measure volume of O₂ produced in a set time (e.g. 3 minutes) for each temperature [1]
- Control variables: same concentration of H₂O₂, same volume of H₂O₂, same size/mass of potato, same pH [1]
Designing a results table
- 1Column headings: IV in the first column, DV (and repeats) in subsequent columns. Final column = mean. Always include units in the heading (e.g. "Temperature / °C").
- 2Repeats: at least 3 repeats per condition for reliability. Calculate the mean. Exclude anomalous results from the mean — identify and circle them.
- 3Decimal places: be consistent. If measuring to 0.1 cm³, all values should be recorded to 1 d.p.
Plot IV on x-axis, DV on y-axis. Use a sharp pencil, mark points with a small × or ●. Draw a line of best fit (smooth curve or straight line as appropriate — do not join dot to dot). Label both axes with quantity and units. Give the graph a title. Use most of the available grid space (choose a scale carefully).
1. Forgetting units on axes. 2. Drawing a dot-to-dot (jagged) line instead of a smooth best-fit curve. 3. Plotting IV on the y-axis. 4. Choosing a scale that leaves most of the grid empty.
| Wind speed / km h⁻¹ | Trial 1 / mm h⁻¹ | Trial 2 / mm h⁻¹ | Trial 3 / mm h⁻¹ |
|---|---|---|---|
| 0 | 2.1 | 2.3 | 2.2 |
| 5 | 3.8 | 3.9 | 3.7 |
| 10 | 5.6 | 5.4 | 5.5 |
| 15 | 7.2 | 3.1 | 7.4 |
| 20 | 8.8 | 8.9 | 8.7 |
- Anomalous result: 3.1 mm/h at 15 km/h wind speed (far below the other two trials of 7.2 and 7.4) [1]
- Corrected mean (excluding anomaly): (7.2 + 7.4) ÷ 2 = 7.3 mm/h [1]
- Trend: as wind speed increases, the rate of transpiration increases [1]
- The relationship is approximately linear / directly proportional (the rate roughly doubles as wind speed doubles from 5 to 20 km/h) [1]
Key evaluation vocabulary
Reliability
The experiment gives consistent results when repeated. Improved by: more repeats, larger sample size, using precise instruments.
Validity
The experiment actually measures what it claims to measure. Ensured by: controlling all variables, using appropriate method, testing only one variable at a time.
Accuracy
How close results are to the true value. Improved by: using calibrated instruments, reducing systematic error, using a more precise measuring technique.
Anomalous result
A result that doesn't fit the pattern. Should be identified, circled/excluded from the mean, and the cause investigated before repeating that measurement.
- Limitation 1: Bubbles vary in size so counting bubbles does not accurately measure volume of O₂ produced [1] → Improvement: collect gas in an inverted measuring cylinder and measure volume of gas over a fixed time [1]
- Limitation 2: Counting is subjective and difficult at high rates; observer error (miscounting) is likely [1] → Improvement: use a data logger with an oxygen electrode to measure O₂ concentration automatically [1]
- Limitation 3 (also acceptable): Temperature of water may fluctuate with changing lamp distance → use a heat filter between lamp and tank, or monitor temperature continuously [1+1]
- Percentage change accounts for/corrects for differences in the initial length of potato cylinders [1]
- This makes valid comparison possible between cylinders that may not have been cut to exactly the same starting length; it standardises the data [1]
Food tests — full summary
| Substance tested | Reagent / test | Positive result | Negative result |
|---|---|---|---|
| Starch | Iodine solution | Blue-black | Orange-brown (unchanged) |
| Reducing sugars (glucose, maltose) | Benedict's solution + heat (80°C water bath) | Brick-red / orange precipitate | Stays blue |
| Protein | Biuret reagent (NaOH + dilute CuSO₄) | Purple / violet | Stays blue |
| Lipid (fat/oil) | Ethanol emulsion test: dissolve in ethanol, pour into water | Milky white emulsion | Stays clear |
1. Benedict's test requires heating — cold Benedict's won't change colour even with glucose present. 2. Sucrose (non-reducing sugar) does NOT give a positive Benedict's test directly — it must be hydrolysed with HCl first. 3. Biuret reagent is added to the FOOD SAMPLE, not heated. 4. Iodine is orange-brown in the negative (no starch) — not colourless.
- Present: starch (positive iodine test → blue-black) [1]
- Present: protein (positive Biuret test → purple) [1]
- Absent: reducing sugars such as glucose (negative Benedict's test → no colour change) [1]
- Absent: lipid/fat (negative ethanol emulsion test → no white emulsion) [1]
Standard osmosis investigation with potato
- 1Cut potato cylinders of equal length (e.g. 5 cm) and equal diameter using a cork borer. Blot dry and record initial mass/length.
- 2Place cylinders in solutions of different sucrose concentration (e.g. 0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³) for 30 minutes.
- 3Remove, blot dry, record final mass/length. Calculate percentage change: % change = (final − initial) / initial × 100.
- 4Plot % change in mass (y-axis) against sucrose concentration (x-axis). The x-intercept (where the line crosses zero) = the sucrose concentration equal to the water potential of the potato cells.
Positive % change = mass increased = water entered by osmosis = external solution had higher water potential (hypotonic to potato). Negative % change = mass decreased = water left by osmosis = external solution had lower water potential (hypertonic to potato). At the x-intercept: no net water movement = solution is isotonic to potato.
- % change = (3.6 − 4.0) / 4.0 × 100 = −0.4 / 4.0 × 100 [1]
- = −10% [1]
- The negative value indicates the potato lost mass; water LEFT the potato by osmosis (the sucrose solution had a lower water potential than the potato cells) [1]