Around 15% of the total marks across all six Edexcel GCSE Combined Science papers (1SC0) assess your knowledge of practical procedures. You do not sit a separate practical exam, but questions about core practicals appear on the written papers. This lesson covers the key Biology core practicals you must know.
What Examiners Test
For every core practical, you may be asked about:
The method and apparatus used.
Variables: independent (IV), dependent (DV), control variables (CVs).
How to make results reliable (repeats, calculating means).
How to make the test valid (controlling variables).
Sources of error and how to reduce them.
How to process and present results (tables, graphs).
Conclusions and whether the data supports a hypothesis.
Safety precautions.
Exam Tip: Learn the structure above as a checklist. For any practical, you should be able to talk about method, variables, reliability, validity, errors, results and safety.
Core Practical 1: Investigating Osmosis in Potato Chips
Aim
To investigate the effect of concentration of sucrose solution on the mass of potato chips (and therefore osmosis).
Method
Cut potato into chips of equal size (e.g. 5 cm × 1 cm × 1 cm) — this is a control variable.
Blot each chip with a paper towel and weigh using a balance (record initial mass).
Place one chip into each of several beakers containing different concentrations of sucrose solution (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³).
Leave for a set time (e.g. 30 minutes) — another control variable.
Remove each chip, blot dry and reweigh.
Calculate the percentage change in mass for each chip.
Variables
Variable type
Variable
Independent variable (IV)
Concentration of sucrose solution
Dependent variable (DV)
Percentage change in mass of potato chip
Control variables
Size of potato chip, volume of solution, time left in solution, temperature, same potato
Key Results and Conclusions
In dilute solutions (lower concentration than potato cells): water moves into the potato by osmosis → mass increases.
In concentrated solutions (higher concentration than potato cells): water moves out of the potato by osmosis → mass decreases.
At the isotonic point: no net movement of water → mass stays roughly the same.
Calculating Percentage Change
Percentage change in mass=initial massfinal mass−initial mass×100
Exam Tip: Always use percentage change, not just the change in grams. This accounts for any small differences in the initial mass of each chip.
Safety
Use a white tile and cork borer or knife carefully when cutting chips.
Handle sucrose solutions with care — mop up any spills to prevent slipping.
Core Practical 2: Investigating the Rate of Photosynthesis
Aim
To investigate the effect of light intensity on the rate of photosynthesis using pondweed (e.g. Elodea or Cabomba).
Method
Place a piece of pondweed in a beaker of water with a small amount of sodium hydrogen carbonate (to provide CO₂).
Position a lamp at a measured distance from the beaker.
Wait 2 minutes for the plant to acclimatise.
Count the number of oxygen bubbles produced in a set time (e.g. 1 minute). Alternatively, collect gas in a syringe and measure volume.
Move the lamp to a different distance and repeat.
Repeat each distance at least three times and calculate a mean.
Variables
Variable type
Variable
Independent variable (IV)
Light intensity (changed by varying the distance of the lamp)
Dependent variable (DV)
Number of oxygen bubbles per minute (or volume of gas)
Control variables
Temperature, concentration of CO₂ (NaHCO₃), type and length of pondweed, colour of light
Key Results and Conclusions
As light intensity increases, the rate of photosynthesis increases (more bubbles per minute).
At high light intensities, the rate plateaus (levels off) because another factor (e.g. CO₂ concentration or temperature) becomes limiting.
Light Intensity and Distance
Light intensity is inversely proportional to the square of the distance:
Light intensity∝d21
So if you double the distance, the light intensity drops to a quarter.
Exam Tip: If asked to plot a graph, plot rate of photosynthesis against 1/d² (not just distance) to get a more linear relationship.
Safety
Do not touch the lamp when hot.
Handle glassware with care.
Wash hands after handling pondweed.
Core Practical 3: Investigating Reaction Time
Aim
To investigate the effect of a factor (e.g. caffeine, practice or distraction) on human reaction time using the ruler-drop test.
Method
The subject sits with their arm on the edge of a table, thumb and forefinger open.
The tester holds a 30 cm ruler with the zero mark at the level of the subject's fingers.
The tester drops the ruler without warning.
The subject catches the ruler as quickly as possible.
Record the distance the ruler fell (in cm) from the position of the top of the thumb.
Repeat at least 5 times and calculate the mean distance.
Convert distance to reaction time using the formula: t=g2d (where d is distance in m and g = 9.8 m/s²).
Variables
Variable type
Variable
Independent variable (IV)
The factor being tested (e.g. with or without caffeine)
Dependent variable (DV)
Distance the ruler falls (converted to reaction time)
Control variables
Same ruler, same hand, same person (ideally), same time of day, same instructions
Key Results and Conclusions
A shorter distance caught means a faster reaction time.
Caffeine may decrease reaction time (faster reactions).
Distractions may increase reaction time (slower reactions).
Improving Reliability
Take at least 5 repeats and remove obvious anomalies before calculating the mean.
Use the same person where possible to reduce variation due to individual differences.
Ensure the tester does not give any cues (verbal or visual) before dropping.
Exam Tip: The ruler-drop test measures voluntary reaction time (not reflex time). This is an important distinction if asked.