Core Practical: Investigating Osmosis

This lesson provides a detailed guide to the required practical on osmosis as required by the Edexcel GCSE Combined Science specification (1SC0). You need to be able to plan, carry out and evaluate this investigation, including identifying variables, recording results, calculating percentage change in mass and drawing conclusions.

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Aim

To investigate the effect of different concentrations of sucrose solution on the mass of potato cylinders, and to determine the concentration at which there is no net movement of water (the isotonic point).

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Apparatus

You will need the following equipment:

• Potato
• Cork borer (to cut uniform cylinders)
• Ruler (to measure cylinder length)
• Scalpel and cutting tile (to trim cylinders to equal length)
• Balance (accurate to at least 0.01 g)
• Paper towels (for blotting)
• 6 boiling tubes (or beakers)
• Boiling tube rack
• Sucrose solutions: 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M
• Measuring cylinder (to measure solution volume)
• Labels or marker pen
• Timer / clock

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Method

Step-by-Step Procedure

1. Use the cork borer to cut 6 potato cylinders from the same potato. This ensures they all have the same diameter.
2. Use a ruler and scalpel to trim each cylinder to exactly the same length (e.g. 3 cm). Cut cleanly on a cutting tile.
3. Blot each cylinder gently with a paper towel to remove surface moisture.
4. Weigh each cylinder on the balance and record the initial mass in a results table.
5. Label 6 boiling tubes with the sucrose concentrations: 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M.
6. Add 10 cm³ of the appropriate sucrose solution to each boiling tube using a measuring cylinder.
7. Place one potato cylinder into each boiling tube.
8. Leave for at least 24 hours (or a minimum of 30 minutes for shorter investigations) at room temperature.
9. Remove each potato cylinder, blot it dry with a paper towel, and reweigh. Record the final mass.
10. Calculate the percentage change in mass for each cylinder.
11. Repeat the entire experiment at least twice more (3 repeats total) and calculate a mean percentage change for each concentration.

Exam Tip: Always blot the potato cylinders dry before weighing — both at the start and at the end. Excess surface water will affect the mass readings and make your results inaccurate.

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Variables

Variable Type	Variable	How Controlled
Independent	Concentration of sucrose solution	Use pre-made solutions of known concentration
Dependent	Percentage change in mass of potato cylinder	Measured using a balance before and after
Control: length	Length of potato cylinders	Cut to the same length using a ruler
Control: diameter	Diameter of potato cylinders	Use the same cork borer for all
Control: volume	Volume of sucrose solution	Measure with a measuring cylinder (e.g. 10 cm³)
Control: time	Time left in solution	Use the same duration for all tubes
Control: temperature	Temperature of surroundings	Keep all tubes in the same location
Control: potato type	Source of potato	Cut all cylinders from the same potato

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Recording Results

Sample Results Table

Sucrose Concentration (M)	Initial Mass (g)	Final Mass (g)	Change in Mass (g)	Percentage Change (%)
0.0	2.50	2.90	+0.40	+16.0
0.2	2.48	2.72	+0.24	+9.7
0.4	2.52	2.52	0.00	0.0
0.6	2.50	2.30	−0.20	−8.0
0.8	2.49	2.18	−0.31	−12.4
1.0	2.51	2.10	−0.41	−16.3

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Calculating Percentage Change

$\text{Percentage change in mass} = \frac{\text{Final mass} - \text{Initial mass}}{\text{Initial mass}} \times 100$Percentage change in mass=Initial massFinal mass−Initial mass​×100

Worked Example

Initial mass = 2.50 g, Final mass = 2.90 g

Percentage change = (2.90 − 2.50) ÷ 2.50 × 100 = 0.40 ÷ 2.50 × 100 = +16.0%

The positive value means the potato gained mass — water entered the cells by osmosis.

Another worked example:

Initial mass = 2.50 g, Final mass = 2.30 g

Percentage change = (2.30 − 2.50) ÷ 2.50 × 100 = −0.20 ÷ 2.50 × 100 = −8.0%

The negative value means the potato lost mass — water left the cells by osmosis.

Exam Tip: Always use percentage change (not just change in mass) because the initial masses may differ. Percentage change allows a fair comparison between different cylinders.

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Plotting a Graph

Plot your results as a line graph:

• X-axis: Sucrose concentration (M)
• Y-axis: Mean percentage change in mass (%)
• Plot each point and draw a line of best fit.
• The point where the line crosses the x-axis (0% change) is the isotonic point — where the concentration of the sucrose solution equals the concentration of the cell sap inside the potato cells.

graph LR
    A[0.0 M: +16%] --> B[0.2 M: +10%]
    B --> C[0.4 M: 0% - Isotonic point]
    C --> D[0.6 M: -8%]
    D --> E[0.8 M: -12%]
    E --> F[1.0 M: -16%]

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Interpreting the Results

Sucrose Concentration	What Happens	Explanation
Below isotonic point (dilute)	Mass increases	Solution has higher water potential than the potato cells → water moves in by osmosis
At isotonic point	No mass change	Water potential of solution = water potential of cell sap → no net movement of water
Above isotonic point (concentrated)	Mass decreases	Solution has lower water potential than the potato cells → water moves out by osmosis

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Evaluating the Practical

Sources of Error

Error	How to Reduce It
Surface moisture on potato	Blot thoroughly and consistently
Unequal cylinder sizes	Use the same cork borer and measure with a ruler
Temperature variation	Keep all tubes in the same location
Inaccurate timing	Start and stop all tubes at the same time

Improving Reliability

• Carry out at least 3 repeats for each concentration and calculate a mean.
• Identify and exclude any anomalous results (outliers).

Improving Accuracy

• Use a balance accurate to 0.01 g.
• Use a precise measuring cylinder (or burette) for measuring solution volume.

Exam Tip: If asked how to improve this experiment, suggest: more repeats for reliability, more concentrations (e.g. 0.1 M intervals) for a more accurate isotonic point, or using a more precise balance.

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Summary

• This practical investigates how sucrose concentration affects potato cylinder mass through osmosis.
• Use percentage change in mass for fair comparison: (final − initial) ÷ initial × 100.
• In dilute solutions, potato gains mass (water enters); in concentrated solutions, potato loses mass (water leaves).
• The isotonic point is where percentage change = 0 (no net water movement).
• Control variables: cylinder size, solution volume, time, temperature, potato source.
• Plot results on a graph with concentration on the x-axis and percentage change on the y-axis.
• Improve reliability with repeats; improve accuracy with precise equipment.

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Deeper Look: Mastering the Osmosis Core Practical

This is one of the most commonly examined biology core practicals. Questions often ask you to describe the method, identify variables, explain anomalies or calculate percentage change in mass. Getting the full marks on this practical requires attention to detail at every step — from cutting the potato to plotting the graph.

Why a Cork Borer?

Using a cork borer ensures that every cylinder has the same diameter. A scalpel cut by hand would produce cylinders of unpredictable width, which would introduce a confounding variable — surface area — into the experiment. Surface area affects the rate of osmosis (more SA = faster water movement), so keeping diameter constant is essential.

Why Exactly 3 cm?

The length of the potato cylinder is a control variable. All cylinders must be the same length so they have: