- Cut to the same length
- Blot dry, then weigh
- Same volume in each tube
Launch practical
About 100 minutes
- Set up the bench with the 12 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 36 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. Fig. 1 Top-pan balance reading to 0.01 g, with a draught shield. The dot on the display is the stable sign.
What it shows
percentage change in mass = (final mass − initial mass) ÷ initial mass × 100
Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane. Potato cells in a solution more dilute than their contents take in water and gain mass; in a more concentrated solution they lose water and mass. Where the curve crosses zero there is no net movement: that concentration matches the inside of the cells.
The graph to expect. The percentage change is positive in dilute solutions and negative in concentrated ones. The curve falls steeply through zero and levels off at both ends: the cell wall limits the gain, and the loss levels off as the cells lose most of the water they can.
Key equation for the exam: percentage change = (final mass − initial mass) ÷ initial mass × 100.
Recall: learn it. A loss gives a negative percentage: keep the sign.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Potato | The plant tissue. | — |
| Cork borer and rod | Cuts cylinders of the same diameter. | — |
| Scalpel | Trims the cylinders and cuts off the skin. | — |
| White tile | A safe surface to bore and cut on. | — |
| 30 cm ruler | Measures each cylinder to 30 mm. | 1 mm |
| Sucrose solutions: 0.0 (distilled water), 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³ | The independent variable. | — |
| 10 cm³ measuring cylinder | Measures 10 cm³ of solution into each tube. | 0.2 cm³ |
| 18 boiling tubes in racks, with labels | One tube for each cylinder. | — |
| Cling film | Covers the tubes so water does not evaporate. | — |
| Forceps | Lift the cylinders out of the tubes. | — |
| Paper towels | Blot the cylinders dry before weighing. | — |
| Top-pan balance | Weighs each cylinder. | 0.01 g |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical slightly differently. Your course setting picks one; you can change it before you start.
Why: The potato, the solutions and the weighings are the same; the lengths, the wording and the exam questions follow your board.
Zero (tare) the balance
With nothing on the pan, press the tare button so the display reads 0.00 g.
Why: A balance that does not read zero when it is empty adds the same error to every mass.
Cut on a white tileSafety
Put the potato on the white tile. Push the cork borer straight down into the potato onto the tile, twisting it. Cut with the scalpel blade pointing down at the tile, away from your fingers.
Why: The borer and scalpel are sharp. Pushing down onto a tile, never towards your hand, means a slip hits the tile, not you.
Choose which potato to use
Bore all 18 cylinders with the same cork borer.
Do: Bore every cylinder from one large potato.
Why: Different potatoes (even of the same kind) have different concentrations inside their cells.
Bore the cylinders
Push the cork borer through the potato, twist it out, and push the core out of the borer with the rod. Make enough cores for 18 cylinders.
Why: The same borer gives every cylinder the same diameter.
Cut the skin off the ends
Cut a thin slice off each end of a core that has skin on it.
Why: Potato skin lets water through much more slowly than the flesh, so a piece of skin slows the water moving in or out.
Trim to the same length
Cut each core into cylinders 3.0 cm long with the scalpel, on the tile.
Do: Measure 30 mm with the ruler for every cylinder.
Why: Cylinders of the same size start with about the same mass and surface area.
Label the boiling tubes
Label 18 boiling tubes with the concentration and a letter (A, B, C) and stand them in the racks.
Why: You must know which cylinder went into which solution, and match each final mass to its own starting mass.
Add the solutions
Measure 10 cm³ of each solution into its three tubes with the 10 cm³ measuring cylinder, from distilled water (0.0 mol/dm³) up to 1.0 mol/dm³ sucrose. Read the bottom of the meniscus with your eye level with the mark, and rinse the cylinder between solutions.
Why: The same volume in every tube covers each cylinder. Only the concentration changes: a fair test.
Choose how long to leave the cylinders in the solutions
Leave every cylinder in its solution for the same time.
Do: Leave them overnight (about 20 hours).
Why: Water moves in or out by osmosis until the tissue nearly stops changing. The longer the time, the bigger and clearer the changes.
Take the readings
Three cylinders in each of six sucrose solutions (0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³), weighed before and after about 20 hours.
Repeats: The method repeats each concentration: three cylinders in each solution, and a mean percentage change for each. A mean reduces the effect of random errors (blotting, differences between cylinders) and lets you spot an anomaly. It does not remove a systematic error, such as never blotting.
Make safe and clear upSafety
Put the potato cylinders and the rest of the potato in the bin. Pour the solutions down the sink and rinse the boiling tubes. Wipe the balance pan and the bench dry. Return the cork borer and scalpel (blade down) to the tray, and wash your hands.
While you take readings
Blotting before you weigh
Before each weighing, roll the cylinder once on a paper towel to take off the liquid on its surface.
Do: Roll each cylinder gently once on the paper towel, the same way every time.
Why: Liquid left on the outside is weighed with the potato, so the mass is too high.
Cover the tubes
Cover the top of each boiling tube with cling film before you leave them.
Why: Water evaporates from an open tube, which makes the solution more concentrated.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Sharp cork borer and scalpel | Cuts to the hand if the borer or blade slips | Bore and cut on a white tile, pushing down onto the tile and away from your fingers; carry the scalpel blade down. |
| Water or solution near the electric balance | Electric shock, and damage to the balance | Blot each cylinder before it goes on the pan, wipe up drips at once and keep the tubes away from the balance. |
| Spilt sugar solution | Slipping on a wet floor; a sticky bench | Keep the tubes in their racks and wipe up spills straight away. |
| Glass boiling tubes | Cuts if a tube breaks | Keep tubes in a rack, not loose on the bench; report any breakage and do not pick up broken glass by hand. |
Variables
- Independent: you change
- Concentration of sucrose (sugar) solution in mol/dm³
- Dependent: you measure
- Percentage change in mass of the potato cylinder in %
- Control: you keep the same
- Same potato. All 18 cylinders from one potato.
- Size of each cylinder. The same cork borer and 30 mm long; skin cut off.
- Volume of solution. 10 cm³ in every tube, covering the cylinder.
- Time in the solution. The same for every cylinder (overnight, about 20 hours).
- Temperature. All the tubes in the same room.
- Blotting. Each cylinder blotted the same way before each weighing.
Results from a specimen run
| Concentration of sucrose solution in mol/dm³ | Cylinder | Initial mass in g | Final mass in g | Change in mass in g | Percentage change in mass in % |
|---|---|---|---|---|---|
| 0.0 | A | 2.03 | 2.35 | 0.32 | 15.8 |
| 0.0 | B | 2.07 | 2.42 | 0.35 | 16.9 |
| 0.0 | C | 2.06 | 2.35 | 0.29 | 14.1 |
| 0.2 | A | 2.04 | 2.17 | 0.13 | 6.4 |
| 0.2 | B | 2.09 | 2.22 | 0.13 | 6.2 |
| 0.2 | C | 2.01 | 2.18 | 0.17 | 8.5 |
| 0.4 | A | 2.01 | 1.80 | -0.21 | -10.4 |
| 0.4 | B | 1.98 | 1.77 | -0.21 | -10.6 |
| 0.4 | C | 2.03 | 1.78 | -0.25 | -12.3 |
| 0.6 | A | 2.12 | 1.59 | -0.53 | -25.0 |
| 0.6 | B | 1.99 | 1.50 | -0.49 | -24.6 |
| 0.6 | C | 2.07 | 1.59 | -0.48 | -23.2 |
| 0.8 | A | 2.12 | 1.52 | -0.60 | -28.3 |
| 0.8 | B | 2.07 | 1.52 | -0.55 | -26.6 |
| 0.8 | C | 1.97 | 1.39 | -0.58 | -29.4 |
| 1.0 | A | 2.12 | 1.56 | -0.56 | -26.4 |
| 1.0 | B | 2.11 | 1.48 | -0.63 | -29.9 |
| 1.0 | C | 2.04 | 1.46 | -0.58 | -28.4 |
| Concentration of sucrose solution in mol/dm³ | Mean percentage change in mass in % |
|---|---|
| 0.0 | 15.6 |
| 0.2 | 7.0 |
| 0.4 | -11.1 |
| 0.6 | -24.3 |
| 0.8 | -28.1 |
| 1.0 | -28.2 |
Processing the data
- Percentage change in mass
- -26.4 %
- Mean percentage change
- -11.1 %
- Shape of the graph
- Positive at low concentrations, falling as the concentration rises, crossing zero and levelling off below zero.
- No change in mass
- 0.28 mol/dm³
- What the crossing point means
- The solution inside the potato cells has about the same concentration as this sucrose solution, so there is no net movement of water.
- Water across the membrane
- 3 of 3 right first time
- Why percentage change?
- The cylinders started with different masses, so the same change in grams means more for a small cylinder.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Concentration with no change in mass (your potato, careful technique) | 0.28 mol/dm³ | 0.29 mol/dm³ |
| Mean percentage change in distilled water (your potato, careful technique, overnight) | 16 % | 16 % |
Across 120 simulated careful groups, concentration with no change in mass (an estimate of the concentration inside the potato cells) (c) came out between 0.231 and 0.332 mol/dm³ (median 0.282). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Use it as it is
Every mass is out by the same few hundredths of a gram (a zero error, a systematic error). It cancels when you subtract to find the change in mass, but each percentage change is slightly wrong because you divide by a wrong starting mass.
The fix: Zero (tare) the balance. A balance that does not read zero when it is empty adds the same error to every mass.
Method: Use whichever potato is nearest each time
The three potatoes have different concentrations inside their cells (by up to about 0.06 mol/dm³). Near the crossing point, repeats can disagree by 10 percentage points or more, so the means and the crossing point are less certain (a random error).
The fix: Choose which potato to use. Different potatoes (even of the same kind) have different concentrations inside their cells.
Method: Leave it on
About two cylinders in five keep a patch of skin on one end. Those change 20–40 % less than their repeats, so they look out of line and the means are too small (each one a random error).
The fix: Cut the skin off the ends. Potato skin lets water through much more slowly than the flesh, so a piece of skin slows the water moving in or out.
Exam answer: Leaving out: Concentration of the sucrose (sugar) solution
The fix: Name what the student changed: the concentration of the sucrose solution.
Exam answer: Leaving out: Any one of: same potato; size (length, diameter, mass or surface area) of the cylinders; volume of solution; time in the solution; temperature; the same blotting
The fix: Name one thing kept the same, and say how, for example the volume of solution (the same in every tube) or the time the cylinders are left. "A fair test" or "similar size" cannot be checked.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Independent variable · Applying knowledge[1 mark]
A student investigated the effect of sucrose solution on the mass of potato. They cut cylinders of potato with a cork borer, weighed each one, left them in different concentrations of sucrose solution overnight, then blotted and reweighed them.
What was the independent variable in this investigation?
Worked answer and how the marks are given
A full-mark answer
The concentration of the sucrose solution.
Mark scheme
- Concentration of the sucrose (sugar) solution Do not accept “concentration inside / in the potato / cell…”, “mass”.
Where marks are usually lost
- Missing: Concentration of the sucrose (sugar) solution. Name what the student changed: the concentration of the sucrose solution.
- Question 2Percentage change in mass · Applying knowledge[2 marks]
Table 1 shows your results for cylinder B, in distilled water.
Table 1 Initial mass in g Final mass in g 2.07 2.42 Calculate the percentage change in mass of the cylinder.
Percentage change in mass = %
Worked answer and how the marks are given
A full-mark answer
change in mass = 2.42 − 2.07 = 0.35 g
percentage change = 0.35 ÷ 2.07 × 100
= 16.9 %Mark scheme
- 0.35 ÷ 2.07 × 100 (change in mass ÷ initial mass × 100) (method)
- 16.9 (%)
- Allow answers from 16.71 to 17.11 % (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as percent, per cent, %.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer -16.9 % (gains 1 mark). The cylinder gained mass, so the change is positive: 16.9 %. (A loss would be negative.)
- Answer 14.5 % (no marks). You divided by the final mass. Divide by the initial mass: 0.35 ÷ 2.07 × 100 = 16.9 %.
- Answer 117 % (no marks). 2.42 ÷ 2.07 × 100 = 116.9 is the final mass as a percentage of the initial mass. Subtract 100 (or use the change in mass): 16.9 %.
- Question 3Planning the osmosis investigation · Knowledge[6 marks]
A student wants to investigate the effect of different concentrations of sugar solution on the mass of potato.
The student has: a potato, a cork borer, a scalpel, a ruler, a white tile, a balance, boiling tubes, paper towels and sugar solutions of different concentrations.
Plan an investigation to find the effect of sugar solution concentration on the mass of potato.
Worked answer and how the marks are given
A full-mark answer
1. Use a cork borer to cut cylinders from the same potato and cut them all to the same length (3 cm) on a tile, with the skin removed.
2. Blot each cylinder dry and record its mass on a balance.
3. Put each cylinder in a boiling tube with 10 cm³ of sugar solution, using at least five different concentrations (0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³).
4. Leave them for the same time (overnight) at the same temperature.
5. Remove each cylinder, blot it dry in the same way and reweigh it.
6. Calculate the percentage change in mass for each cylinder.
7. Repeat each concentration two more times and calculate the mean percentage change.Mark scheme
Indicative content:
- Cut potato pieces of the same size (cork borer, same length) from the same potato
- (Measure and) record the mass of each potato piece
- Place the potato pieces into different concentrations of sugar solution
- Use a range of at least five different concentrations
- Remove the pieces and blot them dry
- Reweigh and calculate the change (percentage change) in mass
- Repeat each concentration (2 more times) and calculate the mean
- Leave them for the same time (at least 10 minutes; overnight) at the same temperature
- Same volume of solution; same blotting technique
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Blotting the cylinders differently each time | Random | Different amounts of liquid are left on each cylinder (and squeezing presses water out), so repeats scatter. | Blot every cylinder the same way: roll it once on a paper towel, without squeezing. |
| Not blotting the cylinders before the final weighing | Systematic | Solution on the surface makes every final mass too high: gains too big, losses too small, and the crossing point too high. | Blot each cylinder gently before every weighing. |
| Differences between cylinders (and between potatoes) | Random | Each piece of tissue is slightly different, so repeats never agree exactly; cylinders from different potatoes disagree more. | Cut every cylinder from the same potato and take a mean of three or more repeats. |
| Skin left on some cylinders | Random | Skin lets water through slowly, so those cylinders change less and look out of line. | Cut the skin off the ends of every core. |
| Water evaporating from open tubes | Systematic | The solutions get slightly more concentrated, so the crossing point comes out slightly low. | Cover each boiling tube with cling film. |
| A balance not zeroed | Systematic | Every mass is out by the same amount; the percentage changes are slightly wrong. | Tare the balance with nothing on the pan before you start. |
| Too short a time in the solutions | Systematic | The tissue has not finished changing, so every change is too small and harder to measure. | Leave the cylinders overnight, all for the same time. |
| Only six concentrations, 0.2 mol/dm³ apart | Random | The crossing point is read between two points far apart, so the curve there is uncertain. | Add more concentrations between the two either side of your crossing point, 0.05 mol/dm³ apart (if it lies between 0.2 and 0.4 mol/dm³: 0.25, 0.30 and 0.35 mol/dm³). |
Repeatable. Your three cylinders at each concentration, done by you with the same method, give similar percentage changes.
Reproducible. Another group, with a potato from the same bag, their own sucrose solutions and their own balance, gets the same pattern and a crossing point close to yours.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 3 | This page | — |
| Edexcel | 1.16 | Same activity | — |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.