Science 7–10 · Year 8

Potato cylinders in sugar solutions: mass change across a concentration series

Science understanding: Biological sciences

PracticalLow risk

The idea

Plant cells take in water through the cell membrane from a dilute solution and lose it to a concentrated one, so potato cylinders gain mass in water and lose it in strong sugar solution, and one concentration in between gives no change.

Safety card

Low riskAn adult supervises

Hazards

Cork borer and scalpel

Controls

Bore into the potato on a board with the hand behind the borer; cut away from the body

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

What you need

Potato, a 5 mm cork borer, a ruler and a scalpel to cut cylinders 40 mm long
Sucrose solutions at 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/L (0, 68.5, 136.9, 205.4, 273.8 and 342.3 g of sucrose made up to 1 L)
Six labelled 100 mL beakers or boiling tubes per group, 40 mL of solution in each
Electronic balance reading to 0.01 g, paper towel, timer

How to do it

Cut 18 cylinders of equal length from the same potato and blot each dry.
Weigh each cylinder to 0.01 g and place three cylinders in each solution; record which cylinder went where.
Leave for at least 40 minutes (overnight gives larger changes), then remove, blot in the same way and reweigh.
Calculate the percentage change in mass for each cylinder: (final minus initial) divided by initial times 100, and the mean for each concentration.
Plot mean percentage change against concentration and draw the line of best fit.
Read the concentration at which the line crosses zero change; at this concentration the cylinders neither gain nor lose water overall.

What you should see

Cylinders in water and 0.2 mol/L gain mass and feel firm; cylinders in 0.8 and 1.0 mol/L lose mass and become floppy. The concentration at which the line crosses zero change is the learner's own measured value. The learner knows it worked when the three replicates at each concentration agree closely and the plot falls steadily from left to right.

What changes

What you change: Sucrose concentration (mol/L)
What you measure: Percentage change in mass
What you keep the same: Same potato
Cylinder diameter and length
Blotting method
Time in solution
Temperature
Solution volume

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Potato in water gains sugar; it gains water.
The cylinder in 1.0 mol/L lost mass because sugar dissolved it.
Zero change means nothing moved; water moved equally in both directions.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-CLS-01 SC4-WS-03 SC4-WS-05 SC4-WS-06
Australian Curriculum v9AC9S8U01AC9S8I02AC9S8I04AC9S8I05

Sources

The pages the author read to write this activity.

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