Investigating the effect of concentration of blackcurrant squash on osmosis in chipped potatoes

Class practical

In this investigation, you prepare a range of dilutions of blackcurrant squash. Cut potato chips, weigh them and place them in the various solutions. Leave them for osmosis to occur. Remove the potato chips from the blackcurrant squash, dry and reweigh. Make qualitative observations or calculate the percentage change in mass.

Lesson organisation

Each working group (or individual students) could carry out the complete protocol. Or, working groups could be allocated tasks for the whole class, for example, setting up the range of dilutions, cutting the potato chips, recording data.

Apparatus and Chemicals

For each group of students

Beaker, 100 cm³, containing approximately 95 cm³ of blackcurrant squash

Beaker, 100 cm³, or boiling tube, 6

White tile

Scalpel

Ruler

Cork borers to cut potatoes, or potato chipper (Note 1)

Measuring cylinder, 50 cm³

Teat pipettes, 2

Distilled water, in wash bottle

For the class – prepared by technician/ teacher

Blackcurrant squash, at a concentration of 556 g dm^-3 sucrose: Ribena™ is ideal, 1 litre is enough for 9 working groups

Large baking potatoes, 1 per working group

Access to balances

Health & Safety and Technical notes

Blackcurrant squash is a foodstuff and therefore low hazard. However, no foodstuffs should be consumed in the laboratory. Take care when cutting the potatoes – cut onto a tile, and be prepared with first aid equipment for cuts.

Read our standard health & safety guidance

1 A potato chipper will quickly cut chips of uniform size. It saves time if the chips are cut (using a chipper) just before or at the beginning of the lesson by a technician. Students still need to trim the chips to fit into the beaker/ boiling tube. If using a cork borer, each group (at least) should use a bores of the same diameter.

2 If you can cut the chips an hour before the lesson and leave them in a large bowl of water, they will be fully turgid at the beginning of the experiment. In this case, all the chips will lose mass in the blackcurrant squash solutions (0% blackcurrant squash solution will stay the same). This simplifies the investigation. Otherwise, the chips in 0% blackcurrant squash solution will gain mass and the rest will probably lose mass.

Ethical issues

There are no ethical issues associated with this procedure.

Procedure

Concentration of blackcurrant squash solution (%)	Volume of blackcurrant squash needed (cm³) to make 10 cm³ of solution (A)	Volume of water needed (cm³) to make 10 cm³ of solution (B)	Volume of blackcurrant squash needed to make 30 cm³ of solution (= A x 3)	Volume of water needed to make 30 cm³ of solution (= B x 3)	Final volume of solution made (cm³)
0	0	10	0	30	30
20	2	8	6	24	30
40	4	6	12	18	30
60	6	4	18	12	30
80	8	2	24	6	30
100	10	0	30	0	30

Equipment and materials for investigating effect of concentration of blackcurrant squash on osmosis in chipped potatoes

Preparation

a Label beakers/ boiling tubes with the concentrations of blackcurrant squash.

b Measure out blackcurrant squash for the dilutions using the volumes shown in column 4 of the table. Measure out water using the volumes shown in column 5. (Columns 2 and 3 show the volumes needed to make 10 cm3 of solution, which is multiplied to get larger volumes in columns 4 and 5.)

c Collect some potato chips or a potato to cut up. Cut or trim your chips, if necessary to fit into your small beaker/ boiling tube. Cut off any potato skin.

Investigation

d Dry the chips on a paper towel.

e Weigh the chips using a balance; record masses in an appropriately designed table.

f Place chips in a beaker/ boiling tube. The solutions should completely cover the chips.

g Leave for at least 15 minutes.

h Remove chips one at a time from the beakers/ boiling tubes.

i Dry the chips on paper towel.

j Reweigh chips and record results in the table.

k Calculate the change in mass and then the percentage change in mass of each set of potato chips.

l Plot percentage change in mass (y-axis) against concentration of blackcurrant squash (x-axis).

Teaching notes

The advantage of using blackcurrant squash is that students can see at a glance which is the most concentrated solution. This means they are less likely to get the concentrations confused in the course of the procedure.

However, for A-level studies you would need to convert the concentrations into molar equivalents, and process the data on graphs showing standard deviation bars in order to assess the variation in repeats. At A-level, graph points should be joined with ruled lines and show error bars. A high-level GCSE group could assess the range of measures using maximum and minimum bars.

From the graph you can read off the ‘blackcurrant squash equivalent’ concentration of the potato cell contents – this will be the concentration at which the mass of the potato stayed the same, or may be interpolated between two concentrations. Undiluted blackcurrant squash contains 23% juice or 556 g dm^–³ sugar. So, 10% blackcurrant squash is a concentration of 55.6 g dm^–³ sugar.

Solutions which draw water out of the potato chips are hypertonic to the cell contents – more concentrated. Solutions from which the potato chips absorb water are hypotonic. And a solution which is in equilibrium with the cell contents is isotonic.

You can modify this activity to show the rate of movement of water by osmosis. To do this, set up 6 beakers/ boiling tubes containing 100% blackcurrant squash. Place weighed, dried potato chips in each beaker/ boiling tube. Every 10 minutes, remove one set of chips, dry on paper towel and reweigh. Find out how long it takes before there is no further change in mass.

Health and safety checked, September 2008