1.5 Experiment 2: Cucumbers and osmosis

Figure 9

In the previous experiment, you determined the water content of a potato and illustrated the rate at which the water is driven off in your graph. You also developed skills in carrying out an experiment. You’re now going to carry out a second experiment looking at the way water gets in and out of cells.

In this experiment, you will be measuring changes in the water content of two slices of cucumber as they are left in two different liquids; distilled water and salty water.

To carry out this experiment, you will need:

• two slices of cucumber
• two glasses
• a knife
• a peeler
• tap water
• distilled, deionised or boiled water
• two tablespoons of salt.

It is best to use distilled water for this experiment, available from most petrol stations and car spares shops. Distilled water is simply water that has had most of its impurities removed by boiling it, then collecting the steam and condensing it in a clean container. An alternative is deionised water, sometimes called demineralised water. This is similar to distilled water, but the manufacturing process does not significantly get rid of organic molecules, viruses or bacteria. If you can’t get hold of either of these, you can use boiled water that has been left to cool to room temperature instead.

While it’s okay to drink small quantities of distilled and de-ionised water, we don’t recommend it. Why do you think the purest form of water might not be good for you? Perhaps you’ll be able to see why at the end of the experiment.

Follow Janet’s instructions in the video (or use your activity booklet [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]   PDF) and remember to keep clear and accurate notes in your journal. Once again, think about the variables that could affect your results.

Download this video clip.Video player: ou_futurelearn_experiments_vid_10056.mp4

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Transcript

JANET SUMNER

This experiment is all about investigating the weird and wonderful properties of water and understanding why water is so vital to life. To investigate one of these life-giving properties, you're going to need one of these-- a cucumber. Now the average adult human body is 50 to 65% water. This cucumber is 95% water. But why is the water in living things so important? Well we're going to use this cucumber to reveal one of the crucial processes of life that's going on in our cells all of the time and in all other living things.

So the first thing to do is to peel the skin off the cucumber. And then you're going to cut two slices, and try and get them as equal in size as you can. Next you're going to need two glasses of water. This one's just filled with ordinary tap water, but I'm going to add two spoons of ordinary salt to make a saline solution.

And this one I'm going to fill with distilled water. Now if you can't get distilled water, you can always use boiled water from the kettle. It's not identical, but it's close enough. But obviously, make sure it's cooled down before you use it. Don't forget to label which glass is which. I've printed this out from the study journal, so I'm going to label this one salt water and this one distilled water.

Now I'm going to weigh the cucumber slices. That one's 26 grams, and that one is 22 grams. I'm going to drop them in the water. Make a note of the start time. That's 4:00 PM. And then I'm going to come back in an hour, weigh them again, and see if anything's happened.

Well that's an hour up, so I'm going to take out the slices, pat them very gently dry without squashing them, and weigh them again. Right. That's the one from the salt water, and that's actually gone down to 24 grams. This is the one in the distilled water, and that has gained weight and gone up to 23 grams. Now I think it's too early to infer anything from these first two sets of measurements, so I'm going to give the experiment another hour, come back, and make another measurement.

Right. That's the two hours up, and the cucumber in the salt water, its weight has decreased to 23 grams, which means in the two hours it's lost three grams. And the one in the distilled water has increased to 24 grams, which means in the two hours, it's gained two grams. So something's happening, but it's clearly happening very slowly, which makes me think that I'm probably going to have to change the parameters of this experiment. Now scientifically that's perfectly acceptable. I've got some early indications that something is happening, but I need to run the experiment for much longer. So I'm just going to make a note of that. And I'm going to leave this experiment running over night and come back in the morning and see if there's an appreciable difference.

Well it's 14 hours later, and I've just done the final measurements. The salt water cucumber has gone down to 21 grams, and it feels quite sort of squishy and flabby. But the distilled water cucumber has gone up to 30 grams. And that feels really firm and turgid, like it's full of water. So this one has lost five grams, and this one has gained eight grams. So clearly something is happening here that has to do with the water solutions that the cucumber slices have been sitting in.

To find out what's happened in this experiment and why it's important to life itself, you'll need to join in the online discussion.

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Based on her initial findings, Janet decided to change the parameters of her experiment and leave her cucumber slices overnight. You may find that you have to do the same. If so put the experiment somewhere where no-one can knock it over, and no pets try and drink it.

You’ll have the opportunity to discuss your findings in the next section.