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Week 1: Water content of everyday goods

Introduction

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Transcript

JANET SUMNER
Hello, and welcome to science experiments. I'm Janet Sumner, and I'll be your guide over the next four weeks. In this course, you're going to be doing some science experiments in your own kitchen and learning some exciting physics, biology, and chemistry on the way.
But this course is not just about doing experiments. It's about developing some of the key skills that it takes to be a successful scientist. These include careful observation, systematic note taking, looking at and discussing your data, and great experimental design. These skills may not sound important or complicated, but fundamentally, they are the root of all the scientific advances we have ever made.
Each week, there'll be an experiment for you to follow. They're all relatively straightforward, but they teach us some key science and reveal some surprises about things that you know, but maybe have never really thought about. Because there are lots of areas of discussion, we hope you'll join us in the course forums to talk about your experiments and to compare results.
One of the key features of a science experiment is the way we make and record our scientific observations. So beginning this week, you'll be starting your own science journal. And to help you with that, we have a PDF template which you can download.
So Week 1. For our first experiment, you're going to work out how much water an everyday food item contains. Now, food stuffs such as potatoes and apples contain water, but have you ever wondered how much? The answer may surprise you, and it has huge significance in how we think about the problems cause, for example, by regional droughts.
Our second experiment involves dropping some cucumber into salty water. By recording what happens over time, you'll learn about an important process called osmosis, which may lead to an alternative green energy source in the future. Good luck with your first experiments. And remember to enjoy the discussion and interpretation of your results in the forums. And I look forward to seeing you again next week.
End transcript
 
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Janet Sumner is your guide through this course. She is a Media Fellow at The Open University with a specialist interest in volcanoes. Janet will appear at the start of each week to tip you off about the coming highlights and challenges, to remind you what you’ve learned and to help you make the most of these four weeks of scientific discovery.

Over the next four weeks you will carry out a series of hands-on experiments. These experiments are designed to get you to:

  • start thinking in a rigorous and scientific way
  • recognise the influence of experiment design and variables
  • think about how the world around you works.

This course is going to assume that you are new to studying science, so don’t worry if you haven’t conducted any experiments before.

The experiments start off simply, but by Week 4 you will be isolating and extracting the DNA of a kiwi fruit! This week, you’ll be focusing on why water is so important to all living organisms and carrying out two different experiments – baking a potato to destruction and examining the process of osmosis in cucumbers.

To test your knowledge you can try the end-of-week and an end-of-course quizzes.

There are plenty of opportunities to communicate with other learners. There are forum threads for activities in each week. Please join in!

Before you start, The Open University would really appreciate a few minutes of your time to tell us about yourself and your expectations of the course. Your input will help to further improve the online learning experience. If you’d like to help, and if you haven't done so already, please fill in this optional survey.

What you'll need

All of the experiments can be carried out with items you would find in a typical kitchen, but before you start, you should probably make sure you have the following:

Shopping list

  • a cucumber
  • a kiwi
  • methylated spirits (or a bottle of vodka!)
  • olive oil
  • a potato
  • salt
  • sugar
  • washing-up liquid
  • yeast
  • distilled water.

Equipment list

  • cling film
  • oven gloves
  • a freezer
  • an ice cube tray
  • kitchen scales
  • a marker pen
  • a microwave or oven
  • a paper clip
  • a printer
  • a ruler
  • a vegetable peeler
  • drinking glasses
  • knife.

Advice for younger learners and homeschoolers

We would like to take this opportunity to remind you of the Conditions of use of Open University websites. To enrol on an OpenLearn course and participate in the forums, you must be aged 16 or over. Adults can use their own OpenLearn account to supervise under 16s on the course, posting comments on their behalf, and assisting with the experiments.

Remember, do not share any personal details such as your home address, email or phone number in any comments you post. You can read more in the OpenLearn FAQs.

1.1 Keeping a study journal

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JANET SUMNER
Hazel, we're asking the learners on the science experiment course to keep a study journal. And I thought I'd ask you - as dean of science - why taking detailed written notes is good scientific practice.
HAZEL RYMER
Well the thing is it's just like the rest of life. It's really obvious that day what those measurements or observations or whatever, what they were for, what you've achieved, what happened. Next day, next week, next month, certainly next year, you have long since forgotten. And that's really why. Just so that you understand how the thought processes is and everything else worked out. What the things were that were affecting the experiment that day.
JANET SUMNER
So what kind of things do scientists write down then? What have you got in your notebooks?
HAZEL RYMER
Oh, in my notebooks.
JANET SUMNER
So this is a bit tatty.
HAZEL RYMER
Well you know, these are real field notebooks. They've got wet, they've been thrown in the mud and everything else, and they're held together with duct tape. Do you know, this is the most important piece of field equipment, is the roll of duct tape.
So the sorts of things - oops - that we would note in the book. Well, we've got phone numbers of local people that are helping us for example. Here I've got the name of a station, it's one of the places I make my measurements. And I've explained how to get there. By the can on the right of the road, two kilometres from the turning of the lake. That sort of thing.
JANET SUMNER
So with that kind of information in it, that means that you could pass this notebook on - say to one of your students - so they've got the local contacts and the locations to go to.
HAZEL RYMER
Exactly. So for field science, there's no point keeping your data hidden away in the notebook just for your own use. In some cases, other people want to go back to that precise location.
Of course these days you can use GPS, but there are other things that you need to note down. Say for example, you might be able to precisely locate a point using GPS, but you would need to know whether the measurement that you were making was up on the top of a boulder, or the bottom, or just around, or whatever it is. So you would have pictures, and you would have a detailed description. So that anybody could then come along and precisely make that same measurement.
JANET SUMNER
So apart from those kind of notes, I'm guessing most of your notebooks are full of numbers.
HAZEL RYMER
They are. They are very nerdy looking notebooks. So these are the sorts of measurements I would make.
I'd say what date it was. I'd say what the measurements were, and here are the numbers, and some other notes about them. And yes it's just numbers, numbers, numbers.
But every now and then, I've got comments such as, instruments fell over, or instrument knocked, or something like that. Because that's what happened, and that could affect the numbers that you make. And also it helps you to remember it.
Because if it's just a string of numbers, you come back next week, next year, it's just a string of numbers. But if you've said, this was where Bill fell over, you think 'Oh yeah, I remember that'. And you can remember what happened. You remember whether it was raining or whatever.
JANET SUMNER
Now it's interesting because we're both volcanologists. We both work on volcanoes. And this just shows how personal every scientist's notebooks are. Because yours are full of numbers, and mine are full of pictures.
HAZEL RYMER
Wow. I must say, yours is very pretty.
[LAUGHTER]
JANET SUMNER
Well I think it just depends on the type of research that you're doing. This is essentially a log through layers of rock. I'm quite fussy because I always have a line down one side of my notebooks. And that's where I record the sample that I've taken, next to the layer that I've taken it from. So I'm guessing that everybody's notebooks are going to be different according to the type of scientific data that you want to record.
HAZEL RYMER
Well that's absolutely right. And do you know the thing is that you can't say that's right and that's wrong or the other way around. It depends.
So long as it does the job, so long as this reminds you of- in this case-- where you collected the samples and what the strata looked like-- what types of things you were seeing-- that's doing its job. And of course, you could hand that over to anybody else. They could go to that locality, and they would be able to identify where to go and collect those same rocks. So the job is to remind you and to be able to help somebody else.
JANET SUMNER
And I guess nowadays with digital photographs, there's a tendency to people to think, well I'm not going to write it all down in this terribly painstaking way. I'm just going to take a photograph. But photos just don't work as well, do they?
HAZEL RYMER
Actually it sounds really old fashioned, but no, I don't think they do work as well. I don't think it's a case of either or, I think you can use both. But there's nothing like a field notebook actually.
Some people record things on their phone, and of course you can write onto your phone as well and do all that at the same time. And then you can email it or whatever to lots and lots of people. But there's something about writing it chronologically through the notebook like this, that does help it somehow to keep it into your mind in a better way.
JANET SUMNER
Yeah. And it's interesting because I've got one as well, that I did for my followup experiments. And I've written down the number of the experiments-- and as you say-- the date and time, and of various conditions and things. But I've got one here that says failed.
[LAUGHTER]
JANET SUMNER
But that's equally as important, isn't it?
HAZEL RYMER
Absolutely it is. A failed experiment is not a failure. So what you were expecting didn't happen, but that's just as important.
And then really, really importantly, you never cross out anything in a notebook, do you? You underline it, or you say not to use or something in your final analysis. But you've written it in the notebook, and it must stay in the notebook. That's a perfectly valid note, whether or not the data turn out to be valid.
JANET SUMNER
Now our books are pretty complicated and pretty detailed. What we're asking the learners to do is a very simple study journal. But that's of equal value, isn't it?
HAZEL RYMER
Absolutely. It's very important to note - as we've said - the time and the date, and so on, where you are, whatever. Environmental conditions. So if you were outside doing an experiment for example - and it was raining - you might get different results from if it was sunny.
All sorts of things can affect an experiment. And sometimes you don't even know what those are until you happen to have noted them all down. Then you can perhaps find variations.
And you can say, Oh actually look, the conditions here were different from this other time. And the two times that you've made those experiments might have been years apart depending on what you're doing. And you wouldn't remember that it was bright and sunny or raining or whatever, unless you'd written it down.
JANET SUMNER
You might even note down at the end the experiment, don't do this next time. Try changing so and so or something.
HAZEL RYMER
Yes. And if you've written it down, you're more likely to remember it. The other thing of course, is it might have been a failed experiment in terms of what you were trying to do at the time. But you've still got those measurements. You've still got those results.
And it might be that another time, what you were actually looking for was whatever happened this time. That you thought was a failure one time, might not actually turn out to be. Some of the fantastic results that have been observed through the history of science, have actually been apparent fail--
JANET SUMNER
People's mistakes?
HAZEL RYMER
--People's mistakes, yes. I accidentally mixed this with that, and 'Oh look what I got'.
JANET SUMNER
What we're asking the learners to do now - in keeping a study journal - could be the start of something big going forward into the future.
HAZEL RYMER
Beginning of their scientific careers
End transcript
 
Interactive feature not available in single page view (see it in standard view).

Keeping a study journal, lab notebook or field notebook is a vital skill for any scientist – beginners to the subject often underestimate their importance.

In the video, Janet Sumner and Hazel Rymer, Dean of Science at The Open University, discuss why your notes are so important. Download the activity booklet for this course, it includes everything you’ll need to make your personal notes on the experiments. If you would rather use your own journal, that’s fine – the type of record that you keep of your experiments is less important than the clarity and detail of your notes.

A good rule of thumb is that your notes should contain enough information that someone else could use them to duplicate your work, or that you could read through them years later and remind yourself of the exact procedures that you followed. It is always far better to have recorded too much information and not need it, than to not record enough and find that a vital piece of information is missing.

1.2 Introducing the experiment

Figure 1

One of the best ways to start thinking more scientifically is by looking at everyday items and experiences in more detail. Your first experiment will be based on an everyday process – cooking.

You will be looking at the relationship between heat and the water content of a potato, measuring the change in water content as heat is transferred to a potato during the cooking process.

If you have ever microwaved a potato before, you will have noticed that steam is expelled during the process. By cooking the potato to destruction you are aiming to drive off all of the water contained within it, enabling you to calculate its water content.

To conduct this experiment, you will need:

  • a potato
  • oven gloves
  • scales (digital scales will give a clearer reading)
  • your activity booklet
  • a pen
  • a ruler
  • a microwave or conventional oven.

If you do not have a microwave oven and wish to use a conventional oven, you will need to have longer cooking times with longer intervals between readings.

Although it can reasonably be expected that most people will have similar results, there will be small differences based on the type of oven used, the type of potato, and the length of the cooking. These are the experiment variables; they are the parts of an experiment that can be controlled, changed or measured. Variables, and their importance in experiments, will be discussed throughout the course.

1.3 Drawing graphs

This experiment requires you to plot your results on a graph. Before we move on to the actual experiment, here is a quick refresher on how to plot line graphs.

Graphs are a great way of presenting numerical data visually; they are used to illustrate clearly the relationship between quantities. There are several different types of graph, but for this course you will only be focussing on how to use a line graph.

The x-axis and the y-axis

The axes on a graph are your reference lines; they carry the scale of your graph and help you locate where to plot each piece of data. They also tell you what variables your graph is illustrating a relationship between.

Line graphs are commonly used to show data that change over a period of time. A simple line graph is drawn with two axes: an x-axis that is drawn left to right across the page, and a y-axis that is drawn up the page.

Figure 2

Scientists like consistency and it is standard practice to put the thing you’re measuring on the y-axis. So, as the change in weight of a potato is what you will be measuring in this experiment, this will be the data plotted on your y-axis. Because you are measuring the change in the potato’s weight over time, the time will be the data illustrated by your x-axis. The graph will therefore show the relationship between cooking time and potato weight.

Once you have drawn the axes of your graph, remember to write alongside them both what they represent and the units they are displayed in, i.e., time (in minutes) and weight (in grams).

Figure 3

Setting the scale of your graph

When drawing your axes, you need to number them appropriately for the experiment you are carrying out, so your y-axis should range from zero to a number just a bit larger than the weight of your potato. Your x-axis should be labelled with the number of minutes of cooking time, probably from zero to about 10 to 15 minutes if using a microwave oven, and zero to maybe 100 minutes if you are using a conventional oven.

Figure 4

Plotting your data

When you obtain a measurement, you write down the weight and the cooking time. To plot this on a graph, you need to find where those two values intersect. For example, if your potato weighed 90 grams at zero minutes, then you would plot your first data point as a dot at the intersection between those two lines.

Figure 5

Once you have all your data plotted on a graph you should join the consecutive points with a line.

Figure 6

1.4 Experiment 1: Potato experiment

Follow Janet’s instructions in the video (or use your activity booklet  PDF) to conduct the experiment. Don’t forget to prick a few holes in your potato and to use oven gloves when handling your hot potato!

Download this video clip.Video player: ou_futurelearn_experiments_vid_1008.mp4
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Transcript

JANET SUMNER
Now it's time to do your first practical science experiment. You don't need much equipment. You need a microwave or a conventional oven, a set of scales, a potato, and a piece of graph paper printed off from your study journal.
It's a very simple experiment. We're basically going to bake a potato to destruction. But it's all about understanding how to set up, observe, and record in a scientific manner. So the first thing is I'm going to weigh my potato and note down the start weight in grams. It's 103 grams.
And now I'm going to stick it in the microwave. Right, in it goes. And I'm going to set the timer for one minute. Now, you can do this in a conventional oven, but obviously it's going to take a lot longer, and you'll probably need to do the measurements every 10 or 15 minutes or so.
[DING]
Right, time's up, so I'm going to take it out and weigh it again, usual health and safety with hot objects. OK, so let's see. And the weight's gone down to 98 grams. So I'll record that. After one minute, 98 grams.
Now I'll pop it back in and give it another minute.
[DING]
That's another minute up, so I'll do another measurement. It now weighs, oh, 84 grams. I'm just going to give it one more minute and see if the weight continues to fall.
[DING]
Weight 74 grams. Now, you're probably going to have to do this seven or eight more times to get an original data set. But what we need to think about next is how you're going to present this data in a way that's meaningful.
One of the most visual ways to do this is with a graph, which is where this comes in. So we have two variables that we're measuring, the weight of the potato in grams, which is changing over the amount of time in minutes that it spends in the microwave or the oven. And I can use this graph paper to show those two variables.
This is the vertical axis, which I'm going to mark weight in grams, and then the horizontal axis, which is going to be my time in minutes. Now I have to decide what scale to put on these axes. So the start weight of my potato was 103 grams. So if I start from naught and mark off every centimetre as 10 grams, then I should have enough space to fit in my weight measurements.
So that's 10, 20, 30, 40, 50, 60, 80, 90, 100, and 110 grams. And then for the time, I probably don't want to microwave a potato of this size any longer than 10 minutes. So I'll make 10 minutes my maximum, and every centimetre on my horizontal axis I'll mark out as a minute.
One, two, three, four, five, six, seven, eight, nine, 10. Now I can start plotting my data and see if a trend emerges. So the start weight at zero minutes was 103 grams about there. Then after one minute, it's gone down to 98. So I move along the horizontal axis to one minute, and then I go up to 98 grams.
Then two minutes, I'm at 84, so up from two minutes to 84. And then finally at three minutes, we're down to 74, which is about there. And I can join up my data points, and you can see I've got a trend starting to emerge.
So what we'd like you to do is finish the experiment. I'm going to do another four or five measurements and plot them. Don't forget to give your graph a title, Potato Experiment, and note whether it was done in the microwave or the oven.
You've done the observation and the recording of the data. Now it's time to do the interpretation of the data by joining in the discussion.
End transcript
 
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It is possible to go too far during the cooking process and cause your potato to smoke and potentially catch fire, so watch the potato during the experiment, and as soon as you see smoke coming from it, or smell burning, you should bring the experiment to a halt immediately.

This is your first opportunity to document an experiment. Remember the advice in the previous sections and record all your observations and results carefully.

You will have the opportunity to discuss your results in the next section.

1.4.1 What's in your graph?

Figure 7

You should now have completed this week’s experiment and should be ready to share your findings with your fellow learners. It is very likely that each of your experiments will have produced different results, but you may be able to find general patterns of agreement.

Activity 1.1 Experiment 1

Timing: Allow about 30 minutes

What was the water content of your potato? To work this out, you just need to subtract the weight of your potato at the end of your experiment from its starting weight. The difference in the starting and finishing weights is the actual weight of the water that was in your potato.

To compare your results with those of other learners easily, you can express your potato’s water content as a percentage. To do this, divide the weight of water in your potato by the potato’s starting weight and multiply the answer by 100. It is likely to come out to about 80%.

Post your results in the course forum thread for this activity  and compare your findings with those posted by other learners. Discuss why you think any differences came about. Consider the variety of potato you used. Does it make a difference? Do waxy potatoes have a higher water content than floury ones? Maybe drying in a microwave is different from drying in a conventional oven?

It might be useful to provide an image of your graph. You can do this by photographing or scanning your graph and attaching the photograph file with your forum post.

Discussions with other people are crucial parts of the scientific process. It isn’t enough to obtain your results and then hide them away; they must be shared and discussed among your peers. Scientists usually do this by having their work critically examined by other scientists to see if it is ready for publication, then publishing their results in scientific journals, where anyone and everyone can examine them. If other people disagree with those results they can carry out research, obtain findings, and publish papers which argue a different case. This is why science produces such a robust body of knowledge. Other people are always trying to spot the flaws in your ideas, and if flaws are there, they are usually found pretty rapidly. A good scientist must always be ready to be corrected.

1.4.2 Why does it matter?

Figure 8

Potatoes originated in South America, and have become a staple food for much of the world’s population. About 4000 to 5000 varieties exist, but most of these are only found in the Andes. In the UK, only about 80 varieties are grown, and only a handful of those are sold by the major supermarket chains.

The amount of water required to grow different crops affects which ones are better suited to drier regions, and which ones are better suited to wetter areas. With drought conditions widespread over many parts of the globe, it is better for farmers in those regions to grow crops more suited to drier conditions and crops that require less water to develop are the preferred choice.

This type of farming even has a name: dryland farming. It is common in the Great Plains of the USA, the deserts of Mexico and the south-western USA, the steppes of Eurasia, Australia, and parts of South America.

This table shows some of the thirstier crops in production around the world. Do any of these numbers surprise you?

Water-intensive cropsTypical water needs (in litres per kg of crop)
Cotton7,000 to 29,000
Rice3,000 to 5,000
Sugar cane1,500 to 3,000
Soya2,000
Wheat900
Potatoes50

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  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.
End transcript
 
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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.

1.5.1 Sharing your results

Figure 11

Now that you have completed your second experiment, you will have a new set of results to discuss. Hopefully the results of your experiment were broadly similar to those seen in the video. The cucumber slice in the salt water should have lost weight, while the one in the distilled water should have gained weight.

Activity 1.2 Experiment 2

Timing: Allow about 15 minutes

Post your results and observations in the course forum thread for this activity:

  • Did the experiment perform as you expected?
  • What do you think caused your cucumber slices to change in the way that they did?
  • If your cucumber slices haven’t behaved in the same way as Janet’s did, can you think of a reason why your results might be different?

Photographs can be really useful for comparing your results, so do add an image to your forum post if you can.

Remember, if your results were unusual this does not make them bad – some of the most significant scientific discoveries have stemmed from mistakes or surprises.

1.5.2 Osmosis explained

Figure 12

You’ve seen the results of your experiment and it should be clear that water is somehow moving in and out of your cucumbers – this process is known as osmosis.

Imagine two solutions of different concentrations, divided by a partition which allows small particles through it, but not large particles. This type of boundary is known as a partially permeable membrane. In this environment, the process of osmosis will occur spontaneously because the concentrations of water molecules on either side of the partition, or membrane, naturally try and equalise.

In your experiment, the water in the glass and the fluid inside the cucumber’s cells are separated by the cucumber’s cell walls which are partially permeable membranes. Salt cannot pass through these membranes, but water can. By adding salt to the water, you made its salt concentration higher and therefore lowered the concentration of water in the mixture. This gives the cucumber cells a relatively higher water concentration than that in the glass. The water in the cucumber cells tries to equalise these different concentrations by moving from the cells to the saltwater solution. As a result, the cucumber loses water and becomes a bit squishy. This environment is referred to as hypertonic.

In your other glass, containing the distilled water, the opposite effect was seen. Water flowed from the pure water (a higher concentration region) into the cucumber cells (which have a lower concentration of water). In this hypotonic environment, the water tries to equalise by moving into the cucumber cells, inflating them, and causing the cells to become firm. This is known as turgor, and it is the turgor pressure in plant cells that keeps them rigid. Without it, plants wilt and their cellular functions will begin to decline.

When the concentrations on either side of the membrane are equal, the condition is known as isotonic, and water moves randomly from one side of the membrane to the other, but with no pressure gradient to drive it, the rate is the same in both directions.

1.5.3 Why does it matter?

Figure 13

You now know what osmosis is and that it’s the process that keeps plants firm but, other than keeping your flowers from wilting, are there any other examples of osmosis in the natural world? Unsurprisingly, the answer is yes, there are many, and here are just a few:

  • As well as keeping plants rigid, osmosis is also the way that plants draw water and nutrients into their roots.
  • If you’ve ever stayed in the bath too long and seen your fingers turn a bit ‘pruney’, it’s because your fingertips have absorbed water through osmosis and become bloated, making them wrinkly.
  • Ever had salted fish? The fish is covered with salt to preserve it. Osmosis is the process whereby the salt draws water from the fish’s cells, drying it out. This, gruesomely, is the same thing that happens when slugs encounter salt.
  • If you have soaked raisins overnight in alcohol for a recipe, the liquid soaked into the fruit by osmosis.
  • Cholera is rare in developed countries, due to clean water supplies and good healthcare, but it used to be one of the most feared diseases in the world. Cholera damages our intestines in such a way to cause osmosis to happen in an unwanted direction. The cells of the intestine become unable to absorb water and instead it flows from the rest of the body into them, causing diarrhoea, dehydration and often death.
  • Industrially, osmosis is used to purify water, at desalination plants where seawater is turned into drinkable water.
  • Osmosis is also used in modern medicine. When patients are treated with dialysis to replace lost kidney function, osmosis is the process which is used to filter waste materials and excess water from the blood.

Now, you may remember that Janet mentioned a green energy source in this week’s guide video. Well, this relates to the use of osmosis to generate electricity. Where rivers flow into the ocean, freshwater and saltwater naturally meet, resulting in a natural mixing of waters of different salinity. Construction of a power plant at such a site allows freshwater and saltwater to be guided into separate chambers, divided by an artificial membrane. At the membrane, the freshwater is drawn towards the seawater. This flow puts pressure on the side of the seawater and that pressure can be used to drive a turbine, producing electricity that produces no greenhouse gases. The only waste product is brackish water (a mixture of saltwater and freshwater), which can be pumped out to sea.

Power plants that utilise osmosis in this way have been trialled, but only as prototypes, as the technology is still relatively new. The first was in Norway, where it generated up to 4 KW; barely enough to keep a couple of houses supplied with power, and the company shelved their development plans. However the technology can still be developed further, if improvements in the efficiency and cost of the membrane can be achieved.

Those are just a few examples of osmosis in real life, can you find some more? .

1.6 Week 1 quiz

Check what you’ve learned this week by taking this end-of-week test.

Complete the Week 1 quiz  now.

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1.7 Week 1 summary

Figure 14

Congratulations, you have completed Week 1 and carried out two different science experiments. You should be starting to get a taste for practical science and sharing data; hopefully you have learned some new things too!

During the week you have covered scientific techniques such as making precise measurements, recording data and observations, plotting graphs, and interpreting results. These are all essential skills for a scientist and you will use them throughout this course.

We hope that you have enjoyed this first week and you should now be ready to tackle Week 2, where you will investigate some properties of different liquids.

To conduct next week’s experiment, you will need:

  • an ice cube tray
  • fresh water
  • salt water (approximately 2 tablespoons of salt added to 500 ml of water)
  • olive oil
  • another liquid of your choice – be creative here, but avoid substances which might be hazardous!
  • four glasses
  • your activity booklet
  • a freezer.

You can now go to Week 2.

Acknowledgements

This course was written by Hazel Rymer.

Except for third party materials and otherwise stated in the acknowledgements section, this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.

Every effort has been made to contact copyright owners. If any have been inadvertently overlooked, the publishers will be pleased to make the necessary arrangements at the first opportunity.

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