Cucumber Chemistry: Moisture capture with desiccants
Have you ever dropped your phone in the water and it stopped working? It would be great if you could somehow dry the phone from the inside before it got damaged, right? There are actually some substances that can absorb water from their surroundings. You might have noticed that when you buy new shoes, electronics or beef jerky, that often there is a little package inside that says “silica gel, do not eat.” This little bag of silica gel protects the product from water damage when it is very humid. Imagine your beef jerky all moist and wet—it wouldn’t have a very good texture!
In this fun activity, you will use a cucumber to explore how different substances can absorb water from their environment. You may be surprised how the cucumber will change when exposed to salt, sugar or baking powder. And you will discover how you might be able to rescue your phone the next time it gets wet!
A material or substance that has the property to attract and bind water from air or its surrounding environment is called hygroscopic. The absorbed water molecules can either be trapped in pores of the material, weakly bond with molecules of the substance or they can form water of crystallization, which is water that occurs inside a crystal structure of compounds like salt. Some compounds even absorb so much water that they dissolve in it and become liquid. These materials are called deliquescent.
But what is all this useful for? Actually, hygroscopic materials are all around you. Your house was probably built using hygroscopic materials such as wood, clay or wool—they help improve your indoor humidity. Because hygroscopic substances do such a good job of absorbing water, they are often used as drying agents or desiccants, like the little silica gel packages. Those are helpful when you want to keep a product very dry. To determine how hygroscopic a material is, usually you measure how much water it absorbs, depending on the relative humidity of the environment. Now let’s get started and explore what substances in your kitchen can absorb water from their surroundings.
- Baking powder
- Optional: Kitchen scale
- Four little plates
- Paper towel
- Watch or clock
- Adult helper
- Find some counter space that you can work on and gather all materials.
- Take the unpeeled cucumber and with the help of an adult, cut 4 equally sized pieces with a knife. The slices should be about 0.5 cm thick. What does the cucumber slice look like? Is it very wet? How does it feel when you touch it? Does it feel crisp, hard or squishy? When you pick it up, does it keep its stiffness?
- Optional: If you have a scale, weigh each of the cucumber slices on your kitchen scale and write down the mass of each slice.
- Put each slice on a different plate; each slice will now get a different treatment.
- For slice 1, take about 1 teaspoon of salt and put it on top of the cucumber slice. The salt should not fall off the cucumber, but be a little pile on top. Feel the salt; is it very dry? What do you think will happen to the salt or cucumber?
- For slice 2, take 1 teaspoon of sugar and build a little pile on top of cucumber slice 2. Touch the sugar with your fingers; how does it feel? Do you think it will feel or look different after a while?
- For slice 3, take 1 teaspoon of baking powder and build a little pile on top of the cucumber. How does the baking powder look and feel? What do you think will happen in this case?
- Cucumber slice 4 will be your control (meaning it receives no treatment and is the one against which you will compare your results). You leave it on the plate like it is.
- Observe all 4 cucumber slices for 30 minutes. Watch closely what happens to the different substances you put on the cucumber. You can also use the teaspoon to press the pile onto the cucumber occasionally. (Be sure to clean the teaspoon for each substance.) Do you notice any texture change of the different substances? What happens to them over time?
- Touch the salt, the sugar and the baking powder every 5 minutes with your fingers (be sure you clean them off each time). If the substances become wet, take a spoon and carefully remove the pile of sugar, salt or baking powder and replace it with the same amount of fresh sugar, salt or baking powder. Make a note of how often you changed the pile for each substance. Which of the substances will be wet first? How often do you think you will have to replace the salt, sugar or baking powder?
- After 30 minutes, remove all the sugar, salt and baking soda from the cucumbers and use a paper towel to clean the cucumber slices of any remaining substance. When touching the different cucumber slices, how do they feel? Do they still feel crisp or did they become squishy?
- Optional: Once all your cucumber slices are clean, weigh each cucumber slice on a kitchen scale and note their mass. Compare this value to the number you wrote down in the beginning. What happened to the cucumber slice during the experiment? Did it become heavier or lighter? Comparing the different substances, which one resulted in the biggest change? Can you think of a reason why?
- Finally, cut your cucumber slices in half and compare the texture and thickness of each slice to your control slice. Did their texture and appearance change? If yes, how? Did the slice thickness change? Which substance had the most noticeable effect?
Extra: You have tested salt, sugar and baking soda in this experiment. Now, can you think of other substances to try that might be hygroscopic?
Extra: During your experiment, the cucumber lost a lot of water. Do you think you will be able to rehydrate the cucumber again? After your experiment, try to put each cucumber slice into a clean cup of fresh water and let it sit there overnight. Do the cucumber slices look different in the morning?
Extra: Can salt, sugar and baking powder also absorb water from the air? Try this alternative experiment to test the hygroscopy of these compounds: Empty three teabags and fill them with the same amount of either salt, sugar or baking powder. Create a high-humidity environment in a pot by boiling some water. Tape your 3 teabags to a piece of aluminum foil and cover the pot with the foil so the teabags are exposed to the water vapor. Wait for a couple of minutes and then check how the salt, sugar and baking powder look. Are they wet? Did the teabags gain weight due to water absorption?
Observations and Results
Did you observe the cucumber slice shrinking? The fresh cucumber should have been very crisp and moist. This is because 96% of a cucumber is actually water. When putting the salt, sugar or baking powder on top of the cucumber slice, you should have noticed that over time they became wet and soaked up the water from the cucumber. You probably had to exchange the salt most often throughout the experiment. After 30 minutes, the cucumber should have lost some weight and the slices may have become thinner and much squishier compared to the control slice.
There are actually several effects responsible for the cucumber shrinking. One is the hygroscopic property of the substances you tested. Salt has the ability to absorb water from its surroundings. Above a relative humidity of about 75%, salt will even become deliquescent, meaning it takes up so much water that it becomes liquid. Sugar is also hygroscopic; it forms weak bonds with the water molecules in its surroundings. In fact, this property is very useful when baking cakes and cookies. Have you ever noticed that a crispy cookie, when left out for a while becomes chewy? This is because sugar absorbs water from the air and makes the cookie moist. Baking soda is less hygroscopic. However, your cucumber slice may still have lost some weight, which means that the baking powder is able to absorb some water from the cucumber.
Besides hygroscopy, osmosis also contributes to the observed water loss in the cucumber. The water inside the cucumber does not contain any salt or sugar, but on top of the cucumber, the water will have a high concentration of salt or sugar. To compensate this imbalance, the water inside the cucumber cells starts moving through the cell membranes into the high concentrations of salt or sugar water on top of the cucumber; this process is called osmosis. It is also the reason why you get so thirsty after eating lots of salty and sweet food. The high amount of salt or sugar in the food “pulls out” the water in your cells, which leads to the brain telling you to drink more. To observe the hygroscopic effect separately, try the teabag activity explained in the “Extra” section above.
Now, coming back to the beginning: How does hygroscopy help your wet phone? Next time your phone gets wet, once you have dried it off from the outside, try putting it in an airtight baggie and add a very hygroscopic substance. The hygroscopic powers of salt filled in teabags or the little silica gel packages might just do the trick to dry it out and get it working again.
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- Collect all the sugar, salt and baking powder and dispose of it in your regular trash. If you have a compost pile, you can put the cucumber slices there; otherwise, throw them into the trash as well.
- Resarchers create novel humidity regulating films to stop water vapour condensation, from foodproductiondaily.com
- This Tiny Sponge is Probably Set to Change the World, from AWESCI
- Science Activities for All Ages!, from Science Buddies