Key concepts
Material science
Food science
Chemistry
Water absorption
Introduction
Have you ever gotten an electronic gadget wet and had it stop working? It would be great if you could somehow dry it out before the internal parts got damaged, right? There are actually some substances that can absorb water from their surroundings. You might have noticed when you buy new shoes, electronics or beef jerky that often there is a little package inside with the warning: "silica gel, do not eat." This little bag of gel protects the product from minor water damage, such as when it is very humid. Imagine your jerky all moist and slimy—it wouldn't have the texture you were expecting!
In this fun activity you will use a cucumber to explore how different substances can absorb water from their environments. 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 an electronic gadget next time it gets wet!
Background
Some materials or substances can attract water from its surrounding environment—they are called hygroscopic. The water molecules they absorb can either be trapped in pores of the material, weakly bonded with molecules of the substance or can form “water of crystallization,” which is water that occurs inside a crystal structure of compounds such as salt. Some compounds even absorb so much water that they dissolve into a liquid solution. These materials are called deliquescent.
But what is all this useful for? Actually, hygroscopic materials are all around you. Some common ones include wood, clay and wool. 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, scientists measure how much water it absorbs, depending on the relative humidity of the environment. And now you can test some of these impressive materials right at home. Let's get started and explore what substances in your kitchen can absorb water from their surroundings.
Materials
- Cucumber
- Sugar
- Baking powder
- Salt
- Kitchen scale (optional)
- Knife (and adult help using it)
- Four small plates
- Teaspoon
- Watch or clock
- Flat workspace large enough to set up all of your materials
Preparation
- Take the unpeeled cucumber and with the help of an adult, cut four equal size pieces with a knife. The slices should be about 0.5 centimeter 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?
- If you have a kitchen scale, you can weigh each of the cucumber slices and write down the weight of each slice.
- Put each slice on a different plate; each slice will now get a different treatment.
Procedure
- Measure out one teaspoon of salt. Feel the salt. Is it very dry? For the first slice, from the teaspoon carefully pour the salt on top of the cucumber slice. The salt should not fall off the cucumber but rather form a little pile on top. What do you think will happen to the salt or cucumber?
- Measure out one teaspoon of sugar. Touch the sugar with your fingers. How does it feel? Now build a little pile with the sugar on top of the second cucumber slice. Do you think the sugar will feel or look different after awhile?
- Measure out one teaspoon of baking powder. How does it look and feel? Pile the baking powder on top of the third cucumber slice. What do you think might happen in this case?
- Cucumber slice four 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 as it is.
- Observe all four cucumber slices for 30 minutes. Watch closely what happens to the different substances you have put on each slice. You can also use the teaspoon to gently press the pile onto the cucumber occasionally. (Be sure to clean the teaspoon between touching it to each substance.) Do you notice any texture change of the different substances? What happens to them over time?
- During the same 30 minutes, touch the salt, sugar and baking powder every five minutes with your fingers. (Be sure you rinse your fingers off between each.) How do they feel? Do they start to change over time?
- If the substances become wet, take a clean 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 became 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 (keeping track of which slice had which substance on in). Use your fingers to clean the cucumber slices of any remaining substance. When touching the different cucumber slices, how do they feel? Do they feel different? Do some still feel crisp or did they become squishy?
- Once all your cucumber slices are clean, if you have a kitchen scale, you can weigh each cucumber slice and note any changes. Compare this value with the number you wrote down in the beginning. What happened to the cucumber slice during the activity? 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 with that of your control. Did their texture and appearance change? If yes, how? Did the slice thickness change? Which substance had the most noticeable effect? What does that tell you about the substance's ability to attract water?
- Extra: You have tested salt, sugar and baking soda in this activity. Now, can you think of other substances to try that might be hygroscopic?
- Extra: During your test of some of these substances, the cucumber lost a lot of water. Do you think you could rehydrate the cucumber again? After your dehydration test, try placing each cucumber slice into a clean cup of fresh water and let it sit overnight. Do the cucumber slices look different in the morning?
Observations and results
Did you observe the cucumber slices shrinking? The fresh cucumber should have remained very crisp and moist over the 30 minutes you observed it. This is because 96 percent of a cucumber is actually water. For the slices with salt, sugar or baking powder on top, you should have noticed that over time the substances became wet and soaked up the water from the cucumber. You probably had to exchange the salt most often throughout the activity. After 30 minutes, the experimental cucumber slices should have lost some weight and the slices may have become thinner and much squishier compared with the control slice. Some of these slices might actually have begun to even look smaller.
There are actually several reasons for the cucumber shrinking. One is the hygroscopic property of the substances you tested. Salt has a strong ability to absorb water from its surroundings. Above a relative humidity of about 75 percent salt will even become deliquescent, meaning it takes up so much water that it becomes a solution. 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 becomes chewy after it is left out for awhile? This is because sugar absorbs water from the air, which makes the cookie moist. Although baking soda is less hygroscopic, your cucumber slice with baking soda on it might 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 much salt or sugar, but water on top of the slice 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; 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 water.
Now we come back to the beginning: How can hygroscopy help your electronic gadget? Next time a device gets wet, once you have turned it off and dried it from the outside, try putting it in an airtight baggie and add a very hygroscopic substance. The hygroscopic powers of salt-filled teabags or those little silica gel packages might just do the trick to dry it out and get it working again.
Cleanup
If you compost, you can compost your cucumber slices. Otherwise, the cucumber slices, salt, sugar and baking powder can be disposed of in the trash.
More to explore
Researchers Create Novel Humidity Regulating Films to Stop Water Vapor Condensation, from Food Production daily.com
This Tiny Sponge Is Probably Set to Change the World, from AweSci
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
