Cucumber Chemistry: Moisture capture with desiccants

Introduction

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!

Background

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.

Materials

Preparation

Procedure

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.

Cleanup

More to Explore

• 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

Credits