Water Jelly Crystals

A repeatable, amazing physical change occurs each time these small, solid nuggets are tossed into some water.

You start with little nuggets that look like rock salt and end up with huge, gleaming, water-filled globs of hydration. Your discoveries using a superabsorbent polymer have just begun! Of course, if you wore a modern diaper at one time in your infant life, then you’ve already had experience with a type of these chemical marvels. This polymer absorbs water – a lot of water – and lends itself to a lot of activities and uses at the same time, too.

Experiment Materials

Experiment Videos

Experiment

1

NOTE: If you’re planning to have colored crystals, it’s easier to start with the color of water you want before you toss in the nuggets. Color Fizzer Tablets are best or food coloring can work.  If this is your first time working with Water Jelly Crystals, don’t use colors so you can see what to expect. You can add color later!

Place 1 teaspoon of the nuggets into a quart-sized plastic bag. Fill it with water and seal the bag tightly. Place the bag into a bowl to catch any slow leaks. Observe the crystals after 10 minutes, 30 minutes, and 1 hour. What changes to the polymer do you see? If possible, let the soaking continue overnight.

2

When you check the bag after a few hours, those tiny, pebble-like nuggets have grabbed onto water molecules and have exploded into huge, globby, water-filled crystals as a result!

Wash your hands first and then scoop a handful of the cool, shimmery, hydrated lumps out of the bag! They can break so handle them gently. The longer you soak the nuggets, the more likely it is you’ll have little or no water remaining in the bag.

3

Using a hand to gently stir the crystals in the bag is a most interesting sensation. You may think it’s cold (and slimy) but leave your hand in the bag for a few minutes and do some “research.” What changes do you notice?

4

Remove three or four crystals from the bag and put them on a plate. Use a ruler to measure their size and compare one of the hydrated crystals to a dry nugget. How do the sizes of the water-filled crystals compare to each other? How do the dry nuggets compare to each other in size?

5

Try to break a nugget into smaller pieces with dry fingers. Now, break a crystal into smaller pieces with your fingers. Compare the results and develop an explanation about what might have happened inside the nuggets. What chemistry probably took place that allowed the nuggets to grow so large? How did they become so soft in just plain water? What is it that “fills” the hydrated crystals? Think about your answers on a molecular level.

How Does It Work

A polymer is a very long chain that’s made up of identical, repeating molecules – they’re the “links” in the chain. Think of these links as tiny sponges waiting to connect with water molecules. In the absence of water, the polymer chain is very tightly twisted and piled up on itself so the water connection points are tightly buried inside the nugget. When water is available, the molecular links on the outside of the nugget grab it and hold on through simple cross-link bonding. The more water is available, the more the polymer has to unwind to get it. Each link gets larger as it hooks up to more and more water. The chain begins to swell on the outside surface and that allows more water to get farther into the polymer to available link points. Water is cross-linked farther in and the polymer expands to make room for it. If enough water is available, this bonding and swelling continues until there are no more available places to collect water. Absorption stops but water that’s linked to the polymer has expanded it to about 300 times it’s original weight and it’s not tightly twisted at all. The crystal is now about 98% water and 2% polymer. That’s a lot of water!

Polymers are very common and you see and use them everyday: silk, wool, nylon, cotton, cellulose, proteins, rubber, PVC, epoxy materials, silicone, hair conditioner, paint, polystyrene, adhesives, gelatin, Silly Putty®, polyester, Kevlar®, CDs, eyeglasses, Teflon®, and the list goes on and on!

Why should you wash your hands before handling the crystals? Goobers and germs! Since the crystal is mostly water, it “washes” your hands as you handle it and the dirt it removes has no where to go – neither do the germs and other microscopic nasties from your hands. This build up can lead to dirty and smelly crystals that are no fun for anyone. Eventually, this oily concoction can cause a reduction in the number of places on the polymer that can hook to water. That means a loss of absorption and less water taken up. Besides, it makes the crystals appear cloudy and dull.

Take It Further

Distilled Water – It’s recommended that distilled water be used to soak the nuggets if possible. Distilled water is usually easy to obtain and not very expensive. It’s about $1 (USD) per gallon at grocery stores and pharmacies. Tap water is OK to use but sometimes, purifying it for people changes how it gets absorbed by the polymer. Distilled water is like rain water and has very little stuff in it but… water. Minerals, chemicals, and other dissolved materials that can change how the polymer bonds with it are removed by distilling. The crystals can be larger as a result.

Disappearing Crystals – The hydrated crystals can “vanish” if you know the secret. Fill a clear container with plain water and dump four or five of your largest crystals into it. They vanish but they’re in there! Since they have very nearly the same index of refraction as that of the water around them (they’re 98% water), light passes straight through them and the water so you can’t see them. If you look very carefully, you can barely see the edges of each crystal where light is bent slightly as it passes through the polymer. Check out Vanishing Jelly Marbles for more ideas.

Rainbow Test Tube – Here’s where colored crystals get into the mix and you combine your science lab with some seriously creative art. Find the details at the Rainbow Test Tube site.

Science Fair Connection

Just Water? – You may be wondering if the polymer can absorb anything other than water. To find out, test any of these liquids in place of the water: tomato juice, orange juice, vegetable oil, milk, soda water, rainwater (or melted snow), hot water, sugar water, salt water, etc. Keep track of your discoveries and you’re on your way to a science fair project.

Cold Crystals – Place a zipper-lock bag of the hydrated polymer in the freezer. Examine the bag of polymer after 12 hours. Here’s a test: compare the length of chilling time and the temperatures reached for frozen crystals to a similar amount of water ice. Maybe you could you use frozen crystals in place of crushed ice the next time you need an ice bag.

Polymer Plants – Check out the activity found at Gardening with Water Jelly Crystals. Grow grass seeds, radishes, beans, or other fast-germinating plants in a half-and-half mixture of hydrated polymer and potting soil. Compare by growing the same kind of seeds you planted in the soil/polymer mixture in just plain soil and compare the growth at two day intervals for one or two weeks. This sounds like another great science fair project! How well do seeds germinate and grow in a cup of hydrated polymer without soil? Test it — you might be surprised!

Safety Information

Since this is a science activity, keep the crystals away from your mouth, ears, eyes, and nose, as usual. However, Water Jelly Crystals (a cross-linked polyacrylamide copolymer gel) are considered not to be a health hazard. They’re non-toxic, safe to handle, and safe for use around pets and young children. They’re NOT to be eaten and can be a choking hazard if used carelessly. Don’t dispose of the crystals (either wet or dry) down the drain as they will certainly clog the pipe. Just toss them into the trash – BUT – remember why they were made!

They’re designed to be environmentally beneficial.  These crystals are intended to be used with plants, so when you’re finished using them for discoveries, you have two good options:

  1. Simply bury clear, hydrated crystals in a planter box or garden. They dry out as plants use the water they hold and then rehydrate when water is present in the soil again.  They’ll support plant growth for about six years.
  2. Let hydrated crystals (clear or colored) completely dry out on a cookie sheet. They shrink back to their original size and can be stored and then rehydrated many times.

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