Growing Spheres

Growing spheres or cubes (hydrophilic water gel) are small polymers that have a strong affinity for water – like a sponge absorbing water. They swell to about 300 times their size from about 3 mm in diameter to about 2.5 cm (1”) in diameter. After use, they can be dried and used again!

These are also available in cubed form which are slightly larger in size and less in number than the spheres (1200+ spheres compared to ~290 cubes in one package).

Preparation

Simply place SOME of the spheres in warm, clean, water in a clear bowl and allow them to sit. In 8 – 24 hours, they will reach their maximum size. If you use distilled water or deionized water, the expansion will be even greater.

growing spheres

When fully expanded, the index of refraction of the spheres is exactly the same as that of water; consequently they are basically invisible in the water. Students will be intrigued with this and you will want them to put their hands into the bowl in order to “lift” some of them up and see them.

Use – with Tomatosphere seeds

Once fully expanded, take a toothpick and make a small hole in the sphere. Insert ONE seed just below the surface of the sphere and place back in the water to allow to seal.

Take the spheres with the seeds and place them in a clear container. Keep the spheres out of direct sunlight and check regularly to watch the germination take place. Note that the polymers are sensitive to sunlight. They will lose their absorbency and cause decomposition when exposed to direct sunlight.

Other learning opportunities with the growing spheres

When you purchase your growing spheres (see below), you will have more than you need for the Tomatosphere project. This gives you the opportunity to use the spheres for different purposes to illustrate various concepts in both math and science. Remember that the ones that you use can be recycled and used again if dried in a location out of direct sunlight.

Use with paperwhite narcissus bulbs and watch the roots grow; place the bulb about ¾ of the way into the sphere.
Push a small wire through the sphere and put it back in the clear bowl – the wire will appear to be completely suspended in the water. This can also be done using a nail or small screw.
Use the spheres to illustrate magnification. Observe a finger THROUGH the expanded growing sphere. The growing sphere on top of the finger will flatten out and when the fingerprint is observed from above, it will appear larger due to the difference in the speed of light travelling through the sphere compared to air.
Use the growing spheres to calculate the amount of water absorbed. You will need a graduated cylinder to see the difference in the water level. Students could also calculate the average amount of water absorbed by a number of spheres in the cylinder.
Discuss with students how growing spheres (polymers) could be beneficial to humans because of their ability to absorb and hold water.
Use different liquids (other than water) to determine the absorption rate of other liquids (soda-pop, milk, vinegar, a water-vinegar mixture, apple cider, salt water, a water-sugar combination).
Use different types of water – salt water, tap water, filtered water, distilled water … or, even the temperature of the water could be varied – to measure the differences in absorption.

Cost of Spheres

The cost will vary depending upon where you obtain your spheres and which size and shape you purchase. However, the price is generally within the $8 - $20 range; Canadian prices tend to be higher due to import duties. Check with the supplier for the most up-to-date prices for the spheres.

Availability of Growing Spheres

The growing spheres are available from a number of sources. They can be purchased from stores that sell floral accessories … occasionally even in “dollar” stores. There are several educational suppliers as well who carry these spheres. Check with your science equipment supplier for your school or board.