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Name: Kathy
Status: student
Grade: 6-8
Location: NJ
Date: December 2007

Question:
Why doe a gummy bear remain the same when placed in salt water and grow when placed in tap water?



Replies:
Believe it or not, both gummy bears and salt water are a mixture of "something" dissolved in water. When one material is dissolved in another, such as salt in water, the salt is known as the "solute" and the water is known as the "solvent". With salt water, the solute is salt and the solvent is water. With gummi bears, the solute is gelatin, and the solvent is also water.

Gummi bears start as a liquidy solution of gelatin and water. As you cool the solution and draw more water out of the gummi bears, they harden into the chewy texture you're used to. Gelatin, unlike salt, is a chain-like molecule that can intertwine and form a solid-like matrix -- that's how gummy bears start as liquid, but solidify as water is removed.

Like gummi bears, salt water is a solution of water, but there is a lot less solute (by mass). Salt also cannot form interlocking chains like gelatin. That's partially why salt water stays a liquid and the gelatin solution solidifies. However, because gelatin molecules are so much larger than salt ions, there may be many fewer (by number) gelatin molecules dissolved in the water. The reason numbers of molecules are important is because it turns out numbers of molecules play a big role in determining if your gummi bear will absorb water or not.

If you put two solutions of water in contact with each other, water will tend to move from the solution with fewer molecules dissolved in it to the solution with more molecules in it. This is known as 'osmosis'. The force that pushes the water is called 'osmotic pressure'. With the gummi bear, if you put the gummi bear in a solution with very few molecules dissolved in it (like distilled water), the water will move into the gummi bear causing it to expand. If you put the gummi bear into a solution of water with many molecules of solute dissolved in it (more solute molecules than are in the gummi bear), then water will leave the gummi bear and move into the water. When water moves into the gummi bear, you can see the bear expand. However, since the gummi bear doesn't shrink much when water leaves it, it appears the gummi bear stays the same.

So the last question to answer is if salt water has more solute molecules in it than the gummi bear (you know by your experiment that it does, but we can prove it with some math too). You can dissolve roughly 400g of salt (NaCl) in 1kg water at room temperature. That's roughly 2/3 of a mole of salt molecules, (google "mole unit" to learn more about what a mole is in chemistry). Because a single molecule of gelatin weighs 10,000 times more than one of table salt, if you had the same mass of gummi bears as the salt solution (1 kg + 0.4 kg), or 1.4kg total, you would have only 1/25th of a mole. So the salt solution has around 10-20 times the number of molecules as the gummi bear. Because there are more solute molecules in the salt water, the water moves out of the gummi bear, and hence the gummi bear does not expand.

Hope this helps,

Burr


Gummy Bears when placed in salt vs fresh water will respond differently by way of osmotic pressure. A great experiment is to place Gummy Bears in both solutions and then different degrees of salt content. Remove them after say 30 minutes let them rest..... is there another change taking place?

Hope this helps.

Prof. Przekop



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