You may have heard that even though you’re surrounded by water in the middle of the ocean, you could still die of thirst. How can that be? Let’s explore!
Materials:
• Gummy Bears
• Several glasses (to hold water solutions)
• 1 Tbsp salt
• 1 Tbsp sugar
• Spoon for stirring
• 1 cup measuring cup
• Cold water
• Optional: kitchen scale
Experiment:
To each of three glasses, add one cup of cold water. To the second glass, add one Tbsp of salt, and to the third, add one Tbsp of sugar. Mix the latter two cups using a spoon and make sure that all solids are dissolved. If you have a scale, individually weigh four gummy bears and then place one bear in each of the three glasses, and keep a gummy bear out of the glasses for your control. Allow the bears to soak for 24 hours and then remove the bears. Compare the bears from the 1st, 2nd, and 3rd glasses with the control (dry bear). If possible, weigh each bear after patting them dry. Did you find any noticeable differences between the size or weight of the bears?
What’s happening?
Gummy bears are made from gelatin, which has long fibers of protein that form a mesh-like network. This network acts like a semipermeable membrane, similar to the membranes in our cells. Water is free to flow through that membrane, but larger particles like salt and sugar often cannot. Nature always tries to balance things out (equilibrium). This means water will flow in or out of the gummy bear to even out the concentration inside and outside. This movement to dilute the solution through the semipermeable membrane is called osmosis. The gummy bear has sugar in it. In the first solution, the water will want to enter the bear to try to dilute the sugar as much as possible.
The dashed line represents the semipermeable membrane of the bear, and it allows water molecules (red dots) to travel through it to dilute the sugar molecules (green dots) that are in the bear.
We see this in the diagram as the membrane begins to grow and bulge out. The bear in the salt water grows the least. Although the water goes in to dilute the sugar and expand the bear, it also flows out of the bear to try to dilute the salt. On the other hand, the gummy bear in the sugar grows as there is more dissolved sugar in the bear than in the glass, so the water will go inside the gummy bear till the solutions are equally dilute in sugar.
This is similar to what happens in your body. If you drink ocean water, the water in your cells leaves to try to dilute the salty water. That makes your cells shrink and dehydrate, which is why drinking ocean water can actually make you thirstier.
Extension:
There are a number of other substances that you could try (baking soda, vinegar, other salts – Epsom, ice, etc.). You could vary the temperature ( be careful here, as the gelatin will dissolve in hot water) or the time. You could try different concentrations of sugar and salt water.
Foods with cells (potato cubes, carrots, melons, apple slices, etc.) could also be explored.
References:
https://www.chillededucation.org/growing-gummy-bears-osmosis-diffusion/
https://www.edplace.com/worksheet_info/science/keystage4/year10/topic/1266/7339/explore-the-process-of-osmosis-in-cells
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
