Water, Salt, and a Shriveled Carrot

Nature abhors a vacuum. Well, actually, nature abhors any kind of gradient, or sharp difference between neighboring things. Whether those things are hot coffee in a cold cup, an inflated tire with a hole punched in it, or salty water in contact with pure water, nature will seek a natural equilibrium. In the case of salty and pure (or mostly pure) water, one process through which nature can achieve a balance is called osmosis.

Osmosis occurs when two solutions of different concentrations are separated by a barrier that only allows water, and not to the salts or other dissolved solutes, to go through. In this case, since the solutes cannot go through the barrier, the water moves through from the least concentrated solution to the most concentrated solution. In this way, nature is seeking a balance between the two concentrations: as water move across the membrane, the concentrations of the two solutions come closer together.

With help from a parent, try this at home:

1. Fill a large glass 2/3 full of water and dissolve about one-half of a cup of salt in it.

2. Take a carrot that will fit all the way into the water and put it in the glass.

3. Watch the carrot over the span of a day or two: it will shrivel up noticeably.

What’s happening in this experiment is osmosis. The water inside the carrot contains some salt and other dissolved substances. But the concentrations of these salts inside the carrot will be less than the concentration of the salt water in the glass. Therefore, water moves from inside the carrot to outside the carrot, seeking a natural equilibrium, and the carrot shrivels up. Try pieces of other fruits or vegetables in combination with more or less salty water. Which combinations show the most shriveled food?

Considering what you’ve learned here about osmosis, can you understand why you should never drink the salty ocean water if you’re stranded at sea without fresh water?