Simple Science Experiment: Osmosis with Potato Slices



Ever cut up some apples or potato slices just to see them turn funky colors and not be as fresh as they could be? What can you do to protect these fruits and vegetables from this happening? What will keep potatoes (or other veggie) fresher: soaking it in regular water or salt water?

This month I’m going to explain this question, by explaining the concept of osmosis. Osmosis is a property of matter that deals with diffusion; a spreading out of particles from high concentration to low concentration (Basically, more stuff balances out with less stuff… like spraying a bunch of perfume in one place? Notice how it travels across the room?).

However, instead of stuff in air, osmosis describes the motion of water going through something. I’m going to give you the experiment, and then we’ll talk about how exactly this water motion occurs.

Materials:

A potato, salt, water (if you have distilled water, that kind is best), a couple of drinking glasses.

Procedure:

  • Fill two glasses with water
  • In one of the glasses add 2-3 tablespoons of salt, and stir it in
  • Slice up a potato into French fry-like pieces
  • Make your observations on these pieces: pay attention to color, how flexible it is, smell, etc.
  • Take a guess about how you think these slices might change by putting them into the different types of water
  • Dunk the pieces in the water, and then let them sit overnight in it
  • Remove the  pieces onto a plate and make your final observations

Explanation:

You will notice some immediate differences in the potato slices. The color of the salted water one is dark brown; not a nice image of how you would like your potatoes preserved! The one in the regular water looks like a nice white freshly cut piece of potato. Moving on to the flexible test, the regular water one again feels firm and crisp (try to break the piece, it snaps!). The salt water potato is bendy and doesn’t snap at all.

Osmosis is the key to understanding this issue. Osmosis is the diffusion of water across a semi-permeable membrane (yikes!) from an area of high concentration of water, to an area of low concentration.

Semi-permeable membrane: a layer that only certain things can go through. For example, parts of the potato that water can pass through.

Salt is the key here. Water will move from an area of less salt to more salt (more water to less water), and so when the potato is placed in the salt water, all the water that is inside the potato (yes, plants have a lot of water inside of them, that’s what gives a plant it’s structure) moves out by osmosis. Thus, the potato gets all flimsy and not crisp anymore. Much like if you were to water all your houseplants with salt water. They would all get flimsy, and then die, and then your parents would be upset so don’t try that at home please.

Experiment further:

Does the process of osmosis work with other pieces of fruits or vegetables? What about the temperature of water? Does that make it get flimsy faster or slower? Lots of things for you to test… remember science is about making observations, testing ideas, and then asking more questions.  

I hope you enjoyed this simple experiment.  If you have more questions about this, or need tips about science fair ideas around this topic (or others), contact the author.

Steve Davala is a middle school science teacher who likes to write and work with Photoshop. He’s got two kids of his own, who both like science (even if what they really like doing best is mixing baking soda and vinegar). Follow him on Twitter or on www.stevedavala.com or email him at steve.davala@gmail.com.  

                                                                            

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