Potato Water

This experiment is a variation of several that we have tried in the past. It deals with something called osmosis. To find out more about this important part of the process of life, you will need:

  • a potato
  • a knife
  • salt
  • a plate

Select a large potato and carefully cut it in half. Place both halves of the potato on a plate, cut side down. Use the knife to carefully dig a hole in the top of each potato. Each hole should be at least big enough to stick the end of your finger into. Leave one just as it is. Into the other hole, put about 1/4 a teaspoon of salt. Set the plate aside and wait a while. How long is "a while"? That is a scientific term for the amount of time that it takes to eat a bowl of ice cream, with sliced bananas and chocolate sauce.

When you are done with your snack, look at the potatoes. The plain potato probably has not changed much. The potato with the salt has changed quite a bit. The hole probably has water in it, and the potato has probably begun to turn brown. Let the potato sit for a few hours and the difference will be even more striking. The salted potato will have lots of water and will be quite brown.

What caused the change? A wonderful process called osmosis. This is one of the driving forces of the cells in all living things. Each cell is surrounded by a thin skin called a cell membrane. This membrane lets water flow in and out of the cell. The flow of the water is controlled by the chemicals dissolved in the water. The water will flow towards the highest concentration of dissolved chemicals. If the concentration of dissolved chemicals inside the cell is higher, then water flows into the cell. If the dissolved chemicals are more concentrated on the outside of the cell, then water flows outwards. The salt you placed into the hole in the potato dissolved in the small amount of juice that was released when you cut the hole. This made a very concentrated salt solution, which pulled more water from the cells. The water pulled from the cells dissolved more salt, letting the salt solution reach more cells, to take more water from the potato.

That explains the water. What about the brown? Pulling that much water from the cells damages them. That causes them to release an enzyme called catechol oxidase, which combines with oxygen in the air to produce the brown color. With time, the plain potato will turn brown too, from the damage caused by cutting, but the salt causes much more cell damage and more browning.

This browning happens with many other fruits and vegetables. In fact, if you have any sliced banana left, it has probably started to turn brown by now. I can't be sure, as I ate all of mine.

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