Caution: This activity uses a knife. Have an adult help you with this activity.
What to do
Pour some water into each bowl.
Dissolve as much salt as you can into one bowl.
Cut the potato into slices, around 5 mm thick.
Place one slice of potato in each bowl.
Leave the potato for about 30 minutes.
What happens to the potato slices?
What's happening?
I found the potato in the fresh water was still crisp and firm, but the potato in the salt water was limp and soft.
The slices of potato are made up of cells, which mostly contain water. Normally, the pressure of the water in the cells pushes out on the walls of the cells and keeps them in the right shape. The potato in the salt water went limp because some of the water in its cells was "sucked out" by the salt in the water.
The membranes inside the cell walls in the potato have tiny holes in them that allow water molecules through, but not salt. If we had pure water on both sides of the cell walls, then the number of water molecules which went through in both directions would be roughly equal. However, the water outside of the cell has quite a bit of salt dissolved in it. This affects the flow of water through the cell walls in two ways:
As the salt molecules move around, they sometimes block the holes, so the water outside the cell can't go through to the inside.
The salt molecules tend to attract water molecules slightly, so they are less likely to go through the holes.
The result is that over time, the number of water molecules going from the inside of the cell to the outside is greater than the number going from the outside to the inside, so the cells loose water. With less water pressure inside the cells, they go limp.
This is an example of a process called osmosis. A barrier that will allow a liquid to go through but not the chemicals dissolved in it, like the cell membrane in the potato, is called a semipermeable membrane. Any time you have a concentrated solution (one with lots of stuff dissolved in it) that is separated from a less concentrated solution by a semipermeable membrane, the liquid will gradually flow into the more concentrated solution. This process is called osmosis.
With our potatoes, osmosis was actually occurring in both bowls. The liquid inside the cells of the potato is slightly salty, so the potato in the fresh water would actually absorb some of the water from the bowl. The cell wall stops the flimsy membrane from swelling up and bursting. The potato in the salt water lost water because the water in the bowl was saltier than the water in its cells.
Osmosis is a vital to all living things. Both plants and animals rely on osmosis to move water in and out of their cells.
Applications
We can't drink lots of seawater, because it is saltier than the water in our cells, so it will suck the water out of the cells of our bodies and make us thirstier (we have other mechanisms like our kidneys to help keep our body fluids balanced).
Osmosis can also be used in reverse. If the pressure on one side of a semi-permeable membrane is greater than on the other, the liquid can be forced through the membrane. This is sometimes used to purify water.
Many foods can be preserved by keeping them in a concentrated salt or sugar solution. The solution will kill bacteria that could otherwise spoil the food, by sucking the water from their cells through osmosis. This was particularly common before refrigeration was invented. The jam you spread on your sandwiches doesn't need refrigeration for this reason.
Slice your potato into 5 mm slices.
Place one slice in fresh water (left) and one in salt water(right)
After 30 minutes, you can bend one one slice of potato easily.
