(Chemistry for ages 10+)
I bet you’ve heard before that the roots of a plant are responsible for absorbing water, but how exactly does that happen? It all has to do with chemistry and a process called osmosis, which means the movement of water.
The video above shows one fun way to experiment with some potato and salt water to see how osmosis works on a larger scale. Here’s how it’s done:
1 large potato or 2 small- to medium-sized potatoes
Adult supervision (Adult supervision at all times please)
- Start by coring the potato. You want at least 2-3 potato strips per cup, so be sure to create 15-20 strips so you have a couple extras just in case any don’t work out.
- After you have created all of the potato cores, you will need to “clean” them up by cutting off any skin or brown spots you see.
- Use the ruler to make each potato strip the same length. The video shows each potato strip being cut to 6 centimeters, though any length is fine as long as all pieces are the same length.
- Now it’s time to make some saltwater. Each cup will have a different concentration of salt, the first of which will be 100 mL of water and have no salt. The remaining cups will each have 100 mL of water with 1 gram of salt, 2 grams, 3 grams, 4 grams, and 5 grams, respectively. Use the graduated cylinder to accurately measure the water you add to each cup, use the food scale to weigh out the proper amount of salt, and be sure to dissolve the salt in the water. Label each cup so you can easily keep track of them.
- Once you have all of your saltwater solutions ready, add 2-3 potato strips (depending on how many you prepared) in each cup. Set a timer for 20 minutes and leave your cups undisturbed during this time.
- When 20 minutes are up, pull the potato strips out of the saltwater and lay them on a paper towel. Be sure to keep track of which potatoes came out of each solution. Using the ruler, measure the length of each potato strip and record their length. The video shows a great example of how to set up a table for recording your results. What do you notice as you measure the potato strips? Have they gotten longer? Shorter? Stay the same?
- The video shows how you can take these findings even further and chart your results into a graph. This is a great way to visualize your results and see how the different solutions impacted the size of potato pieces. Graphs are a very useful tool in the sciences, so try it out and see what you come up with!
Osmosis is the movement of water, but more specifically, it is the movement of water across a semipermeable membrane (like a cell membrane), from high concentration to low. When salt is added to your water, there is a lower concentration of water (because of the salt) than in just plain water.
Potatoes are made up of plant cells that also have water inside them, but they also have their own salts and minerals. Water will move into the potato if there are more salts within it than in the solution the potato is in (likely what you saw with the plain water), which will make the potato grow larger (it swells up with water).
This means the potato is in a hypotonic solution. You can also think of this as more water outside of the potato than in, so it moves from a higher concentration of water to a lower one. On the other hand, we have a cases where the potato is in a hypertonic solution, which means there is more salt in the solution (less water) than in the potato (more water).
In this case, water will move from its higher concentration in the potato to the lower concentration in the saltwater. This will result in the potato pieces shrinking because they are losing water to the environment they are in. There is also the possibility of an isotonic solution, which means the concentrations of salts and water are the same in both the potato and the water it’s in.
When these concentrations are the same, water doesn’t have to move one way or the other so the potato pieces will stay the same size. You might have seen this in some of your potato strips.
This can all seem pretty complicated but osmosis is an important chemical property that makes life possible. For example, the plant cells that make up the roots of a plant will have a hypertonic environment within their cells (they have more salts and less water present) so water from the ground (a hypotonic environment) will move from the dirt into the cells, giving the plant the water it needs to survive.