Osmosis is the chemical process of diffusion, involving the transfer of solvent with a lower concentration of a certain solute through a semipermeable membrane, and into the area containing a higher concentration of that solute. It sounds like a confusing concept to teach younger children just getting into the sciences, but it’s actually quite simple, and can be demonstrated with only a potato, some water, and a little bit of salt
In fact, this osmosis experiment is a great way to teach any new biology student the gist of diffusion and the process of osmosis, regardless of age. If you’re looking to gain a strong, foundational understanding of biology, check out this introduction to basic biology course first.
What is Osmosis?
Say you have a container, with a separator down the middle. The separator is a semipermeable membrane, or a biological membrane that allows for the passage of certain molecules. The container is filled with water on both sides, with the side on the left containing a low concentration of salt, and the container on the right containing a high concentration of salt.
This particular solution is saltwater, where the water is the solvent and the salt is the solute. All this means is that the saltwater is more water than salt, and while that’s true for both sides, one side has more salt than the other.
Over time, the water on the left, containing less salt, will diffuse through the semipermeable membrane into the side of the container with a higher concentration of salt. Water molecules can pass through this membrane, but the salt can’t. The concentration of salt will remain the same on both sides, but now the concentration of water on the side with more salt will be greater.
This is due to a process called osmosis. You can learn more about a related process, diffusion, in this guide. Knowing basic cell structure is important for fully understanding osmosis. Check out this course on GCSE Biology for more in depth instruction.
Since osmosis and diffusion occur at the cellular and molecular level, it can be tough to picture, but with the help of a little osmosis experiment, you can actually see – to an extent – the process in action. Hands on experiments like this are a great way to engage students in learning. Check out this course on how to guide unmotivated students down a happy and fun academic path for more tips.
What You’ll Need
- two plates
- one raw russet potato
Since this experiment requires a knife to cut the potato in half, an adult should always be present during this experiment. Or if you’re a teacher, you can cut the potatoes and seal them in bags for the class beforehand, though it’s recommended that the potatoes are cut fresh. The plates should also be elevated enough on the sides that pouring water on them won’t cause them to overflow, and will create a nice pool for the potato slice to soak in.
Knowing a bit of chemistry is sure to make the learning experience here much smoother. Learn the principles of chemistry in this course.
First, lay out your two plates and fill them both with the same amount of water. It’s important that both sides have the same amount of water. Measure out two tablespoons of salt and sprinkle it into the water in only one of the dishes. Make sure you label which dish has the salt, and which dish doesn’t. This is vital.
Next, slice the potato. Remember, don’t let small children do this step themselves! The rest of the experiment is safe and easy for children of all ages, but slicing the potato should be done by an adult. If you are a science teacher conducting this experiment for an elementary or middle school class, check out this teaching course on how to construct lesson plans and keep younger students engaged.
Make sure you slice the potato down the middle, lengthwise. You want there to me as much of the raw potato exposed as possible. Place one potato half with the flat side down onto one dish, and the other half down on the other dish. Then, you wait. You should allow about two to three hours for the potatoes to soak in the water.
What you’ve essentially setup is a catalyst for osmosis. One potato half is soaking in a pool of freshwater, while the other potato half is soaking in a pool of salt water. What do you think will happen? If you’re teaching a class, this might be a good time to have students write down their hypothesis, or to cover relevant material in a text book so that when you return to the experiment, students will have an understanding of what has occurred.
After a few hours has passed, flip both potato halves over and have the students observe what has happened. The potato that was soaking in the freshwater will be slightly more dense, and with the same fresh white-ish color as before. The potato has soaked up some of the water.
The potato that was soaking in the saltwater will look much different. Its flesh should be slightly discolored, and the potato will be much softer than the one sitting in the fresh water. What occurred here was osmosis. The cells inside the potato have had the water sucked out of them by the high concentration of salt in the water of the surrounding dish. (For a more advanced understanding of cell structure, check out this course on biotech basics.)
While the water in the potato might contain a very small amount of sodium, it is nowhere near as concentrated as the salt inside the dish. This means the water inside the potato cells have passed through the semipermeable membrane of the cell walls and out into the dish of water. The collapsed, dehydrated potato cells are the reason for the mushiness and discoloration.