Osmosis is the process of fluid passing through two semi-permeable membranes to create an isotonic state, or equal amounts of fluid on either side of the barrier. It is the primary method of cellular water transportation in all living things. The process of osmosis can be demonstrated by conducting a few simple, inexpensive experiments in the classroom or kitchen.
Osmosis in Celery
Food coloring and a stalk of celery demonstrate how plants use osmosis to absorb water from the soil. Fill a clear cup with about 20 drops of red food coloring, then place a fresh celery stalk with the leaves intact in the cup. In just a few minutes, the food coloring will begin to leach up through the celery stalk into its leaves.
A carrot is a taproot --- a root that grows straight into the ground. There are two experiments you can perform with carrots that demonstrate osmosis. In the first experiment, fill a glass half-full with water. Cut the tip of a carrot off and put it in the water. Add 10 drops of red food coloring to the glass and place the glass in front of a window for a few days. Cut the carrot in half and look inside. The tubes of the carrot are died red, showing how water was absorbed from the bottom of the carrot to the top. In the second experiment, you will need one carrot, two glasses, 3 tablespoons of salt, string and water. Fill both glasses half-full with warm water, then dissolve the salt into one of the glasses. Mark the glass "salt." Break the carrot in half and wrap a string around the broken end of each piece. Place the carrot halves, broken and tied side down, into each glass of water. The next morning, pull the carrots out of the water. The carrot soaked in salt water will have shrunk while the carrot soaked in plain water will have bloated.
Because an egg is a giant cell, it is an excellent model for demonstrating animal cell osmosis. You will need three de-shelled raw eggs, three glass jars, vinegar, water and corn syrup. To de-shell the eggs, soak them overnight in vinegar. The shells will dissolve, leaving only a thin membrane behind. Place one de-shelled egg in a jar filled with water and another in a jar filled with corn syrup. Place the third egg in a jar filled with the vinegar used to de-shell the eggs. Osmosis is demonstrated by the size of the egg. The egg in pure water will expand as it soaks up the water around it while the egg in corn syrup will shrink as water flows out of the egg. The egg in vinegar has achieved an isotonic state, so the egg will remain the same size.
To demonstrate how water moves naturally from areas of low salt concentration to areas with high salt concentration, you will need two potatoes, two glass dishes, salt and water. Fill both dishes with an inch of water. Add 2 tablespoons of salt to one dish, and label it "salt." Cut the potatoes in long quarters that are flat on one side. Place each of the cut potatoes in one of the dishes. After 20 minutes, look at the potatoes. Nothing has changed in the potato soaked in regular water, but the potato in salt water has turned to mush. This is a result of osmosis --- water left the potato to balance out the high concentration of salt in water surrounding it.