A celery experiment is a good introduction to osmosis. Fill a glass with water and add a little food coloring. Cut the bottom off a stalk of celery that has its leaves, and place it in the water. Leave it overnight, but check the celery every couple of hours to see if there are any changes. The children will be able to see how osmosis happens because as the celery absorbs the colored water, it will change color. This will be most evident in the leaves, and if you cut the bottom off of the celery stalk. This experiment can also be done with a white carnation or even a carrot.
Fill two bowls with water, and add 2 tbsp. of salt to one of the bowls. An adult can cut a potato in half, and then place each potato half, cut-side down in the bowls. Let the potatoes soak for a few hours, flip them over and look at the difference between the potatoes. The potato in the fresh water will look basically the same because the salt and chemicals in the potato will absorb the water. The potato in the salt water will lose water because the water is saltier than the potato. This causes it to shrivel up and become soft. This experiment can also be completed with a carrot.
Cover a potted plant with a clear plastic bag, ensuring no air can escape. Wait several hours and ask the students to observe what has happened. Water droplets will have appeared on the inside of the plastic bag. This is water that has exited the plant through osmosis, also known as transpiration in this case. The water escapes from the plant into the air because the air has less water than the plant. The plastic bag makes this visible.
Get five small potted plants, and water each plant with about 1/4 cup of water each day. Add 1 tsp. of different types of detergent to the water for four of the plants. This can include dish detergent, laundry soap, hand soap and bath soap. Water the fifth plant with plain water. Keep the plants in the same spot, and write down your observations each day. The plants that get the water with soap will gradually wither and die because the detergents prevent osmosis from occurring.