Osmosis Mini-Lesson Vanderbilt Student Volunteers for Science Fall 2016 Training Presentation

Important! Please use this resource to reinforce your understanding of the lesson! Make sure you have read and understand the entire lesson prior to picking up the kit! We recommend that you work through the kit with your team prior to going into the classroom. This presentation does not contain the entire lesson—only selected experiments that may be difficult to visualize and/or understand.

Introduction Ask the students if they know what osmosis is. Discuss the motion of water molecules striving to achieve a balance or equilibrium between a low area of concentration and an area of higher concentration of water. Introduce the students to the demonstration they are about to see. Tell the students that the are going to see how water can move into and out of a potato slice. Tell them that potatoes contain water and minerals. One of the minerals in a potato is salt.

Potato Illustration Give each pair 1 plate with three potato slices on it: one from the 1% salt solution (e.g. freshly cut, the control), one from the 40% salt solution, and one from the distilled water. 2. Ask students to observe all three potato slices. Tell the students to very carefully feel how rigid or floppy the potatoes are. (Warn them to NOT break them.)

Potato Illustration With your guidance, students should observe the following: The potato slice in the distilled water is stiffer , indicating that more water molecules went into the potato than came out, because there was a higher concentration of water outside of the potato. The potato slice in the salt solution is limp, indicating that more water molecules came out of the potato than went in, because there was a higher concentration of water inside the potato.

Potato Illustration Share the following explanation with students (adapt to the age of the students), along with drawing the diagram on the board: Osmosis refers to the movement of water molecules across a membrane trying to achieve equilibrium. If the concentration of salt is greater in the solution than in the potato, then there is less water in the solution. This means that the water from the potato will pass out of the potato in effort to achieve a balance. If there is less salt in the solution than in the potato (so more water outside of the potato), then water will pass into the potato.

Potato Illustration Share the following explanation with students (adapt to the age of the students), along with drawing the diagram on the board: In all cases, water is moving across the membrane to establish equilibrium. The water goes in or out of the potato to equalize the concentration of salt in the solution.

Osmosis in Sodium Polyacrylate Have pairs join together to form groups of four. Give each pair a large (16 oz) cup containing 1 tsp sodium polyacrylate and a 10 oz cup containing 200 mL water. Tell them to pour the water into the cup with the sodium polyacrylate. Observe that all the water is absorbed (forms a gel) immediately. This is osmosis - the water moved into the white powder.

Osmosis in Sodium Polyacrylate Tell them to take out about 2 tsp of the gel and put in to the 3.5 oz cup. Add 1 tsp salt and stir. Observe that the gel will return to liquid. This is osmosis again – the water moved out of the gel. Tell the students that this is similar to what happens when the potatoes are placed in water. When the potato is put into the distilled water, it will absorb the water. The water is trying to dilute the salt inside the potato. When the potato is put into salty water, it will lose water. The water in the potato moves towards the salty water to try to dilute it.

Osmosis in Sodium Polyacrylate Side Note: Uses for sodium polyacrylate include high absorbancy disposable diapers and moisture absorbent for automobile and jet fuels. When the superabsorbant polymer is distributed in sandy soil, it improves the soil’s ability to retain moisture and improves its ability to support agriculture. This polymer absorbs about 300 times its weight of tap water (800 times its weight of distilled water because the ions in tap water reduce the absorbing properties of the polymer). The addition of the salt (sodium chloride) breaks the "gel" polymer apart as water leaves the polymer to dilute the salt concentration outside the polymer network. Show the students the dried beans and the beans that have been soaking in water overnight the soaked and unsoaked dried fruit.

Osmosis in Beans and Fruit Fruits and beans are dehydrated to maintain freshness Show dried beans and fruit, as well as beans and fruit that have been soaked in water Water was absorbed through osmosis

Osmosis in “Orbs” “Orbs” are sold in garden stores They absorb water over time and swell Can be returned to their original size if left to dry

Osmosis in “Orbs” (cont.) A cup of polymer beads will be given to the teacher to keep, as shown Add water to until it’s ¾ full Students will be able to observe osmosis over the next few days as the orbs progressively increase in size