Beacon Lesson Plan Library

Making Hypotheses

Mark Howell


Students practice making hypotheses about what they believe will occur as they perform an osmosis experiment in class.


The student knows that investigations are conducted to explore new phenomena, to check on previous results, to test how well a theory predicts, and to compare different theories.


Each group needs the following materials:
-Three 250mL beakers
-3 Potato cubes (precut by teacher)
-1 Ruler
-300 mL water
-1 Glass stirring rod
-1 Wax pencil


1. Gather the materials.
2. Purchase a bag of potatoes and cut them into equally-sized cubes.
3. Refrigerate the cubes until they are needed for the experiment.


1. Prior to gathering materials to begin the activities, arrange the students into groups of three, and then ask the class what the terms osmosis and hypothesis mean. Discuss the definitions as a class. Ask the students if they have ever seen the process of osmosis in action before, and if they have ever made a hypothesis about something. Ensure the students that today they will be participating in activities in which they will see how osmosis works, and that they will also be practicing their skills in making hypotheses.


2. Obtain three 250 mL beakers and fill them with 100mL of water. Using the wax pencil assign each beaker a number, 1, 2, or 3. Then add a handful of salt to beaker #1, a pinch of salt to beaker #2, and leave beaker #3 alone. It will contain pure water.

3. Obtain three potato cubes from the teacher that have been previously cut into equally-sized cubes.

4. Measure the perimeter of one side of each of the cubes and record the measurement on paper.

5. Label these measurements as “Pre-Submersion Measurement.” (You may need to model this for students.)

6. Place the potato cubes into the beakers of water (one cube in beaker #1, one in beaker #2, and one in beaker #3) with the side that was measured facing up toward the ceiling.

7. Ask the groups to write down their predictions as to what they think will happen to the potato cubes after they have been submerged in the different solutions.

8. Allow the cubes to remain in the beakers for 30 minutes.

9. During the 30-minute time period, which the students can keep track of using the classroom clock or a student's watch, the students write down hypotheses as to what size they believe the potato cubes will be when they are removed from the three beakers.

10. Hypotheses should be recorded next to the numbers labelled “Pre-Submersion,” and labelled “Our Hypotheses.”

11. During the 30 minutes, circulate and observe the groups' different hypotheses.

12. After 30 minutes, have the students remove the potato cubes and measure them once again on the same side.

13. Record the results and label them “Actual Size After 30 Minutes.”

14. Ask for quiet and discuss the students' measurements. Find out how close they were according to the hypotheses they made. Explain that no one was expected to be exact because a hypothesis is an educated guess.

15. Tell the students that if they were close that they did a great job hypothesizing, and those that were not close also did a great job, because again, a hypothesis is only an educated guess and is usually not accurate. Either way, everyone got to practice this important skill that is used in many of the experiments they will participate in during their present and future science classes.

16. Ask students to take notes, and reveal the reason for the final measurements, explaining how osmosis accounted for the change in the size of the cubes after the 30-minute time period. The potato cube that was in the high concentrate of salt shrinks because the cells in the potato contain a higher concentration of water than the salt solution they are sitting in. The reason for this is the cells push water through their cell membranes to try and equalize the solution and thus shrink due to the loss of water that was initially there. The potato cube that sits in the plain water will swell because the concentration of water outside of the potato cells is greater and in this case the cell membranes will pull water in to attempt to equalize the solution, and therefore the cells will grow larger in size due to the excess water they bring in. The cube in the weak salt solution will remain the same because the concentration of water is relatively equal both inside the potato cube cells and the outside salt water solution.


Students must have a paper showing the measurements they were asked to record as they conducted their investigation of the potato cubes. They will also be asked to record on those papers a short description stating how the cells inside the potato responded according to the solutions they were placed in. They can use the notes they took in step 16 to help them while they are writing. They must also include in their writing what the purpose of doing an experiment is.


Beets can be used for this experiment.
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