Summary

In this lesson, students will build upon their understanding of solutions and concentration. They will observe the diffusion of food coloring dye in water and then perform an experiment focused on how solutions of different concentrations will affect the movement of water across a semi-permeable membrane.

Grade Level

Middle School and High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• Scientific and Engineering Practices:
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this lesson, students should be able to

• Define diffusion and osmosis.
• Understand how concentration differences can affect the process of osmosis.

Chemistry Topics

This lesson supports students’ understanding of

• Solutions
• Concentration
• Solute
• Solvent
• Diffusion
• Osmosis

Time

Teacher Preparation:

• Demo: 15 minutes
• Lab: 25 minutes

Lesson: 40 minutes (leave the osmosis set-up overnight)

Materials

• Copies of April 2007 ChemMatters article, “Paintball”
• Copies of the Reading Guide

Diffusion Demo:

• Two 1000mL Beakers
• 600ml water (distilled or deionized) in each beaker
• Hot plate
• Food coloring
• Thermometer (optional)

Osmosis lab: (Per lab group)

• Dialysis tubing, 6 inch long, soaked in water overnight
  • Recommend 33mm width as it is easy to handle.
  • For easy dispensing tie knots at one end after the tubing has softened
• 800 mL beaker
• ~ 500 mL deionized or distilled water
• 200mL of 10% sodium chloride solution
• Rubber band
• Thistle tube (or a Buret without the tip)
• Disposable Beral pipet

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Teacher Notes

• In this lesson, the students will explore the concepts of diffusion and osmosis.
• Students should have prior knowledge regarding how to prepare a solution and how to describe the amount of solute present in the solution compared to the amount of solvent.
• During this lesson, students will read the April 2007 ChemMatters article, “Paintball”. The article involves the formation of a mixture in a gelatin membrane.
• Teacher preparation for the demonstration will require about 15 minutes to collect materials and heat up a sample of water.
• Teacher preparation for the lab will require about 25 minutes to prepare sodium chloride solution and dialysis tubing.

Lesson Overview:

• First review the parts of a solution:
  • Solute: substance that dissolves
  • Solvent: substance that does the dissolving
• Describe the process of solvation using this animation of salt dissolving in water.
• Review related content:
  • Concentration: describes the amount of solute present in the solution.
• Qualitative considerations:
  • Concentrated: large amount of solute present
  • Dilute: smaller amount of solute present
• Quantitatively:
  • Percentage by mass: determined by the mass of solute/volume of solution
• Next, give students a copy of the April 2007 ChemMatters article, “Paintball” to read and complete the Reading Guide. Answer key has been provided for teacher reference.

Demonstration:

• Since the article describes how the paintball is made of a gelatin membrane, which will increase in size, when the paintball is placed in water. A demonstration of diffusion would be helpful to illustrate how diffusion occurs.
• Procedure:
  • Set up two 1000 mL beakers, one with hot water and the other with cold water. The temperatures are not important; they just need to be different.
  • Add two drops of food color into each beaker, try to do this very close together/ at the same time.
  • Students should complete the demonstration handout while observing.
  • Project a copy of the student handout or recreate it on the classroom board.
  • See the Answer Key for expected results and discussion points.
  • Explain Diffusion: Molecules will tend to move from a high concentration of substances to a lower concentration.
• Next, introduce how water may move through a semi-permeable membrane.
  • Define osmosis: a process where diffusion occurs through a semi-permeable membrane.
  • The membrane will allow smaller particles to go through. Water will be able to move through such a membrane, if there is a concentration gradient across the membrane.
  • Use this animation of osmosis to help students understand the process.

Osmosis Lab:

• The “Osmosis Lab” will give the students an opportunity to experiment with how the paintball may change in volume when placed in solutions of a different concentration.
• To simulate the gelatin membrane, a dialysis tubing is use in this experiment:
  • The students will set up a concentration gradient through a dialysis tubing (a semipermeable membrane). The dialysis tubing can be purchased from any scientific supply company which sells biological supplies, such as Flinn.
  • The tubing must be soaked in water in advance of use.
  • When the tubing is soften, you can easily tie a knot at one end. You may wish to set up an example for the students to see or if time is limited, you can tie the ends of the tubing before distributing them to each group.
• Teachers must prepare a saturated solution of sodium chloride by adding 1 gram of sodium chloride per 1 mL of water.
• For a class of 24 with students working in pairs, prepare 200 mL of solution by dissolving 200 g of sodium chloride in 200 mL of water.
• The expected observation of the lab is that the liquid level in the tubing will go up because water will diffuse through the membrane.
• If the thistle tube or buret is not available, you can alter this experiment by using a longer length of dialysis tubing and tie both ends and have the students find the mass of the tubing before and after the activity. Note that the students will need to dry the outside of the tubing before collecting each mass.