Diffusion (Kathy Shenassa)

Title: Effects of Density and Temperature on the Movement of Molecules

Principle(s) Investigated: Diffusion rates, kinetic theory, molecular motion, density, salinity, concentration

Standards :

MS- PS1-4:

Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.Clarification: qualitative molecular-level models of solids, liquids, and gases to show that adding or removing thermal energy increases or decreases kinetic energy of the particles until a change of state occurs.

HS-PS1-6:

The concept of stability and change in systems. Diffusion from high to low concentration denotes how systems tend towards achieving equilibrium, and how changes in the system will affect the rate of molecular motion, through change in kinetic energy (heat) or physical barrier (salinity).

Materials: Include a list of materials and sources from which they may be obtained.

Materials:

Each group receives:

• 3 glass beakers filled with equal amounts of 1) hot water 2) room temperature water 3) ice water
• 2 additional beakers filled with 1) plain water 2) salt water (both room temperature)
  • 1 vial of food coloring
  • timer

Procedure:

Divide the class into groups of 3-4 students. There will be 2 parts to this experiment:

Part 1:

On the Hypothesis form, fill out your predictions.

Place the 3 beakers of different temperature water on a white background. Using a dropper, place a single drop of food coloring onto the surface of each beaker, and record the time it takes for the color to become uniform through the container in This chart

Part 2:

Place the 2 beakers of plain and salty water on the white background. Repeat the procedure above, placing one drop of food coloring into each beaker. Record the time it takes for the color to become uniform in each container and again record in This chart

Student prior knowledge: What prior concepts do students need to understand this activity?

Students should be aware of the concepts of molecular motion, and the tendency of matter to reach equilibrium. Students should understand that molecules will move from areas where they are in higher concentration to lower concentration (down the concentration gradient) until an "equilibrium" is achieved, where they are equally distributed. Students will contemplate how various factors such as temperature and concentration of other solutes in the medium can influence the rate of diffusion. Students can connect this knowledge to understand the importance of diffusion in cellular function.

Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.

This experiment is designed to demonstrate the principle of diffusion, one of the basic . Contrasting the rate at which food coloring will diffuse in hot vs cold water will enable students to visualize the movement of molecules through a medium and their tendency to go from the areas of high concentration to low concentration until equilibrium is achieved. They will see that by contrast, in a cold medium, the molecular movement is more restricted due to a lower kinetic energy and diffusion will occur slower. They can also visualize that when the medium is more saturated (saltwater), it would be more difficult for the food coloring to move around, and should take longer for diffusion to occur.

Questions & Answers: Give three thought-provoking questions and provide detailed answers.

1) We see that temperature and density of the medium can affect the rate of diffusion. Can you think of any other factors that can influence diffusion in a cell?

Answer: the size of the molecule can affect how fast it would diffuse in a medium. A larger molecule will have a harder time moving through other molecules, and therefore would diffuse slower than a smaller molecule.

2) Explain why diffusion rates increase with temperature.

Answer: Molecules in a liquid move randomly, and bump into each other at random. Increasing temperature will increase the kinetic energy of the these molecules, increasing the speed (velocity) at which they move and collide. Therefore, with higher temperatures, the molecules will move faster and achieve equilibrium faster. Conversely, lowering the temperature will slow down their movement, and diffusion will slow down.

Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.

1) It is easier to dissolve sugar in hot milk than it is to dissolve it in cold lemonade. And when you stir your beverage, you are adding kinetic energy, allowing the molecules to move faster and achieve equilibrium faster.

2) Diffusion is a key component of cellular functions, from the exchange of oxygen and carbon-dioxide in the lungs, to absorption of food in the intestines and neuronal communication.

3) We can observe the diffusion of gases when smell of food cooking will diffuse through the house, or when we observe a helium balloon deflate because helium diffuses out. Have you noticed that in hot weather, you are able to smell bad smells (garbage, sweaty sock) better?!

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.

The experiment we performed today is explained in this video:

https://www.youtube.com/watch?v=NvZPV2iR1pU&nohtml5=False

A clear explanation of osmosis and diffusion:

https://www.youtube.com/watch?v=zuNMVzTeCtw&nohtml5=False