Demo: Density Flask Flip

Curriculum Context 

•  Grade 2 Physical Science - Properties of Matter
• Identify the properties of solids, liquids, and gases
• Investigate changes to the properties of matter when it is heated or cooled
• Investigate the interactions of liquids and solids

• Grade 7 Physical Science - Chemistry
• Conduct investigations into the properties of matter

Materials Needed

• 4 x 1L Erlenmeyer flasks (or soda bottles)
• 1L warm water
• 3L cold water
• 2 colours of food colouring
• 1 tablespoon of salt
• Clear bin
• 1 piece of acetate paper

Directions and Questions

Salt vs. Fresh Water

Illustration done using iDraw and Wacom tablet

1. Fill two Erlenmeyer flasks with water. Add salt to one of the flasks. Which of these liquids do you think is more dense?
2. Dye the water different colours using food colouring (e.g. red for salt & blue for fresh).
3. Place a piece of acetate paper onto the flask with the fresh water in it, and flip it over so that its opening is on top of the opening of the flask of cold water with the piece of acetate paper in between.
4. Ask students to make hypotheses: Which do you think is more dense, salt water or fresh water? When I slide the plastic out, what do you think will happen?
5. Slide the acetate paper out.
6. Flip the two flasks over, keeping their openings connected, so that the salt water is now on top. What are you observing? Why do you think that is happening? Does this remind you of anything?

Yellow = fresh water,
blue = salt water

Blue = salt water,
yellow = fresh water

Cold vs. Warm Water

Illustration done using iDraw and Wacom tablet

1. Fill one Erlenmeyer flask with hot water, and another with cold water. Which of these liquids do you think is more dense?
2. Dye the water different colours using food colouring (e.g. red for hot & blue for cold). (Optional: Ask a student to touch each of the flasks and confirm the temperature difference. Alternatively, use thermometers to show the difference.)
3. Place a piece of acetate paper onto the flask with the hot water in it, and flip it over so that its opening is on top of the opening of the flask of cold water with the piece of acetate paper in between.
4. Ask students to make hypotheses: Which do you think is more dense, hot or cold? When I slide the plastic out, what do you think will happen?
5. Slide the acetate paper out.
6. Flip the two flasks over, keeping their openings connected, so that the cold water is now on top. What are you observing? Why do you think that is happening? Does this remind you of anything?

Warm = red, cold = blue

Cold = blue, warm = red

Explanation 

Salt vs. Fresh Water
When the fresh water flask is on top, the fresh water will mostly stay in the top flask, and the salt water will mostly stay in the bottom flask. This is because the fresh water is less dense, so we can think of it as being less heavy. Within the 1L volume, there is more mass, because we added the salt. (The salt dissolves in water, providing an example of a solid-liquid interaction.) After flipping the flasks, the salt water will sink to the bottom because it is denser, and the fresh water will float upward because it is less dense. A similar process takes place in estuaries, where a river connects with the ocean. 

Hot vs. Cold Water 
When the hot water flask is on top, the hot water will mostly stay in the top flask, and the cold water will mostly stay in the bottom flask. This is because the hot water is less dense, so we can think of it as being less heavy. The molecules in the cold water are closer together than the molecules of the hot water. After flipping the flasks, the cold water will sink to the bottom because it is denser, and the hot water will float upward because it is less dense. A similar process, called convection, takes place during the seasonal mixing of lake waters, and in ocean currents.

Safety Considerations

• Water could make the floor slippery. Have students stand back, and use a clear bin to contain the experiment.
• Glass flasks could break if dropped. Have students stand back, and use a clear bin to contain the experiment.  

Next Steps

• Students could engage in a hands-on exploration of density. One idea would be making density columns using pipettes, test tubes, and salt water of various concentrations (coloured with different colours of food colouring).
• Connections to habitats and associated processes, such as lakes an estuaries, could be elaborated on.  

 

References

• Spangler, S. (2013). Colorful Convection Currents. Retrieved January 18, 2015, from http://www.stevespanglerscience.com/lab/experiments/colorful-convection-currents  
• University of Alaska Fairbanks [UAF]. (2008). Retrieved January 18, 2015, from http://www.arcticclimatemodeling.org/lessons/acmp/acmp_58_SeaIce_SaltwaterScience.pdf