Activity: Discovering Water Density
Summary: Students explore the phenomenon of thermal stratification by watching the teacher demonstrate the different densities of hot and cold water. They will complete the student data sheets and discuss the results as a group.
- 50 minutes
- 4 glass canning jars
- Quart or pint size
- Jars with a shoulder (narrow band at the top) work well
- Blue and yellow food coloring
- Index cards
- 2 large basins
- Warm and cold tap water
- Start by filling one jar approximately 1/8” from the top with cold tap water (if tap water is not very cold, add one or two ice cubes and allow to melt).
- Add yellow food coloring to the cold water jar and set jar in basin.
- Fill second jar with warm tap water approximately 1/8” from the top.
- Add blue food coloring to the warm water jar.
- Be careful water is not hot enough to burn. Maximize temperature difference by using very cold water rather than very hot water.
- Invert the jars, warm on top of the cold:
- Cover the top of the warm blue water jar with an index card.
- Hold the jar with one hand and use the lengths of your fingers on the opposite hand to hold the index card over the top of the jar.
- Carefully, in one quick fluid motion, turn the jar upside down and on top of the cold water jar, while sliding the index card (and your fingers) away. With the warm blue water jar inverted over the cold yellow water jar, the warm blue water should remain on top and the cold yellow water should remain on the bottom.
- About an eighth of a cup of water may spill while turning the warm jar on top of the cold jar.
- Alternate method: Carefully invert the warm blue water jar on top of the jar in the basin with the index card in place. Slide the card out from in between the two jars.
- Allow the jars to stay as they are during the remaining steps.
- Next, repeat the procedure using water with uniform temperatures.
- Fill two jars with warm tap water.
- Place one jar in the remaining basin.
- Add blue food coloring to the other jar.
- Invert the jar with warm blue water over the jar with warm clear water.
- When the warm blue water jar is inverted over the warm clear water jar, the blue and clear water should immediately begin to mix.
- After both jars have reached a uniform color, empty the jars and repeat with procedure with cold water.
- Return to the blue and yellow water stacked jars from the beginning of the activity.
- As water temperatures have become more uniform during the class period, the water in the upper and lower jars has begun to mix.
- Discuss the principles of water temperature and density (found in Lesson: Layer Upon Layer – Principles of Water) with your class.
- Ask them to describe what is happening in the jars.
- Have students complete the downloaded data sheet.
This extension shows the differences in rate of mingling of warm water compared to cool water. Using two jars, you will invert a jar of cold water over a jar of warm water — mixing happens slightly faster when inverting cold over hot water.
- Fill one jar approximately 1/8” from the top with warm tap water.
- Add blue food coloring to the warm water jar; set the jar in a basin.
- Fill the second jar approximately 1/8” from the top with cold tap water (add one or two ice cubes and allow to melt if tap water is not very cold).
- Add yellow food coloring to cold water.
- Invert the jar of cold yellow water over the jar of warm blue water.
- Warm blue water should begin to move up into the upper jar and cold yellow water should move down into the lower jar.
- The temperature of the water in the jars becomes uniform very quickly. The effect is similar to starting with uniform temperatures, but there is a subtle difference in how the water mixes.
- Discuss the difference with your class.