Magic water and convection

This lesson will give students a demonstration of how heat affects water particles.

A lesson plan for grade 6 Science

By JoAnne Pearson

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• Learn more about physics and physical science and science.

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Learning outcomes

Students will:

• collect data by questioning
• use the data to determine that differing temperatures cause the reaction observed

Teacher planning

Time required for lesson

30 minutes

Materials/resources

• four baby food jars
• two note cards
• blue and yellow food coloring
• hot water
• cold water

Pre-activities

Students will need to be skilled at making observations. They need to know how to ask questions that are answerable with “yes” or “no” to gather data to support the inferences they make from their observations.

Activities

1. Set up the experiment ahead of time.
   • Put one drop of blue food coloring in two of the baby food jars. Put one drop of yellow food coloring in the other two baby food jars.
   • Add hot water to one of the yellow jars and one of the blue jars. Fill the other yellow and blue jars with cold water.
   • Place a note card across the top of each of the blue jars, invert carefully, and balance each blue jar on top of a yellow jar. Be sure the necks are lined up exactly. You should have both blue jars (one hot and one cold) on top of both yellow jars (one hot and one cold) with a note card separating them.
   • Label one set of jars “Number 1” and the other “Number 2.”
2. Students draw a diagram of the two sets of jars as they appear once the experiment is set up, labeling them accordingly.
3. Students write a hypothesis stating what they think will happen when you remove the note cards that are separating the jars in each set.
4. Direct students to watch carefully. Slowly remove the cards from the jars (The set of jars with the hot water on the bottom will mix and become green. The one with the cold water on the bottom will remain the same or mix slightly where the two jars meet.)
5. Students record their observations, once again labeling each set of jars.
6. Ask the students to figure out exactly what you did to cause such different results. They should ask you questions that you can answer with either “yes” or “no.” They should eventually arrive at a series of questions about temperature such as “Are they all the same temperature?” or “Are both yellow jars the same temperature?”
7. Once students have determined that the difference was caused by the different temperatures and the location of the different temperatures, you are ready to discuss “why this works.”
8. Ask the students “What effect does temperature have on density?” Discuss the fact that when energy is added to an object (heating), its molecules move faster or vibrate more. This increases the space between the molecules and allows fewer to fit into a given space so the density is decreased. When energy is removed from an object (cooling), its molecules move slower or vibrate less. This decreases the space between molecules and allows more to fit into a given space. When you remove the card between the set of jars with the cold (denser water) on the bottom, they mix because there are many particles in the space. When you remove the note card between the set of jars that has the hot water (less dense) on the bottom, the denser water on the top sinks into the less dense water on the bottom.

Assessment

Students should be able to write a paragraph describing the results of each set of jars and why what they observed happened. They must state that the jars that mixed and turned green did so because the warm water with more space between the particles was in the bottom jar, which allowed the denser cooler water in the top jar to flow down and mix. The jars that did not mix had the cooler water in the bottom jar, leaving no space for the upper particles to flow. Use a rubric to score the paragraphs.

1. Students only state what they observed.
2. Students can correctly explain the placement of the jars by temperature.
3. Students can demonstrate an understanding of the principles of temperature and particle spacing.
4. Students can demonstrate an understanding of the principles of temperature and particle spacing using data from the experiment to prove their understanding.

Supplemental information

Comments

• Be careful when inverting the jars on top of each other. Many times the hot water jars are very hot to the touch.
• Place the experiment on a tray or paper towels in case some water escapes from the jars.
• Baby food jars are suggested, but if you do not have any available any four jars of the same size will work.