To explore the effect of heating and cooling on the dispersal of food coloring in water.
The primary purpose of this experiment is to engage students in an activity that will allow them to observe that hotter conditions can speed up changes in materials. Students will predict whether food coloring disperses more quickly in hot, cold, or room temperature water, and then carry out a short activity to explore their predictions.
To elicit student responses and to help find out what the students already know about the benchmark idea, ask the following questions.
- Have you ever made a cup of tea? How did you make it?
- Do you think that the temperature of the water matters when you are making tea?
- Why is tea brewed in hot water instead of cold water?
You can also ask these questions as part of a demonstration in which you brew a tea bag in a cup of hot water. If you choose to do this, it is best to use a clear glass cup so children can observe the changes in the water as the tea is brewed.
The purpose of this activity is to let students experience the effect of heat on the dispersion of a drop of food coloring.
Begin the activity by asking students to predict what will happen if they put a drop of food coloring into a cup of water. Then, place a drop of red food coloring into a clear cup filled with water and hold it up. Ask the children to describe what they see. You may want to walk around the room so that all the students can observe the food coloring being dispersed into the water.
Then, say to students, "Do you think that the same thing would happen if we put food coloring into hot water? Or cold water? Would there be a difference? Let's find out."
If students have difficulty answering this question, ask them to think about what happens when you make tea.
Next, students will conduct an activity in which they drop food coloring into a container of room temperature water, cold water, and hot water, and compare the rates of dispersion of the food coloring in each. Following are notes on the activity; see the Hot and Cold Colors teacher sheet for the suggested procedure.
- It will be important to discuss the procedure with the students before beginning the activity (the instructions on the student worksheet may not be clear without guidance).
- It is suggested that students work in groups of four. You may want to make sure that the suggested lab roles are assigned and followed within each group.
- It takes about 30 seconds for the food coloring to dissolve in the hot water. The actual dispersion activity should not run longer than two minutes.
- A common data table can be composed on the overhead, board, or computer, where students can write their data and observations. (See student worksheet for a sample data table.)
- See the student worksheet for a complete list of materials. Please note: the containers can be juice glasses, measuring cups, beakers, etc.; and if stopwatches aren't available, students can use the second hand of a clock.
- Students will need access to cold water, hot water, and room temperature water; a pitcher can be used for the cold water, and a coffee pot or teakettle for the hot water. You may choose to have these available at a central location in the classroom, or at individual workstations.
When you are ready to begin the activity, place students into groups of four and give each student a student sheet.
Start by asking students to answer the Prediction question on the student sheet:
- "Which will change color faster when a drop of food coloring is added: cold water; hot water; or room temperature water? Explain your answer."
Then have students read the worksheet and follow the procedure to complete the activity.
When the activity is complete and all students have answered the questions on the student sheet, discuss the results with the class.
After students have discussed the results of their investigation, lead the class in a discussion about changes that you want to slow down. For example, butter or ice cream melting; food spoiling; sugar dissolving; and so on.
Ask students: How might you slow down these changes?
The assessment activity presents students with a question in which they can apply the idea in the lesson to a real life situation:
- "Write a paragraph describing something you witnessed in the last week that demonstrates how heating or cooling affected a material (consider what happened in your kitchen, backyard, etc.)."
Ask students to read this question (found on the Hot and Cold Colors student sheet) and then write a paragraph explaining their answer. What is most important is that each student demonstrates the understanding that heating and cooling affect materials, and that some changes occur faster under hotter conditions. A secondary evaluation may be how well the paragraph is constructed.
The International Boiling Point Project
Students can participate in this global science project that has as its purpose to discover which factor in the experiment (room temperature, elevation, volume of water, or heating device) has the greatest influence on boiling point.