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LAB in Observations, Density, Physical Change, Balancing Equations, Chemical Change. Last updated June 4, 2018.


Summary

In this lab, students add food coloring to a mixture of oil and water and record their observations. They then add an Alka-Seltzer tablet, record their observations and answer a series of questions about the chemical and physical changes that took place.

Grade Level

High school

Objectives

By the end of this lesson, students should be able to

  • identify chemical changes.
  • identify physical changes.
  • write and balance a chemical equation.
  • compare the densities of different substances.

Chemistry Topics

This lesson supports students’ understanding of

  • Density
  • Observations
  • Chemical change
  • Physical change
  • Balancing equations

Time

Teacher Preparation: 15 minutes

Lesson: 40 minutes

Materials

  • Water
  • A clear plastic bottle
  • Vegetable oil
  • Food coloring
  • Alka-Seltzer tablet
  • Dish soap

Safety

  • Always wear safety goggles when working with chemicals in a laboratory setting.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

  • Have a large container at the front of the room to discard the used vegetable oil.
  • See suggested rubric in the answer key document.

For the Student

Lesson

Background

In this experiment, a chemical reaction will be observed when an Alka-Seltzer tablet is dissolved in water. Alka-Seltzer is a solid mixture of two substances, sodium bicarbonate (NaHCO3) and citric acid (H3C6H5O7). When dissolved in water, the substances react to form water, carbon dioxide (CO2), and an ionic compound (chemical formula Na3C6H5O7).

Materials

  • Water
  • A clear plastic bottle
  • Vegetable oil
  • Food coloring
  • Alka-Seltzer tablet
  • Graduated cylinder

Procedure

  1. Pour 100 mL of water into the plastic bottle.
  2. Pour in twice as much vegetable oil—you don’t need to measure it, just estimate.
  3. Wait until the oil and water have separated. Record your observations.
  4. Add 10 drops of food coloring to the bottle (choose a color).
  5. Watch as the food coloring falls through the oil and mixes with the water. Record your observations.
  6. Cut an Alka-Seltzer tablet into five or six pieces and drop them into the bottle. Record your observations.
  7. Clean up: After the reaction has stopped, slowly pour most of the oil into the designated container. Try not to pour in any colored water. Add a squirt of dish soap to the remaining oil and water in your bottle, swish it around to mix, and dump the resulting solution down the drain. Put the empty bottle in the trash, NOT the recycling bin.

Observations

Step

Observations

Oil is added to water

Food coloring added

Alka-Seltzer tablet added

Conclusion

1. Read the last two sentences of the background again: Alka-Seltzer is a solid mixture of two substances, sodium bicarbonate (NaHCO3), and citric acid (H3C6H5O7). The substances react to form water, carbon dioxide (CO2), and Na3C6H5O7.
a. List the reactants:
b. List the products:
c. Write the balanced chemical equation.

2. a. Explain how this experiment demonstrates density.
b. Which material had the highest density?

3. Give an example of a physical change that you observed during this experiment. Explain how you know it was physical.

4. Give an example of a chemical change that you observed during this experiment. Explain how you know it was chemical.