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LAB in Conservation of Mass, Chemical Change. Last updated June 12, 2017.

Summary

In this lab, students perform three simple experiments, recording the mass of a substance before and after a reaction.  They then explain how the law of conservation of mass was not violated, even if the mass changed.

Grade Level

Middle and high school

Objectives

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

  • explain the law of conservation of mass.
  • determine whether or not a chemical reaction has taken place.

Chemistry Topics

This lesson supports students’ understanding of

  • Conservation of mass
  • Chemical changes

Time

Teacher Preparation: 15 minutes

Lesson: 40 minutes

Materials

  • Two 100-mL beakers
  • Dixie cup
  • Popcorn kernels
  • 0.5-M lead(II) nitrate
  • 0.5-M potassium iodide
  • 1-M hydrochloric acid
  • Safety goggles
  • Balance
  • Marble chips
  • Paper lunch sack
  • Microwave or popcorn popper

Safety

  • Always wear safety goggles when working with chemicals in the laboratory setting.
  • Food in the lab should be considered a chemical, not for consumption.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • 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

  • Make sure students return all of the popcorn to the bag after popping.
  • Students should find the mass of the chemicals by subtracting the mass of the beakers and cups from the total mass.

For the Student

Lesson

Background

Chemical changes are changes in chemical bonds. In a chemical reaction, the identity of the substance changes. You can usually recognize a chemical change by seeing evidence of this new substance. Evidence includes color changes, precipitates (solid formed from two liquids) forming, gas (bubbles) released, or energy changes in the form of heat, light, or sound (think fireworks).

The Law of Conservation of Mass states that in chemical reactions mass cannot be created or destroyed. The mass of the substances that you start with have to equal the mass of the substances produced.

Safety

  • Always wear safety goggles when working with chemicals in the laboratory setting.
  • Food in the lab should be considered a chemical, not for consumption.

Prelab Questions

  1. Wood burning is a chemical change. How do you know this?
  2. After burning, the mass of the wood has changed. Does this violate the Law of Conservation of Mass?

Procedure

EXPERIMENT 1

  1. Obtain two small (100-mL) beakers. Add approximately 20 mL of lead(II) nitrate to one beaker.
  2. Add approximately 20 mL of potassium iodide to the second beaker.
  3. Place both beakers on the balance and find the total mass of beakers and chemicals. Record observations.
  4. Pour contents of beaker one into beaker two. Find the total mass of beakers and chemicals. Record mass and observations in data table. Clean and dry beakers when finished.

EXPERIMENT 2

  1. Using the same beakers from experiment one, fill beaker one with a small handful of marble chips (up to approximately the 25-mL mark).
  2. Add approximately 25 mL of hydrochloric acid to beaker two. Place both beakers with substances on balance and record total mass.
  3. Leaving both beakers on the balance, carefully pour the hydrochloric acid from beaker two into beaker one. Leave both beakers on the balance and record your observations.
  4. Wait two minutes and record total mass again. Rinse marble chips with lots of water and pour into strainer. Clean and dry both beakers.

EXPERIMENT 3

  1. Obtain a Dixie cup and record the mass.
  2. Fill the Dixie cup about half way with popcorn kernels and record the mass. Determine the mass of the popcorn kernels alone.
  3. Obtain a lunch bag and record the mass.
  4. Using a hot air popper, heat the popcorn and collect the popped kernels in the lunch bag. Add any unpopped kernels back into the bag as well.

Record the mass of popcorn and lunch bag. Determine the mass of the popcorn alone.

Data

Experiment I

Before

After

Total mass of beakers & chemicals

g

g

Observations

(color/texture)

Experiment II

Before

After


TOTAL Mass of beakers & substances

g

g

Substance

Mass

Dixie Cup

g

Dixie Cup with Kernels

g

Kernels only (Calculate)

g

Lunch Bag

g

Lunch Bag with Popcorn

g

Popcorn (Calculate)

g

Analysis

EXPERIMENT 1

  1. Did a chemical reaction take place? How do you know?
  2. What change did you notice in the mass of the substances before and after your experiment? (Explain using data)

EXPERIMENT 2

  1. Did a chemical reaction take place? How do you know?
  2. What change did you notice in the mass of the substances before and after your experiment? (Explain using data)

EXPERIMENT 3

  1. Did the mass of the popcorn change before and after popping? (Explain using data)
  2. Did the volume of the popcorn change before and after popping?

Conclusion

Write a paragraph summarizing how these experiments demonstrate the Law of Conservation of Mass. Include evidence from each experiment in your summary.