Summary

In this lab, students perform seven different investigations in order to develop an understanding of conservation of mass.

Grade Level

High School

Objectives

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

• Explain the difference between the system and surroundings
• Explain the difference between a “closed system” and an “open system”
• Define mass as the amount of “stuff” in a substance and that the particles of the substance give the substance mass
• Explain that mass is conserved in a closed system and that if the universe is considered the system that mass is conserved during any process
• Make and defend a claim given data in a histogram
• Provide reasonable sources of error for data that do not follow a trend observed during an investigation

Chemistry Topics

This lab supports students’ understanding of

• Conservation of mass
• Matter
• Observations
• Systems vs. Surroundings
• Error Analysis
• Particulate Nature of Matter

Time

Teacher Preparation: 30 minutes
Lesson: Three 50 minute periods for lab and discussion

Materials

• Balances (0.1 g or 0.01 g precision)
• Test Tubes
• Beakers
• Crucible tongs
• Container with lid
• Bunsen Burner or Lighter (to ignite steel wool)
• Watch glass
• Balloon
• Erlenmeyer flask
• Steel Wool
• Ice
• Sugar or salt (to dissolve in water)
• Baking Soda and Vinegar (to produce a gas)
• 0.1 M sodium carbonate and 0.1 M calcium chloride (to form precipitate)

Safety

• Always wear safety goggles and lab aprons when handling chemicals in the lab.
• 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.
• When steel wool is ignited it should be done over a watch glass or flame resistant container that can then be massed. It can melt weigh boats. Small pieces may fly off a short distance.
• The precipitate reaction used here is safe and nontoxic. Avoid the use of toxic ions or ions that require special disposal such as lead (II), silver, or mercury.

Teacher Notes

• This lab is designed to be done as one of the first activities of the year in a first year course.
• Students will perform seven different investigations. You may choose to have them complete all of them at once or place them in to groups as you see appropriate for post-lab discussion.
• Creating a central area for material distribution aids in facilitation so each lab group does not need all of the materials. Students do not all have to be doing the same experiment at the same time.
• The use of centigram (0.01 g) balances is recommended for an introduction to error, precision, and significant figures as students may observe a 0.01 g change when no change in mass actually occurred. Students always complain about the last digit “ticking back and forth” and not settling on one value. If you do not wish to introduce this at this time, the use of decigram (0.1 g) balances is appropriate.
• You may choose to give students the detailed procedure below or demonstrate the goals of each investigation and develop a procedure as a class during pre-lab discussion. The latter is recommended so that students have an understanding of what is going to happen and don’t get lost in procedure.
• Because this is an introductory lab before students have accepted the particulate nature of matter, this is not an appropriate time to discuss physical vs. chemical changes.

Melting Ice

• Use small pieces of ice and have students work on other investigations while their ice melts as it could take some time.
• This investigation produces the most consistent results as no transferring of material is required between mass measurements.
• Get the students to notice that liquid water takes up less space than solid water, this is a good example to come back to when density is discussed in future lessons.

Forming a Precipitate

• Use two small test tubes and support them on a balance by placing them in a small beaker.
• Students will usually predict the mass will increase because “solids weigh more than liquids”
• Some students will suggest subtracting the mass of the empty containers so they are only recording the mass of the reagents while others will recognize this is not necessary if all materials are massed before and after. Either method is appropriate, use this as an opportunity to discuss proper techniques of massing by difference with them and the importance of massing all containers before and after the reaction.
  

Dissolving Salt or Sugar

• The best technique is to use a container with a lid, place the substance in the lid for massing before and then massing the substance, container, and lid afterwards.
• Students usually record a small mass loss during the investigation due to improper transfer when pouring the substance into water.
• Some students conclude this mass loss is because the substance dissolved “disappeared.” If so, provide the proper explanation as an alternative and see if they accept it. If not, this may be an appropriate time to boil the solution as a demonstration to show the solute remains.

Pulling apart Steel Wool

• Students usually record a loss of mass in this investigation due to small pieces of steel wool not falling on the watch glass. When you observe this ask them why the mass decreased, they are usually quick to offer the proper explanation.

Burning Steel Wool

• The best technique is to hold steel wool with crucible tongs over a watch glass and having a second student hold the flame of a lighter or Bunsen burner to the steel wool.
• Students are usually very intrigued by the bright glow and sparks produced, but make sure to coach them not to be distracted and remain aware of safety precautions.
• The piece of steel wool should be heated for 5-1o minutes. Doing so ensures a significant mass increase >0.1g
• Students usually predict the mass will go down.
• The mass will increase due to the oxidation of the iron in the steel wool. Many students may suggest that particles of fire attach to the steel wool.

Baking Soda and Vinegar (no lid)

• Do trial runs to know how much you should use for the beaker/container you’re working with so there are not spills. Update the student handout to indicate the required amount.
• Any reaction that produces a nontoxic gas may be used, but this produces consistent results and occurs quickly with visible gas production. The use of antacid tablets is another cheap alternative, but they should be crushed to a powder to increase the rate of reaction.

Baking Soda and Vinegar (with lid or balloon)

• This should be done as a demonstration to avoid unwanted explosions and student difficulty getting a balloon over the mouth of a flask. This demo and video from the AACT library may be helpful to reference.
• When paired with the first investigation, this variation highlights
• The difference between an open and closed system
• The fact that gases are matter, they are made of particles, and they have mass. A prevailing misconception among students is that “gasses weigh less than liquids or solids” or that they have no mass at all
• You can also tie this back to the Burning Steel Wool Investigation if students are struggling with where the added mass came from.