Beacon Lesson Plan Library

Conservation of Mass

J Keener


Students observe a chemical reaction, determine that a gas has mass, and confirm the law of conservation of mass and energy.


The student understands that there is conservation of mass and energy when matter is transformed.

Creates potential solutions to industry problems using math and/or scientific concepts and communicates solution using industry appropriate language arts and graphic skills.


-Erlynmeyer flask
-Weighing paper
-Graduated cylinder
-Baking soda


1. Discuss reaction types, mass, and conservation of mass and energy with the class prior to teaching this lesson.
2. Collect listed materials.


-balance a chemical equation and understand that mass is conserved in all reactions
-understand that there are a variety of reaction types; in all cases mass is conserved
-know that matter may exist as a solid, liquid,or gas; all forms of matter have mass
-know that a balanced chemical reaction may be represented as symbols and numbers, and that while compounds may change, mass will be conserved
-know that mass is not conserved in a nuclear reaction (fission, fusion); however, mass and energy are conserved overall

1. Divide students into groups of three or four.

2. Instruct students to collect the following materials: 2 flasks, spatula, graduated cylinder, balloon, a weighing scale for student use, vinegar, and baking soda.

3. Have students label and accurately determine the mass of each flask, and the mass of the balloon.

4. Give students access to a weighing scale. Ask them to weigh 2 grams of baking soda and place the soda into flask #1. Repeat the procedure for flask #2.

5. Have students measure 10 ml of vinegar and accurately determine its mass.

6. Instruct students to predict what the final mass would be if they combined the 2 grams of baking soda with the10 ml of vinegar. Record predictions.

7. Have students combine the vinegar with the baking soda in flask #1 and swirl until the foaming stops.

8. Allow students to determine the combined mass of the flask, vinegar and baking soda. Ask them to compare these numbers to their original predictions. (The true values will be less than the predicted because a gas is produced which escapes into the atmosphere.)

9. Allow students to share results and to discuss possible explanations.

10. Have students repeat steps 4-7 again. This time, a balloon will be attached to the top of the flask as soon as the vinegar is added. The flask will be swirled and observations will be recorded until the foaming stops.

11. Allow students to determine the mass of the balloon, flask, vinegar and baking soda, and to compare the mass to the combined original masses of the balloon, flask, baking soda and vinegar.

12. Allow students to share results and discuss possible explanations.

This lab may be done with common fizzing antacids from the store. A similar reaction may be observed, and the components of a seltzer tablet may be analyzed.


The following questions should be answered after successful completion of the lab:
1. What explanation accounts for the observation that the mass of the reactants and the mass of the products in reaction #1 were not equal?

(answer: A gas is produced which escapes and is not weighed.)

2. What explanation accounts for the observation that the mass of the reactants and the mass of the products in reaction #2 were equal?

(answer: The gas produced is collected and weighed.)

3. What gas may have been produced in this reaction?

(answer: Carbon dioxide may have been produced.)

4. What reaction type is illustrated in this lab?

(answer: Double displacement is illustrated.)

5. What law is obeyed in this lab?

( answer: Law of conservation of mass and energy is obeyed.)

The following questions may be used to asses student understanding:

1. Which of the following states of matter does not have a measurable mass?
a. solid
b. liquid
c. gas
d. none of the above

(answer d. All forms of matter have a measurable mass.)

2. Which of the following statements is true of a chemical reaction?
a. Mass is converted into energy.
b. Energy is converted into mass.
c. Mass and energy are conserved.
d. Mass and energy are not involved in chemical reactions.

(answer c. Mass and energy are conserved in all chemical reactions.)

3. What do coefficients in a balanced equation represent ?
a. The conservation of compounds in a reaction.
b. The conservation of mass in a reaction.
c. The conservation of energy in a reaction.
d. The conservation of heat in a reaction.

( answer b. The coefficients in a balanced reaction represent the conservation of mass.)

In all reaction types, the law of conservation of mass and energy must be obeyed. This is best represented through a series of symbols and numbers that demonstrates the conservation between products and reactants.

The law of conservation of mass and energy may be demonstrated though the observation of the five simple reaction types: combination or synthesis, decomposition, single displacement or singe replacement, double displacement or double replacement, and combustion.

Mass is not conserved in nuclear reactions (fission,fusion). In these cases, a small amount of mass is converted into energy. For this reason, the law is written for mass and energy together, rather than separately.

In this lab, vinegar (acetic acid) reacts with baking soda (sodium bicarbonate) to produce carbon dioxide, water and sodium acetate. The carbon dioxide may be captured in the balloon.


Percent composition may be determined by comparing the mass of the gas in the balloon to the mass of the starting compound.
Stoichiometric connections may be made between the amounts of gas produced and the amounts of starting materials. Limiting reactants may also be illustrated.
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