LAW OF CONSERVATION OF MASS (LAB)

Conservation of Mass  Part One and Two

http://www.online-stopwatch.com/

Students will explore and demonstrate the Law of Conservation of Mass by measuring accurately the mass of the system before and after the reaction.  Students will be familiar with the chemical reaction produced from the mixture of vinegar and baking.  You may want to consider replacing the baking soda with Alka Seltzer or denture cleaning tablets.  Accuracy in measurement needs to be emphasized.  Part One needs to be timed, which will lead to Part B of the investigation.  Check freshness of baking soda and vinegar for the best reaction.  If using Option B, ensure that the resealable bag is tightly closed.  You may choose to pre-measure the materials.

The Law of Conservation of Mass indicates that mass cannot be created nor destroyed.  This means that the total mass of the reactants in a chemical reaction will equal the total mass of the products.  If a gas is produced, students often do not include its mass because they cannot “see” the gas.  In this investigation, we will be collecting the gas.  It is important to time the reaction, as it leads to part two of the investigation. 

You may also refer to the baking soda as sodium bicarbonate (NaHCO₃) and vinegar as acetic acid (CH₃COOH).

In Part Two, students will change the temperature of one of the reactants.  Depending on your classroom situation, you may either allow students to use cool or gently heated vinegar.  Teachers should do the heating.  Another option would be to cool the baking soda.  Your students should be ready for a higher level of open inquiry.  You can provide the question, allowing students to write the procedures (pending your approval).  Some classes may need scaffolding.  Student directions are not provided.

Depending on the amount of materials and time, you will want students to post and compare data.  This can be accomplished either electronically on a projected spreadsheet or on chart paper.  Students should analyze the entire class’s data to devise conclusions.  This is an opportunity to replication and repetition.

Standards

SC.8.P.9.1   Explore the Law of Conservation of Mass by demonstrating and concluding that mass is conserved when substances undergo physical and chemical changes.  High (EOC)

SC.8.P.9.2   Differentiate between physical changes and chemical changes.  Moderate. (EOC)

SC.8.P.9.3   Investigate and describe how temperature influences chemical change.  High (EOC)

SC.8.N.1.2   Design and conduct a study using repeated trials and replication.  High (EOC)

SC.8.N.1.4   Explain how hypotheses are valuable if they lead to further investigations, even if they turn out not to be supported by the data.  High (EOC)

SC.8.N.1.3   Use phrases such as “results support” or “fail to support” in science, understanding that science does not offer conclusive ‘proof’ of a knowledge claim.  Moderate (EOC)

Purpose

To allow students to explore the Law of Conservation of Mass.  Also a chemical change is demonstrated.  An extension of this lab will allow students to design and manipulate a variable to explore how temperature influences chemical change.  Nature of Science benchmarks will be reinforced.

Materials

FLASK
VINEGAR
BAKING SODA 
HOT PLATE



STUDENT DIRECTION
SHEET

Part One

Option A:  balance, timer, flask, balloon (that will fit snuggly over opening of flask), vinegar, and baking soda.

Option B:  balance, timer, small containers (bathroom-sized disposable cups) 2 per group, large resealable bags (quart sized), baking soda, and vinegar. 

Option C:  Materials in Option A or B, but substitute Alka Seltzer or denture cleaning tablets for the baking soda.

Student Direction Sheet   Option A

SAFETY TIP:   WEAR GOGGLES

Part One

1. Obtain materials from your teacher.
2. Measure 50 mL vinegar into flask.  Measure 10 g baking soda into the balloon
3. Place the balloon over the flask WITHOUT COMBINING baking soda and vinegar.
4. Measure and record the initial mass.
5. Start timer
6. Put the contents of the balloon into the flask, without removing the balloon.
7. Stop the timer when the reaction is complete.
8. Record the time.
9. DO NOT REMOVE THE BALLOON and measure the mass. ACCORDING THE LAW OF CONSERVATION OF MASS, THE ENDING MASS SHOULD BE THE SAME AS THE INITIAL MASS

                                       REPEAT, BUT  ADD HEAT TO THE (50 ML) IN THE FLASK ON A HOT PLATE 






                                                                             

Student Direction Sheet   Option B

SAFETY TIP:   WEAR GOGGLES

Part One

10. Obtain materials from your teacher.
11. Measure 50 mL vinegar into container A and 10 g baking soda in container B
12. Place both containers into resealable bag WITHOUT SPILLING.
13. Completely close bag.
14. Measure and record the mass of the bag and its contents.  DO NOT SPILL
15. Start timer
16. Through the bag, pour the vinegar into the baking soda.  DO NOT OPEN THE BAG.
17. Stop the timer when the reaction is complete.
18. Record the time.
19. DO NOT OPEN THE BAG and measure the mass of the bag and its contents