Name:
Date:
Period:
The Law of Conservation of Mass
Introduction: Chemical reactions result in changes within matter. Often they are accompanied with visible
signs of energy release and changes in the appearance of the substances. In this experiment a chemical
reaction will be carried out in a closed bottle. As it is conducted, consider the following questions: How are
compounds altered and what does their mass relate? Where do the atoms come from and where do they go?
Will the total mass of the chemicals change because of the reaction? What is meant by the Law of
Conservation of Mass?
Purpose: In a brief statement, explain the reason for the experiment
.
Hypothesis: In this experiment, NaHCO3 (baking soda) will be reacted with HCl (hydrochloric acid) in a
closed container. What will happen to the total mass of the chemicals? Using the If–Then–Because format,
write a testable hypothesis about the reaction with regard to how the mass of the bottle and its contents will or
will not change during the reaction and the reasoning for your prediction.
Materials:
balance
test-tube
100 mL graduated cylinder
NaHCO3 (baking soda)
weighing paper or weighing boat
2 L pop bottle and cap
scoopula
1M HCl (aq) (hydrochloric acid)
matches
wooden splint
Procedure – Follow the procedure below and make careful observations in the data table provided.
1) Rinse out a 2 L pop bottle with cap. Shake as much water out as possible. Make sure the outside is dry.
2) Measure 51.8 mL of 1 M HCl (aq) using a graduated cylinder. Remember to measure to the bottom of the
meniscus. Pour the acid into the 2 L pop bottle.
3) Mass 4.35 g of NaHCO3 on creased weighing paper or a weighing boat.
4) Pour the sodium bicarbonate into a test-tube. Use the creased weigh paper to get all the material into the
tube. Don’t lose any. Make sure the outside of the test-tube remains clean.
5) Carefully, at an angle, slide the test-tube of sodium bicarbonate into the 2 L pop bottle without letting the
acid or baking soda mix. Keep the tube upright so it doesn’t spill. Screw the cap on tightly so no gas can
escape during the reaction.
6) Mass the 2 L bottle with everything in it and record. Do not start the reaction yet!
7) Observe the capped bottle carefully. Write down anything you observe including: flexibility of the plastic
bottle, chemicals present, color and state of matter of all chemicals, and any other substances in the bottle that
may or may not take part in the reaction to come.
8) Now, mix the sodium bicarbonate with the acid. To do this, carefully tip the bottle horizontally, swishing
the acid into and out of the test-tube of sodium bicarbonate. Keep swishing back and forth until the reaction
seems to be finished and all the white powder is gone. Find and record the mass of the 2 L bottle.
In the data table, describe what happens during the reaction. Note any changes, such as color, temperature,
and states of matter of the substances and physical characteristics of the bottle when the reaction is complete.
What happened to the mass of the bottle? Compare it to your data from procedure 6.
9) Answer questions 1 – 4 in the Question section before continuing.
10) Pour out the “air”. Read this step in its entirety before doing anything: Hold the 2 L bottle upright.
Completely loosen the lid but leave it on bottle. Using a match, light a wooden splint and hold the flame near
the mouth of the bottle. Open the bottle, tip it down at a slight angle and put the flaming splint into the bottle
without getting burned. Do not let go of the splint! Smoke will be produced when the flame goes out.
Remove the splint from the bottle and slowly pour out the gas/smoke just as you would a liquid. Watch as it
sinks in the air, but don’t let anything else drip or fall out. Now, allow the bottle enough time for the gas
inside of it to come to equilibrium with the air in the room. Put the cap back on the bottle, mass it and record.
Describe any changes you observed during this procedure.
11) Rinse out the bottle and test-tube. Neatly return and organize your lab materials on your lab table.
12) Answer questions 5-8.
Data:
Mass (g)
Observations
Before
Reaction
After Reaction
After Pouring
Out the Gas
Questions:
1. The baking soda did not dissolve, although that’s what many people think.
What do you think happened to the acid and baking soda during the reaction? What is your evidence?
2. Was the mass of the bottle changed significantly after the reaction? Explain why or why not.
3. Since the cap was screwed on tightly, did any mass or matter escape the bottle? How do you explain any
discrepancies in mass when comparing the before and after reaction mass data?
4. Name at least two things that can get into and out of the bottle without the cap being opened. (Not
necessarily matter…)
5. What gas do you think was poured out? What evidence is there for your answer? (Oxygen is required for
combustion, CO2 prevents combustion.)
6. One fundamental property of chemistry is the conservation of mass. Explain what conservation of mass
means, using your experimental results as evidence.
7. The complete balanced equation for this reaction is:
HCl + NaHCO3 → NaCl + H2O + CO2
Count the number of atoms of each element on each side of the equation (reactants and products). What do
you notice?
How does your observation above relate to the results from the lab regarding the mass of the bottle?
8. Write a conclusion to this lab with regard to whether or not the written hypothesis was supported by the
results. Be sure to include a claim, evidence from the data and the reasoning as to why the evidence supports
the claim.
M. W. Walker/J.L. Daniel
Jul 11, 16