Demonstration: Law of Conservation of Mass
Purpose: In a chemical reaction, how does the total mass of the products compare to the total
mass of the reactants.
Background: We will carry out this experiment in a sealed container so that if any of the
products of the chemical reaction are gases, the gases will remain trapped in the flask. Your
teacher will demonstrate for you the arrangement of the flask and the test tube for this
experiment.
Procedure:
1. Pour enough sodium hydroxide into a flask so that the bottom of the flask is covered with
it.
CAUTION: SODIUM HYDROXIDE IS CORROSIVE. BE SURE THAT YOU ARE WEARING
YOUR SAFETY GOGGLES FOR THE ENTIRE EXPERIMENT AND THAT YOU DO NOT
RUB YOUR FACE IF YOU GET SOME OF THIS ON YOUR SKIN. ON THE OTHER
HAND, IF YOU DO GET SOME OF THIS ON YOUR SKIN, DO NOT PANIC. WHILE IT
IS CORROSIVE, YOU DEFINITELY HAVE TIME TO RINSE IT WITH WATER BEFORE
ANY DAMAGE IS DONE. AT THE END OF THIS LAB, BE SURE TO WASH YOUR
HANDS.
2. Pour copper(II) sulphate into the small test tube until the test tube is about half full. In
words, describe the contents of the flask and test tube in the table below.
3. Tilt the flask to one side and carefully slide the test tube down inside the flask until it
comes to rest at the bottom. Do not let the two chemicals mix. Place a stopper firmly on
the mouth of the flask, sealing the flask.
4. Determine the mass of the flask, test tube and chemicals using the electronic balance at
the front of the class. Record it below.
5. Tip the flask so that the two chemicals will mix. Find the mass of the flask apparatus now
that the chemicals have been allowed to mix. Record the mass below.
6. Remove the stopper form the flask and dispose of the chemicals in the waste container
provided by your teacher.
Observations:
Mass of chemicals and glass apparatus before mixing ______________
Mass of chemicals and glass apparatus after mixing _______________
Description of sodium hydroxide
solution before mixing
Description of the copper (II)
sulphate before mixing.
Description of the mixture after being
mixed.
Discussion/Conclusion: These should be answered on a separate piece of paper, in full sentences.
1. When the solutions were mixed, did a chemical change occur? What evidence backs up
your conclusion. Be specific.
2. How did the mass of the reactants and glassware before the reaction compare with
the mass after the reaction?
3. If you noted a change in mass, explain why the mass changed.
4. If we consider the chemical reaction in this experiment to be typical, what would you
conclude about the total mass of reactants in comparison to the total mass of the
products in the chemical reaction?
5. Suppose that you performed a reaction in a flask by mixing two liquid chemicals
together, but this time you did not stopper the flask. If one of the products is a gas,
predict whether the mass of the products and glassware would be greater than or less
than the mass of the reactants and glassware. Explain the reasoning behind your
answer.
6. State the law of conservation of mass.