Coservation of Mass

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Updated on Feb 04, 2012

Grade Level: 9th - 12th; Type: Chemistry

The goal of this experiment is to learn about conservation of mass by conducting a chemical reaction inside a sealed environment. Students can weigh the reactants before, during and after the reaction to determine if mass was conserved. The difference between weight and mass will be considered.

  • What chemical reaction occurred in this experiment? Can you write the equation and balance it?
  • What is the difference between mass and weight?
  • According to the laws of conservation of matter, the end mass of the reactants should be the same as before the reaction took place. If your end weight was lower, can you explain why?
  • If the weight decreased after the gas evolved, how do you explain thisDoes the weight increase later? Why?

According to the principal of conservation of mass, the mass of the products of a reaction should be the same as the mass of the reactants if the reaction takes place in a sealed system. This is largely synonymous with conservation of matter. In this experiment, a sealed system is created by a sealed zip-lock bag. Students can measure the weight of the bag before, during and after the reaction. The evolution of gas inflates the plastic bag in all directions, and gives rise to interesting questions about the difference between mass and weight.

Check to see if your teacher can supply these small amounts of reagents and let you use a triple beam balance.

  • Quart size zip-lock bag
  • 10 ml phenol red indicator
  • Small paper cup or film canister
  • Small graduated cylinder
  • 25 ml water
  • 5.0 g of solid CaCl2
  • 5.7 g of solid Na2HCO3

  1. Weigh a 1 gallon zip-lock bag. Write down this weight.
  2. Measure 25 ml of water and the water to the cup or film canister. Using a graduated cylinder, measure 10 ml of of phenol red. Add this to the container and water. Weigh the container, water and phenol red. Write down this value.
  3. Measure 5.0 g of solid CaCl2 and put it in one corner of the plastic bag.
  4. Weigh 5.7 g of solid NaHCO3 and put it in the other corner of the bag, taking care to isolate it from the CaCl2.
  5. Place the cup of water in the center of the bag, taking care to keep the reagents in their separate corners.
  6. Squeeze as much air as possible out of the bag. Seal the bag as completely as possible. There should be a closed system in the bag.
  7. Tip the water and combine the ingredients by massaging the outside of the bag. The bag will get warm and a gas will evolve. Record your observations of this reaction.
  8. Weigh the bag as the reaction is occurring.Weigh the bag again when the gas has evolved.Weight the bag after the heat dissipates.

Terms/Concepts: Conservation of mass; Conservation of matter; Weight, mass


Cy Ashley Webb is a science writer. In addition to having worked as a bench scientist and patent agent, she judges science fairs in the San Francisco bay area. She loves working with kids and inspiring them to explore the world through science.

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