Conservation of Mass – Teacher Version

Key Concepts:  Matter is the “stuff” that makes up an object. All things (living and nonliving) are made up of matter, and this matter gives objects their mass.  Mass is a measurement used to quantify the amount of matter in an object.  The Law of Conservation of Mass states that mass is neither created nor destroyed during chemical reactions.

 When looking at a chemical equation (e.g. 6CO2 + 6H2O ==> C6H12O6 + 6O2) things of the left side of the arrow are called reactants, whereas things on the right side of the arrow are called products.  The mass of the products created during a chemical reaction will always be equal to the initial mass of the reactants; what you put in is what you get out.

Materials: 1. ¼ cup water (from the tap is fine) 2. 2/3 cup flour 3. 2 tsp. salt 4. Scale (Note: If triple-beam or digital balances cannot be obtained, a rudimentary scale can be constructed out of a wooden bar with two Ziploc bags [see picture below]. When the masses in the two baggies are the same, the wooden bar should be horizontal. JellyBelly jelly beans can be used to calibrate the bags as each one weighs approximately 1 gram.

http://lpchscience.pbworks.com/Projectile-Lab Part 1 – Conserving Mass in Two States of Matter (Liquids and Solids)

Procedure: 1. Make observations on the materials (quantity, powder v. liquid, color, odor, etc.). Record your observations in your data table below. 2. Weigh each of your items individually and record in your data table below. a. 2/3 of a cup is approximately 150mL. Since 1mL weighs approximately1 gram, one cup weighs roughly 150g. b. ¼ cup of a cup is approximately 50mL. Since 1mL weighs approximately 1 gram, ¼ cup weighs roughly 50g. c. One teaspoon is approximately 5mL. Since 1mL weighs approximately 1 gram, one teaspoon weighs roughly 5g. There are two teaspoons, so together they weigh roughly 10g. 3. Add up all of the items’ weights, so you have a total initial weight. There should be an initial weight of approximately 210g. 4. Dissolve salt in water. 5. Add flour. 6. Knead for about 5 minutes. 7. Weigh the final product and record in your data table.

Title: Conserving Mass In Two States of Matter (Part I) Initial Observations: Student observations will vary.

Initial 2/3 Cup Flour ¼ Cup Water 2 tsp. Salt Mass (g)

Final Mass (g)

Concept Questions: 1. What were your observations as you mixed the reactants? Student answers may vary. 2. What were your initial (reactants) and final (products) masses? Calculate the difference in mass (if any). Are there any discrepancies between your starting and ending masses? Students should report their initial and final masses from their data tables above. The difference in mass can be calculated by using the formula:

Final Mass – Initial Mass = Difference in Mass

There should not be any major discrepancies between the initial and final masses, unless there was a major spill or complication with the weighing apparatus.

2. What are some possible sources of error? How do you think they affected the results? Some possible sources of error include both mechanical and human error, such as inexact weighing of materials (balance error), spilling of water (or other materials), or poor quality of materials. The way these sources of error could have affected the final result is that there was not an exact measure of materials, so the process of transferring various different elements (materials) into a compounded element (dough) is affected. Thus, the final (products) mass does not equal the initial (reactants) mass of all the items.

Part 2 – Conserving Mass in Three States of Matter (Liquids, Solids, and Gases)

Materials: 1. Several non-sterile nitrile examination gloves 2. 1 large spoon 3. 1 small bowl or container 4. 1 Alka Seltzer tablet 5. 50mL of White Vinegar 6. Rudimentary Scale (Note: Use the same scale constructed for Part 1 above if a triple-beam balance or digital scales are unavailable).

Procedure: 1. Make observations on the materials (quantity, powder v. liquid, color, odor, etc.). Record your observations in your data table below. 2. Weigh each of your items individually –except for the bowl and spoon– and record in your data table below. a. 50mL of vinegar weighs approximately 50g as 1ml is approximately equal to 1g. On the rudimentary scale, you weigh vinegar inside the glove (then subtract the glove’s weight). b. Consult the box/bottle of Alka Seltzer used for the approximate weight that students should find for this material (though if the Alka Seltzer tablet is weighed with the rudimentary scale, it has an approximate mass of 3 jelly beans). c. The glove weighs approximately 4 jelly beans (which is roughly 4 grams).

3. Add up all of the item’s weights, so you have a total weight. 4. Empty 50mL of White Vinegar into glove. 5. In the small bowl (or container) crush Alka Seltzer tablet into a powder form using the spoon. 6. With spoon, scoop and insert crushed Alka Seltzer tablet into opening of glove (where it cannot come into contact with the vinegar yet). 7. Mix contents into the glove. As you do this, make a double-knot at the opening of the glove, closing it tightly. 8. As glove inflates due to the reaction, prepare to weigh entire glove. Record mass again. Inflated glove in rudimentary scale shown below:

Title: Conserving Mass In Three States of Matter (Part II) Initial Observations: Student observations will vary.

Initial Mass (g) Alka Seltzer Glove Vinegar

Final Mass (g) Concept Questions: 1. What were your initial (reactants) and final (products) masses? Calculate the difference in mass (if any). Are there any discrepancies between your starting and ending masses? Explain. Students should report their initial and final masses from their data tables above. The difference in mass can be calculated by using the formula:

Final Mass – Initial Mass = Difference in Mass

The total accumulated mass of the materials should not have changed throughout the experiment. There could have been mechanical or human error as with Part I, but no major discrepancies between the initial and final mass values should be expected from either procedure.

2. What are some sources of error in this experiment? How do you think they could have affected the final result? Some sources of error include spills, incomplete transfer of materials from the nitrile finger to the flask, or mechanical errors such as a gas leak (from the flask), scale malfunctions, etc. The lab is supposed to demonstrate the Law of Conservation of Mass, so if any of the material matter were lost during the process due to human errors, then that means the total accumulated mass would change as well.

3. What do you think would have happened if the reaction had taken place within an open glove? Tying the glove served the purpose of trapping the gas produced by a chemical reaction between the reactants and products within the glove. If the glove had been open instead of closed, the byproduct (the gas) of the reaction would have escaped into the air, thus allowing mass to be lost, not conserved.

[ADVANCED] 4. How did the experiment demonstrate the Law of Conservation of Mass? Mass was conserved throughout the entire experiment because the reactants’ chemical reaction yielded products with the same mass as the initial total value. This means that even when elements transform into other compounds, the total mass does not change because mass is conserved.

5. What is the importance of trapping the gas? The gas is a byproduct formed during the chemical reaction between the liquid and the powder. Molecules in the gaseous state trapped within the nitrile glove are far apart from one another and moving very quickly. If all molecules of an element have mass, gas not only has mass like liquids and solids, but contributes to the total mass found in the products as well. Trapping the gas is important because if it was allowed to escape, a certain amount of mass would have been lost into the air rather than being conserved.