The law of conservation of mass states that mass in a closed system will not change before and after a chemical reaction. Mass isn't created or destroyed, it just moves places. Seems simple enough, but this law can be difficult to demonstrate. It seems nearly impossible to get PERFECT data every time. And when the measurements are off by two hundredths of a gram, students are often yelling "Ah-Ha! Lavoisier was wrong!" (Sigh.....) BUT, it can lead to good discussions on where any change in mass could have come from.
Start with reviewing the difference between physical and chemical changes. (Chemical changes include: gas, color change, precipitate, temperature change, or light). Get some play doh and roll it into a ball. Place it on the scale and ask students if they think the mass will change if you change the shape of the play doh. You could also use legos or anything else you have handy.
Once they've seen that physical changes don't cause a mass change, move on to chemical changes.
Here are some labs you can use for different grade levels to teach the law of conservation of mass.
Grades 5-6: GLOW STICKS
In this experiment students compare the mass of a glow stick before and after it has been cracked and activated. You should get relatively good data for this lab, but as you can see in the picture the mass decreased by a hundredth of a gram. Glow sticks can be found on amazon, at the dollar store, or you can stock up on them after Halloween when they go on clearance.
Grades 7-8: ALKA SELTZER BAG
In this experiment, students compare the mass of water and alka seltzer in a Ziploc bag before and after it reacts. I'll admit this is a tough one to get great data because the bag is porous and some gas will escape during the reaction. However, it's best to use a bag instead of a sealed container because with a sealed container the lid can pop off from the pressure. I've found thicker name brand bags work better than store brand, so don't skimp to save a few bucks.
One thing you can do after the alka seltzer and water reacts is to let students open the bag, release the gas, and re-weigh (see the last picture on the right). They will see a decrease in mass and learn that gases (in this case, carbon dioxide) have mass. If you'd like to check out the lab worksheet, click here.
Grades 9+: STEEL WOOL
In part 1 of this experiment, students compare the mass of steel wool before and after pulling it apart. Since it is only a shape change and not a chemical change, it's relatively easy to get perfect data as long as they pull it apart over the scale (small fragments will fall off).
In part 2, students burn steel wool and compare the burned mass to the initial mass. (Bunsen burners will give you better data than using matches or candles). What is interesting about this experiment is that the steel wool actually GAINS mass after burning. This is because as it burns it combines with oxygen in the air to form iron oxide. The addition of the oxygen atoms causes an increase in mass. It's a great experiment to get students thinking about chemical reactions and where this additional mass could have come from. If you'd like to check out the lab worksheet, click here.
Do you have any other ways you demonstrate the law of conservation of mass with students? Leave me a comment!