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!
Diffusion is a topic that is covered in biology (when we introduce cell membranes) and chemistry (particle movement and kinetic theory). I came across this "smelly balloons" activity on Flinn's website and thought it was such a fun way to introduce diffusion. In this activity, students will smell balloons that have different flavor extracts inside. Their task is to guess the scent and explain particle movement. How is the smell passing through the latex barrier? (Note: Prior to this activity you will need to make sure none of your students have latex allergies. Bust out those lab safety contracts they signed at the beginning of the year!)
For this activity you will need:
- Balloons (an assortment of colors is ideal)
- Eyedroppers or pipettes
- A variety Flavor extracts* or different odor substances
- String and tape (optional)
*Flinn's directions say to use a variety of flavor extracts. Flavor extracts can be pricey, so if you don't have very many at home you have some other options. You can use essential oils, perfume, or cooking spices. I used vanilla extract, lemon extract, eucalyptus oil, and cinnamon (mixed with a little bit of water to make a slurry).
States of matter is a topic that is covered in middle school, and reviewed again in high school chemistry with more depth. I've compiled some resources to help you teach this concept! Take a peek!
FOR MIDDLE SCHOOL
1. I created this powerpoint when I taught phase changes to my 8th and 9th graders. It is editable and can be adapted for higher or lower grades. It also includes a foldable!
2. This PHET simulation goes over the basics of phase changes and students can visually see what happens to the atoms as you heat them up and cool them down.
3. I read the book series "Stop Faking It" when I was in my first few years of teaching. In the air, water, and weather book he talks about how students can act like air particles when teaching high and low pressure. This also works great for teaching students about properties of solids, liquids, and gases. For solids, have students huddle up close and vibrate. Then yell out "liquid!" and have students spread out more and shake their arms and legs a little more. Then yell out "gas!" and have students run like crazy around the room waving their arms in the air. I know it sounds silly, and I thought students would hate it... but they ate.it.up!
4. This virtual lab from my.hrw is great for middle school, but does require flash. Be sure it loads on student devices before assigning.
5. I love card sorts! They are a quick and easy way to review new content. HERE is a quick card sort activity on the states of matter.
FOR HIGH SCHOOL
6. This PHET interactive is similar to the one listed above, but includes phase change diagrams. Great for high school students!
7. American Association of Chemistry Teachers has a simulation and quiz students can work through that can be found here.
8. This NOVA interactive website allows students to see particle movement in water, carbon dioxide, and hydrogen. It also requires flash, so make sure to check the link before using with students.
9. When I taught 9th grade physical science we reviewed states of matter and looked at phase change graphs. Typically teachers have students boil water and graph the temperature change, but I wasn't comfortable getting out hot plates with my squirrely freshman. So instead of dealing with hot plates I had students turn water into ice (without using a freezer!) and graph the temperature change. Check out this video to see the basics of how this lab works:
If you are interested in downloading this lab, you can find it here.
Even though I have taught life science most of my career and not physical science, I still feel it is important for students to understand polarity and electronegativity. If students don't understand polarity, they won't understand why the cell membrane has a hydrophobic and hydrophilic region. Understanding polarity also helps them understand protein folding and so many other macromolecule concepts. Because of this, I spend time before my cells unit reviewing the periodic table, bonding, and polarity. Here are a list of some resources you may find helpful!
The website Middle School Chemistry has a ton of free chemistry lessons and labs aimed at lower secondary grades, including this one on why water is a polar molecule. You can check out the lab here.
This resource is a cartoon called "The Bare Essentials of Polarity" and uses polar bears and penguins to help students visually understand polarity and electronegativity. I use it every year and get so many "ah-ha" moments by the end. You can download the cartoon here.
Similar to the cartoon listed above, I created this review worksheet that uses a tug of war as an analogy for polarity. It would be a great homework assignment following your polarity lesson. You can check it out here.
Have kinesthetic learners? In this lesson plan from TeachChemistry.org, students model electronegativity and polarity of molecules.
If you have access to computers, PHET has a free interactive website where students can play around with and observe the polarity of different molecules. You can access the interactive here.
Want to add a little coloring to your polarity lesson? Check out this polarity color-by-number activity to give your students a bit of a brain break. You can find it here.
My friend at Science with Mrs. Lau created this free electronegativity reference page you can print and have students keep in their binders. You can download it here.
I haven't tried this last lab, but it looks like a blast! Students use food coloring, paper, and shaving cream to make observations about polar molecules. You can check it out here.
I hope one or two of those links are useful to you! If you have any other favorites, drop them in the comments, I'd love to check them out!
One of my favorite case studies to examine with students is the tragedy that occurred at Lake Nyos. Located in Camaroon, Africa, Lake Nyos is a lake that formed in a volcanic crater. While villagers thought the volcano was dormant, it was slowly releasing carbon dioxide into the lake. One night in 1986 the carbon dioxide built up enough that the lake overturned and all the carbon dioxide was released into the atmosphere. Since carbon dioxide is more dense than air, thousands of villagers and livestock died in their sleep that night of asphyxiation.
While it is a devastating story to learn about, it is good in the sense that it can be applied to so many science concepts. Biology teachers can bring it up when learning about the carbon cycle. Earth science teachers can discuss the story during their volcanoes unit. Physical science teachers can use it to introduce density of gases. It's a phenomena that is so versatile!
I begin the lesson by showing this video clip from National Geographic on Youtube. It gets the students 100% engaged and doesn't reveal why this mystery fog killed the villagers:
Following the video clip I have students read an article I wrote about what happened at Lake Nyos and the science concepts behind it. You can find the article HERE if you would like to download it (appropriate for grades 7-10).
Then at the end of class I like to end with a demo showing how carbon dioxide is truly more dense than air. All you need are 3 birthday candles, some clay or play-doh, a container, baking soda, and vinegar.
Cut two of the candles shorter so all the candles are different heights. Stick them to the bottom of a container with clay. Sprinkle the bottom of the container with baking soda and light the candles. Have students predict what will happen when you pour some vinegar into the container. Students will observe the lowest candle extinguishing first because the dense CO2 that is being formed stays nearest to the bottom of the container. (I do it under the document camera so all students can watch, but if you trust your students with matches you can have them do it in small lab groups instead).
I hope your students enjoy this lesson- I know mine do! It's simple, engaging, and a story your students won't forget.