I Heart Density

I HEART density. It’s the most important concept we teach in physical science, because it helps us understand the real world. Even if kids don’t go into STEM careers, a good understanding of density can help them navigate life and understand what they observe.

Why does the wind blow? Density. Why do helium balloons go up? Density. What makes earthquakes and volcanoes? Thunderstorms? Ocean currents? Hot air balloons? Density. Lava lamps work because of density. When you make an Arnold Palmer with tea and lemonade, the lemonade stays on the bottom of the cup because of density.

Middle school and high school students can work with density in different ways – calculating, measuring, and experimenting. Here’s a quick rundown on the possibilities.

Calculating Density

Help them understand mass and volume first. Measure, observe, use units. Measure the volume and mass of regular solids, irregular solids, and liquids. Don’t spend much time on the difference between mass and weight! For this, mass is like weight. It describes how much stuff there is in an object. If you model by always using the word mass, students will eventually pick up that habit. And volume is how much space the object takes up. If they have a solid grasp of mass and volume, they won’t be as confused when you introduce density.

Note: I use d instead of ρ for physical science because it’s easier for the middle schoolers to remember. If your students are more experienced with variables, use ρ. 

See how the “m-over-v” looks a little like a heart? I “heart” density because it’s so important in science, and I “heart” this easy way to remember the formula.

Then practice. Practice calculating density. Use algebra and practice calculating mass. Use algebra again and practice calculating volume.

If your kids are strong in algebra, show them the dimensional analysis. If not, teach them how to look for unit clues in the problem. In other words, if you’re given a density in kilograms per liter and asked to find volume, the result will be in liters.

Measuring Density

Regular objects (cubes) are easy. Measure the mass with a balance and the volume with a ruler. (Students should have practiced with both of these tools before. Rulers are surprisingly troublesome — a topic for another post.) If you use a standardized set like these Density Cubes, students can check their result with the real density of the substance.

Irregular objects (rocks, bolts, clay) are easy IF students have practiced finding volume by displacement. Another topic for another post!

Liquids: This time, measuring the volume is simple with a graduated cylinder. Determining the mass of a liquid requires subtracting the mass of the empty container. I make my middle school and high school students practice this the long way, rather than using a tare function.

Experimenting with Density

Density experiments, demonstrations, and discrepant events are a great way to start the year in a physical science class. Here are a few of my favorites:

Density Columns (of course): Great as an engagement activity and/or a followup experiment when students know how to find the density of a liquid. Here’s my roundup of density columns that you can try.

Will it Float? This ridiculous David Letterman bit is popular with 8th-9th grade physical science students. I like to show one of these videos each day, followed by a round of the “Classroom Edition” of the game. I have a 5-gallon plastic aquarium just for this purpose. Some fun things to test: bouncy balls of different sizes, a can of Coke vs. a can of Diet Coke, peeled vs. unpeeled oranges, pumice stones (the larger the better), ironwood samples, and bowling balls. (An 8-pound bowling ball will float!) Kids will want to hold the items being tested, and will naturally do something that geologists do to field test density – hefting! Hefting involves tossing the item a little in your hand, to get an idea of its mass. (Note: watch those Letterman clips first and decide if it’s appropriate for your classroom. You’ll see what I mean.)

Cartesian Divers: Combine the concept of density with gas laws, specifically the relationship between pressure and volume of a gas. There are tons of variations on Cartesian Divers! Here’s a good Steve Spangler video tutorial where he emphasizes the importance of getting ALL of the air out of your bottle. Here’s the kit I like to get for my classes. You can also use glass eyedroppers, pen caps, and (believe it or not) soy sauce packets. I’ll probably do a whole post on this some day, but, for now, here’s another good video that includes an important component of this project – telekinesis! It also has a good review of how density relates to floating and sinking.

Solar Bag: This one also relates to gas laws, specifically the relationship between temperature and volume. Choose a cool but sunny day for this. Fill the solar bag with air, close it up, and wait. As the sun heats the air inside, it expands. Its volume increases while its mass stays the same. This decreases the density until, eventually, it is less dense than the ambient air. This causes it to float up! This demo made me famous at my school.

Image attribution: Fir0002/Flagstaffotos

I could go on. Teach density, and don’t skimp. Observe, experiment, measure, calculate, and surprise. Density will creep up again and again as students continue through their science education.

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