What Is Density?

Everyday Einstein explains how density works. Plus, two fun experiments you can try at home.

Lee Falin, PhD
4-minute read
Episode #42

What Is Density?

One of my family’s favorite wintertime drinks is hot chocolate. Nothing warms you up after a hard day of sledding and snowball fights like a steaming hot cup of cocoa. Of course no cocoa is complete without marshmallows and, aside from being fun and tasty, marshmallows in hot chocolate provide us with an opportunity to discuss density..


The density of an object is how much mass it has divided by how much space it takes up. Sometimes scientists refer to this as “mass per unit volume,” but real people don’t talk that way. Let’s imagine that you’re packing your car for a long road trip with your family. As you cram in the sleeping bags, backpacks, strollers, camera, pillows, stuffed animals, food and other necessities, you are increasing the mass of the car. Every item you add makes the car more massive.

Unfortunately for the person trying to make everything fit into the car, the volume of the car does not increase. No matter how much stuff and how many children you try to fit into that thing, the car still takes up the same amount of space.

Since the mass of the car has gone up while the volume has stayed the same, we have increased the density of the car.

Don’t Be So Dense

When we want to compare the density of two objects, it’s helpful to keep that car analogy in mind. Let’s say we have a block of wood that measures exactly two inches on every side and we also have a chunk of brick that measures two inches on every side. If you’ve ever handled bricks or wood you’ll probably know instinctively that the brick is going to be heavier than the wood, even though they are the same size. The reason for this is because the brick is denser than the wood. In other words, more stuff is crammed into that brick than is crammed into the wood; it has more mass than the wood.

Floating in a Sea of Chocolate

Ah, the life of a marshmallow. Destined to float in a sea of warm chocolate. But what makes a marshmallow float while other things, like rocks, sink to the bottom of hot chocolate? (I don’t advise adding rocks to your hot chocolate).

As you might have guessed from the subject of this episode, the reason marshmallows float in hot chocolate is because of density. Marshmallows are less dense than hot chocolate so they float, whereas rocks are denser (and considerably less tasty) than hot chocolate, so they sink into the warm chocolatey goodness.

You might also have noticed this effect in certain kinds of salad dressing. Since many dressings are made of oil and vinegar, and since oil is less dense than vinegar, left to itself, the oil will float on top of the vinegar. No matter how much you shake up the dressing, it will eventually separate again.

Full of Hot Air

Something else that can affect density is temperature. Usually, if you take a gas or liquid and heat it up, it becomes less dense than it would be at a lower temperature. This is what makes hot air balloons float. By heating the air inside the balloon, it becomes less dense than the surrounding air and begins to float, just like a marshmallow in hot chocolate. This is the science behind the popular phrase “hot air rises.”

Do Try This at Home

There are two fun density experiments you can try at home. The first is to get a clear glass and mix in some molasses or dark corn syrup, some vegetable oil, and water. Watch how the layers of liquid separate to reveal which are denser than others. Now try dropping various small objects into the glass, maybe a rock, a piece of plastic, a rubber ball, a raisin, or a marshmallow, and see where they fall on the density scale.

Another fun experiment is to heat some water in a pan, pour it into a jar, and add a few drops of red food coloring. Then put some cold water into a jar that is the same size and add a few drops of blue food coloring. Place a piece of cardboard over the hot water jar and very carefully (preferably over a sink), flip it over, and set it on top of the cold water jar so that the mouths of the two jars are touching. Then carefully slide the cardboard out from between the jars and if the fates are smiling upon you, the red hot water should stay on top of the blue cold water.


I hope you take a few minutes to try out those experiments. While it’s fun to learn science by reading or listening to people talk about it, the real magic happens when you try things on your own. If you liked today’s episode, you can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

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Hot Chocolate image from Shutterstock

About the Author

Lee Falin, PhD

Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech. 

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