Field of Science

A fluff piece on density

We're covering density in my introductory chemistry this week - which prompted this post.

Density is the ratio of mass to volume and often (though not always) one of the easiest physical properties of a substance to measure. Introductory chemistry labs often feature an exercise where the mass of an object is determined using an analytical balance or somesuch and the volume is determined by displacement (often in a graduated cylinder).

In certain of my teen-aged sons' circles it's in fashion to write in public - to take your notebook (computer or spiral bound) and head to the local coffee shop. You can write the scene while being seen. Personally, I write in public as a last resort. Last week, while waiting to meet a friend for coffee I did haul out my iPad to see if I could inhale some of the fumes of caffeine and inspiration wafting around and some first thoughts for a column hammered out. No coffee for me, hot chocolate - with a serious mound of whipped cream on top.

While I'm all for decadence when it comes to chocolate, hot or otherwise, the whipped cream was a practical touch. My friend is notoriously late, and I wanted the chocolate to stay hot until he arrived.

The ability of a material to conduct thermal energy - heat - depends to some extent on its density. On a molecular level, heat transfer is mostly about collisions between molecules. If you are a molecule with lots of thermal energy, you are generally stuck with it all until you collide with another molecule!

Air at 20oC (68oF) feels warm (yes, I know it's about 3oC here right now, but a girl can dream, can't she?) while water at the same temperature feels refreshingly cool. Water is about a thousand times denser than air, so there are many more molecules in contact with your body surface. The more molecules boucing off a surface, the more opportunities there are for energy transfer. If you're hot, you'll get cooler faster by fully immersing yourself than by standing in a cool room. Conversely, if you want something to stay hot, surround it with air, not water. Air is a good thermal insulator, relative to water, because it is far less dense.

The whipped cream atop my hot chocolate is full of air, which lowers the density and so it floats on top of the cocoa. Heat transfer to the airy cream isn't very efficient due to the low density. Since the air trapped inside the cream isn't moving around (and thereby not presenting fresh molecules to take away the energy presently in the liquid in my cup), the system quickly equilibrates, with most of the heat in the system staying there.

Trapped air (or other gas) is a great insulator, but trapped nothingness works even better. Put my cocoa into a vacuum chamber and it should stay nice and toasty (though there would be other complications - but that is another post!).


  1. Good points about density! We don't get into this much depth in my intro chem class.

    I'm surprised you still teach intro chem. Haven't you been teaching for many years?


  2. Thanks, Julie!

    Even after 25 years, I teach everything from intro chem to graduate courses - as do all my colleagues, even the one who just retired from teaching after 53 years!

  3. Nice post Michelle... I think it's great that you still teach intro level chemistry courses...

    As a young professional chemist, I owe most of my enthusiasm and love for my trade to my experienced and knowledgeable teachers throughout high school and college. Thanks for what you do.


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