Why Do Things Float?

In the balloon family, helium balloons are the party animals. They don’t sink to the floor and just sit there (yes, we’re looking at you, air-filled balloons!). They float, move with air currents, and only very reluctantly come down to earth. How is a helium balloon able to keep the party going? An important part of the answer is its personality density.

The density of something shows the relationship between mass and volume: D = m/v. Mass is a measurement of how much matter is in a substance or object. It’s also a measurement of its resistance to changing motion. A grape has very little mass. Pick it up, toss it around, put it in your mouth and squash it. It’s not going to resist change much. (Well, maybe it’ll let out a little wine.) Try to do that with a boulder. (No! Don’t! You’ll hurt yourself.) The point is, a boulder is not easy to move when it’s sitting still, and don’t get in the way if one is rolling downhill.  It has a LOT of mass.

Volume refers to how much space something takes up. You just ate that grape. Still hungry? Thought so. The grape didn’t take up much space, did it? Not a whole lot of volume there. Now back to the boulder. Get a group of friends together, and you could all hide behind it. (Wait, I still see you. Pull your foot in. OK. Better.) That boulder takes up a lot of space. It has a large volume.

Density is a property of all substances, and it doesn’t depend upon how much there is. A tiny twig from an oak tree has the same density as an entire limb. Density takes into consideration both how much matter is in a substance or object and how much space it takes up.  So to find density, divide mass by volume. Don’t forget your units! With density, the standard unit for mass is kilograms (kg), which is helpful for measuring boulders and the like. Grams (g) can be used for small things (grapes!).

For volume, the standard unit is meters cubed, or m3, but centimeters cubed (cm3), liters (l) and milliliters (ml) are common.  Let’s move past the grapes and boulders and check out the densities of some ordinary substances:

·      Water: 1.00 g/cm3

·      Milk: 1.03 g/cm3

·      Gasoline: 0.70 g/cm3

·      Ice at 0° Celsius: 0.93 g/cm3

·      Gold: 19.3 g/cm3

·      Air at standard temperature and pressure (STP): 0.001293 g/cm3

·      Carbon Dioxide at STP: .001977 g/cm3

·      Helium at STP: 0.000178 g/cm3

So now you know why helium is such a party animal. With such a low density, all it wants to do is keep those Sponge Bob and Captain America balloons flying.

If you can’t get enough of the density of substances, check out:http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&ved=0CGIQFjAK&url=http%3A%2F%2Fwww.mrnorton.com%2FChemistry%2FTables%2FTABLEOFDENSITYFORSOMECOMMONMATERIALS.doc&ei=PDmLU-SSEceQyAT76IHwBg&usg=AFQjCNFJCtIX5c7Z289v1SUL85gbRlsjjA&bvm=bv.67720277,d.aWw