# Q & A: Density

Q:
How do you measure density?
- Andry H. (age 14)
Poulsbo Jr. High, Poulsbo, WA
A:
Density is the amount of mass per volume. In order to measure it, you would need to find the mass of the object and how much space it occupies.

For liquids, you would find the mass of a graduated cylinder. Then add the liquid in the graduated cylinder and weigh the graduated cylinder with the liquid. You would subtract the mass of the graduated cylinder from this value to find the mass of the liquid. Since you are using a graduated cylinder, you can find the volume of the liquid by measuring it with the graduated cylinder.

For solids, weigh the solid to find the mass. If the solid is a shape like a cube or a rectangle, it will be easy to find the volume of the solid. If the object is an irregular shape, first place water in a container where you can measure the volume of the liquid. Then place the object in the water and find the change in the volume of the liquid. This change in volume is the volume of the solid.

For gases, it's much harder to make the measurements directly by weighing, because the densities are low. If you have the right equipment, you can weigh a container with a known volume of the gas, then remove almost all the gas with a vacuum pump and weigh the empty container. Often, you know enough about the gas (what types of molecules it's made of, what its temperature and pressure are, and how the density varies with temperature and pressure) to figure out what its density is without having to directly measure it.

Angela

(published on 10/22/2007)

## Follow-Up #1: Density of solids

Q:
Do solids have a high or low density
?
- Rebekah
Derry
A:
Solids have a wide range of densities.  Balsa wood for example has a density about one fifth that of water.  Iron on the other hand has a density more that seven times that of water and that of the metal osmium, the most dense of all, is more than 22 times that of water.

The reason for the differences are due to 1) the atomic weight of the constiuent atoms, and
2) the relative spacing of the atoms within the material.

LeeH

p.s. "Balsa wood isnt really a solid (a regular crystal), but a messy combination of several different components, I gues including some air pockets. Another example of a fairly low-density solid is ice, just a little less dense than liquid water.
Mike W.

(published on 10/22/2007)