WHAT IS DENSITY?

"I know what density is! It's how heavy something is for its size! Like a pebble can weigh as much or more than a much larger piece of styrofoam - because the pebble is denser!"
"You got it! Density is the mass of an object divided by its volume! The formula is Density = Mass/Volume or, more usefully for our purposes, Density = Weight/Volume or D = W/V!"
"I guess density is useful in identifying minerals?"
"Yes, sir! If you've got a pure mineral knowing its density can distinguish it from similar looking materials! Gold, for example! It has a density between 15 and 19 g/cc (grams per cubic centimenter)! Pyrite (and some other minerals) looks like gold but its density is much less!"
"So how do you measure density!"
"You remember when you played with pebbles when you were a kid! Most of the pebbles you played with were probably quartz and weighed about 2.7 g/cc! Rocks (or minerals) with that density seem normal to you! You'll know right away if a mineral is unusually dense! Try picking up one that contains a lot of lead! You'll know right away that something's up!"
"You mean I have a built in density determiner!"
"You sure do! But some density differences between minerals are more subtle! You need to weigh them and determine their volume to get their density!"
"Weighing them, OK! You can use a scale! But getting their volume?! Minerals can be so irregular in shape!"
"One step at a time, Lou! Let's talk about weighing them! For medium size samples, a triple beam balance is useful!"
"The triple beam balance has a pan to put the specimen on; three sliding weights that slide along three scales (beams); and a pointer that points to zero when everything balances!"
"You start off with the pan empty, the weights all to the left at zero, and make sure the pointer points to zero! Then you put the specimen on the pan!"
"Then you slide the weights until they counterbalance the specimen! The top two scales are notched and the weights must each rest in a notch! The top scale is in tens of grams: 0, 10, 20, etc. The middle scale is in hundreds: 0, 100, 200, etc. The bottom scale has no notches. The weight can rest anywhere along the scale! The scale is in units: 0, 1, 2, 3, etc. It also shows tenths of units! For this specimen, the hundreds scale is at 100, the tens scale is at 50, and the units scale is at 5.8. So the weight of the specimen is 155.8 grams!"
"What about volume!"
"We get volume by immersing the specimen in a graduated cylinder that is partly filled with water and seeing how much the water level rises!"
"Cool! I see you've colored the water red so I can see it better!!"
"Here's a closer look so you can read the starting level of the water! The cylinder has a scale with marked divisions every ten ml (milliliters) and labeled divisions every hundred ml!"
"Next, you immerse the specimen completely in the water! The water level rises to 920 ml!"
"The level looks less than 920 to me! More like 917 ml - although it seems to vary along its length!"
"It's hard to read it exactly! So it's safest to round the reading off to the closest division on the scale - in this case, 920 ml!"
"So the volume of the specimen equals the difference between the starting level of the water (900 ml) and the final level of the water (920 ml)! That's 20 ml! But don't we want the volume in cubic centimeters (cc)??!!"
"For our purposes a cubic centimeter and a milliliter have the same volume, so it's OK! So the density is weight (155.8 g) divided by volume (20 cc) = 7.79 g/cc!"
"How accurate is that??"
"Good question! Our weight is probably pretty accurate, but the volume reading is much less accurate. If we round off the density to 7.8 that would be less precise but more realistic, although not necessarily correct! If the specimen were pure, and the measurements were made very carefully, the answer should be between 7.4 and 7.6 g/cc! But it's in the ball park! An error of only 2 parts in 75 is good enough most of the time!"
"Accuracy, precision, error.... Sometime I gotta go over all that stuff...!!"
"Be my guest!! Just press the button!"
"OK!! Now it's time to see if you can distinguish some minerals on the basis of their density!"

© 2010, David Leveson/Revised by G. Rocha and Michelle O'Dea