Can density be used to identify a substance?

Density and identity

No. Density is a valuable attribute for identifying substances, but a frog in a bottle of liquid will have the same density as the liquid. Likewise, knowing the density of a liquid will not tell you for certain what the liquid is.

Consider--If you have two bottles of different liquids; how many ways can you think of to modify the chemistry without changing the density? Would adding something of higher density plus something of lower density work? Yes, of course it would.

A chemist will use density as one piece of information to help identify a substance, but measuring the melting point, boiling point, IR spectra, and the reaction to other chemicals, along with many other tests, including how it appears, is usually necessary to figure out what is in the bottle.

Eureka!

Archimedes was one of the best of the ancient Greek mathematicians. It is rumoured that in his time, there was some Greek city-state king who had a gold crown. But the king suspected that the goldsmith had kept some of the gold for himself and substituted cheaper metal.

Archimedes knew that if the crown was weighed, and if its volume could somehow be measured, it would be possible to calculate its density. If it had a different density from gold, then it was not gold. But although he knew how to calculate the volumes of simple shapes like cubes and cylinders, he didn't know how to measure the volume of something as complicated as a crown (the shape of which is highly irregular).

One day, as Archimedes was getting into his bath, he realised that his body was displacing water, and that the density of any object could be measured by immersing it in water. Here's how:

The crown would be hung on a beam balance and weighed. Next, the crown (on a string) would be weighed while submerged in a tub of water below the scale. The difference between the two weights was simply the weight of the water displaced by the crown's volume, and then he knew--

Density=(the crown's weight in air) divided by (the crown's weight in air minus its weight submerged in water) Let's run through an example to see how this works:

Archimedes weighs the crown in air and gets 193 grams. He divides this by the crown's weight in water (which will be --because we wrote the question-- 183g). So density in grams per cc =193/(193-183)=19.3

The density of gold is 19.3g/cc, so apparently the king was not cheated.

Archimedes was so excited that he ran down the street naked, shouting "Eureka!" which means roughly "I've got it!" Unfortunately, history does not record whether or not the crown turned out to be pure gold.

This simple but brilliant experiment is often taught incorrectly. It's an example of an algebraic calculation where the complicated stuff cancels out. Note that the crown must being homogeneous and completely submerged, but no complicated volume determination enters the calculation. Pretty Cool...!

First answer by Searchman. Last edit by Eric M Jones. Contributor trust: 506 [recommend contributor]. Question popularity: 49 [recommend question]