A 2.5 kg brick sits on top of a piece of aerogel weighing
only 2 grams. Photo courtesy of NASA/JPL.
Density is the amount of mass per unit volume of a substance. The Greek mathematician and scientist Archimedes is credited with using the concept of density to determine whether or not a crown made for King Hiero was made of pure gold. Legend has it that the king thought his goldsmith was cheating him by substituting silver for some of the gold he was given to make the crown, so he asked Archimedes to test the crown without destroying it. Archimedes was stumped until one day when getting into his bath he caused the water to overflow. He realized that he could put a weight of pure gold equal to the weight of the crown into a bowl and add water until it was filled to the brim. Then when he removed the gold and put the crown in it would make the water overflow if it contained silver because silver is not as dense and therefore it would take up more volume. It’s said that upon this discovery, Archimedes ran through the street naked shouting “Eureka!” which means “I found it!” in Greek. No word on what happened to the goldsmith.
Osmium is the densest naturally-occurring element, being twice as dense as lead. In nature, osmium is found alloyed with platinum. Alloys of osmium and platinum are used where hardness and durability is needed such as electrical contacts or in the tips of fountain pens. The least-dense metal, lithium, is so light that it floats in water. But the record for the lightest solid is held by Aerogel which weighs only three mg per cubic centimeter, or about one tenth the density of the lightest Styrofoam!
Manufactured by JPL in Pasadena, California, it’s nearly transparent with a blue cast and is surprisingly sturdy. It’s made of pure silica but is 99.8% porous. It makes an excellent insulator—almost 40 times better than fiberglass. Aerogel was used on NASA’s Stardust Mission that returned to Earth in 2006 after a flyby with a comet. Its purpose was to collect dust particles for analysis back on Earth. These dust particles travel so fast that they would penetrate most solids and be vaporized—yet when the collide with Aerogel they are buried in the material and come to a relatively gradual stop so that they can be collected.
1) True of false: An alloy of gold and silver is less dense than pure gold.
2) The least-dense metal is __________________.
a) aluminum b) lithium c) silver d) Aerogel
3) True or false: Osmium is twice as dense as gold.
4) Which of the following is not true about Aerogel?
a) Aerogel is man-made b) Aerogel is very delicate c) Aerogel is lighter than Styrofoam by an order of magnitude d) Aerogel was made for NASA.