Peter Eager, age 11, of Sherrill, N.Y. , or is question:
What is density?
The number of different materials in the universe is countless. But all of them are fragments of matter such as molecules, atoms and atomic particles. They
May be solids, liquids, gases or plasmas. Yet all kinds and states of matter have certain properties. All matter has mass, or weight, and density.
A measuring cup of mercury weighs 13.5 times more than one of water. A pot made of iron is almost three times heavier than its twin model made of aluminum. These weight differences are due to density, a quality of matter which is related to weight and volume. Weight is the pull of the earths gravity upon an amount of matter. Volume is the amount of space which the material or matter occupies.
Mercury is 13.5 times denser than water and iron is almost three times denser than aluminum. The density of an object is its weight divided by its volume. Suppose we have a boulder which occupies a volume of 100 cubic feet and its weight is seven tons, which is 14,000 pounds. Its density per cubic foot is 140 pounds.
The density of water per cubic foot is 62.4 pounds, of lead 705 pounds, of air 0.08 pounds.
The differences, of course, depend upon the amounts of matter or material packed into the same volume. Matter is made of atoms perhaps there are more atoms in a denser substance, but not necessarily. A liter of air and one of hydrogen contain the same number of atoms. But the weight and therefore the density of the air is almost 15 times greater.
Atoms, of course, come in assorted weights and sizes. Perhaps the denser materials are made of the big, heavy atoms. This may be part of the story. The iron atom is twice as big and more than twice as heavy as an atom of aluminum. And iron is almost three times denser than aluminum. The gold atom is almost twice as heavy as the silver atom, and the density of gold is almost twice that of silver„ But this cannot be the whole story. The mercury atom is heavier than the gold atom, and gold is almost twice as dense as mercury. The density of a substance is caused by the weight of its individual atoms and how closely these atoms are packed into a given space or volume.
For measuring the density of solids, scientists use the cubic centimeter, which is about half a thimbleful. The density of gases is measured by the liter, which is about a quart. Specific gravity is another way of measuring density. In this . Method, a volume of, say, iron or aluminum is compared with an equal volume of water. The specific gravity of iron is 7.80, of aluminum 2.70. Iron is 7.8 times denser and aluminum 2.7 times denser than water.