Density

As the protoplanets grew through accretion, the materials that composed them were heated due to the energy transmitted by collisions, as well as the gravitational forces acting on the material at the center of the planets. The early planets would have become molten masses of melted material, which would have allowed gravity to pull the planets into spheres (just as the wax in a lava lamp does when it breaks free and floats upwards). But changing into liquid material also allowed the planets to undergo another process, and in order to understand this better, one must first have a thorough understanding of the measurement of density.

It turns out that they both have the same amount of mass; a kilogram of mass is equal to a kilogram of mass, regardless of the substance being measured. But a kilogram of sand is much smaller than a kilogram of feathers because sand is more tightly packed together.

Density is different than mass. Density, by definition, is the amount of mass contained in a unit of volume. In other words, a comparison of densities is a comparison of the weights of two substances of identical sizes. A brick weighs more than a glass marble. But if you compared a piece of brick that had been carved into a sphere the same size as the glass marble, you would find that the marble weighs more than the sphere made of brick. The marble is denser than the brick.

Different metals on the periodic table have different densities- gold is more dense than silver, for example. Since gravity pulls on objects based on their mass, denser objects experience a greater gravitational pull than objects of lower densities. In an environment where all of these substances were liquid - like the early planets - the denser substances would have sunk towards the center of the Earth, while less dense substances would have been displaced towards the surface. As the Earth cooled and began to solidify, these layers would have been "frozen" in place.

Complete the density review activity below.

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