Density

Density. When density of an object is how much mass is in a certain amount of space. The density of an object is equal to its mass divided by its volume. This relationship is shown below:

Density = mass/Volume or D = m/V

There is more mass in the same space for the object on the left below. So, the object on the left has a higher density.

Mass: The mass of an object is how much material (or "stuff") it is made of. An object's mass is basically the sum total of all of the atoms and molecules that make up the object. It also depends on what types of atoms make up the object.

For example, an object that is made up of 10⁶ (one million) atoms of carbon has less mass than an object made up of 10⁶ (one million) atoms of iron. This is because each atom of iron has more mass than each atom of carbon (the mass is mostly in the center, or nucleus, of atoms).

(See the mass unit for more information about mass.)

Volume: The volume of an object is amount of space taken up by the object.

An atom is very, very small, and has a very small volume.
At the other extreme, a planet (like Earth) is very, very large, and has a large volume.

Back to Density. The density of an object is the amount of material it is made of divided by the space it occupies: mass of object / volume of object. The densities of some common materials are shown below.

(Back to the buoyancy unit.)

Q1: The density of distilled (pure) water is about 1 gram per cubic centimeter (g/cm³). If a tablespoon of salt is added and mixed into a glass of distilled water, which of the following will be true:

a) The density of the salt water will be equal to 1 g/cm³.

b) The density of the salt water will be less than 1 g/cm³.

c) The density of the salt water will be greater than 1 g/cm³.

d) It is not possible to know.

You are right! If you add salt to water, the density of the (now) salt water will increase. This happens because the mass of the salt and water increases, but volume of the water when salt is added does not change.

Actually, if you add salt to water, the density of the (now) salt water will increase. This happens because the mass of the salt and water increases, but volume of the water when salt is added does not change.

Density and state of matter. For the same type of substance, the density of the substance is in general greatest when the substance is in solid form. The same substance is generally less dense in its liquid state. And, you guessed it, the density of the substance is lowest when the substance is in gas form. The relationship between density and states of matter is shown in the picture below:

image of solid state, liquid state, gaseous state