Density

If the weather forecaster tells you that the pressure is falling, what is the air density doing?

decreasing

Metric units for density

g/cm3

The density of water is:

0.1g/cm^3

How phase effects density:

All substances exist in one phase or another. Some substances, such as water and minerals, can exist in more than one phase, depending on the temperature and pressure of their surroundings. The density of such a substance depends upon its phase. For example, the density of liquid helium is different than the density of helium in its gas phase. The same is true of liquid hydrogen. The molecules of an element are farther apart in the gas phase. The gas will usually have less mass within a given volume.

Density

Amount of matter packed into a unit of volume; equal to mass divided by the volume of the substance. Matter has another physical property that is important to study: this quality is called density. Density is a measure of how much matter is packed into a given volume.

How pressure effects density:

Another factor that influences density is pressure. When a bicycle tire is pumped up, the volume changes very little, but the tire contains much more air. Therefore, as the pressure on a gas is increased, the density of the gas is increased. A trash compactor is another example of increased pressure that causes an increase in density. As more and more trash is added, volume does not increase. The increase in pressure results in more trash being packed into the same space. As this happens, density will increase. Density is directly proportional to pressure: D ~ P

What does this mathematical expression mean? D ~ P

Density is directly related to pressure.

What does this mathematical expression mean? D ~ 1/T

Density is inversely related to temperature.

How temperature effects density:

Temperature greatly affects volume. As temperature increases, the volume of most solids and gases increases. Therefore, when density of a substance in any particular phase is given, its temperature must also be given. As temperature increases, density will decrease. Therefore, density is inversely proportional to temperature. This relationship is inverse, or opposite: as the temperature increases, the molecules bounce further apart, and density decreases. D = 1/T. The relationship of density to temperature and pressure is: D = p/t

Density is affected by:

Temperature. This relationship is inverse, or opposite: as the temperature increases, the molecules bounce further apart, and density decreases. Pressure is directly related to density. As the pressure increases, matter is crushed into a smaller volume, thus increasing the density. As a substance changes from a solid to a liquid to a gas, its density generally decreases. Water is a notable exception. Icebergs float, and freezing water expands in cracks of rocks, forcing them to break and crumble. The solid form of water is less dense than the liquid form.

What is a common use for density?

The comparison of one substance to another. Density compares masses of equal volumes, such as the mass of a cubic centimeter of iron with the mass of a cubic centimeter of wood.

When a cold front passes your location, the temperature drops rapidly. What happens to the air density?

increases

The density of ice is:

less than that of water

Density equals:

mass/volume

If a substance besides water is changed from a gas to a liquid and then to a solid, what happens to the particles?

they get closer together