Comparing Densities Using the Particle Theory
Science 8
The particle theory can help us to understand why solids usually have great er densities than liquids, and liquids have gr ater densities than gases: the particles in solids are tightly packed together, held by the attractive f orces between particles. There is relatively little spa ce between the particles (Figure 1), so the substance tends to be dense. Liquids have a little more space between the particles, so are slightly less dense.
Gases have very large spaces between the particles, so they have the lo west
density.
When a solid is heated until it melts, it expa nds slightly and becomes less dense.
Figure 1: This diagram shows how the particles of a substance gain en ergy and start to move as they are heated. We are looking at a fixed volume of the substance. You can see that the density decree ases as the temperature increa ses.
solid + heat → liquid + heat → gas

Unless we are told otherwise, the density given is for a substance in its most common state at room tem perature . Copper, for example, is solid, and oxygen is a gas. There are a couple of exceptions to the
"solids are most dense" rule. Merc ury , a metal that is liquid at room temperature, is more dense than many solids and over 13 t imes as dense as water.
What Portion of an Iceberg Is Submerged?
Water, also, is a special case . At some temperatures, liquid water is more dense than solid ice!
Approximately 90% of an iceberg is un der the surface of the water. Only about 10% shows above, which is why icebergs are so hazardous to ships.
Using Pure Water as a Standard
One cubic centimetre holds 1 ml of water and 1 ml of pure water has a mass of 1 g. The density of water is 1 g/mL or 1 g/cm 3 . This value is used to compare the densities of all other fluids and solids.
Comparing den sities allows us to determine which substances will floa t on top of other substances. For example, carbon dioxide gas, a byproduct of respiration, is den ser than air. As a result, air flo ats on top of carbon dioxide, but both air and carbon dioxide sink below helium gas, which has a very low density.

Comparing Densities Using the Particle Theory
Science 8
The particle theory can help us to understand why solids usually have greater densities than liquids, and liquids have greater densities than gases: the particles in solids are tightly packed together, held by the attractive forces between particles. There is relatively little space between the particles (Figure 1), so the substance tends to be dense. Liquids have a little more space between the particles, so are slightly less dense. Gases have very
large spaces between the particles, so they have the lowest
density. When a solid is heated until it melts, it expands slightly and becomes less dense.
Figure 1: This diagram shows how the particles of a substance
gain energy and start to move as they are heated. We are looking at a fixed volume of the substance. You can see that the density decreases as the temperature increases.
solid + heat → liquid + heat → gas

Unless we are told otherwise, the density given is for a substance in its most common state at room temperature. Copper, for example, is solid, and oxygen is a gas. There are a couple of exceptions to the
"solids are most dense" rule. Mercury, a metal that is liquid at room temperature, is more dense than many solids and over 13 times as dense as water.
What Portion of an Iceberg Is Submerged?
Water, also, is a special case. At some temperatures, liquid water is more dense than solid ice!
Approximately 90% of an iceberg is under the surface of the water.
Only about 10% shows above, which is why icebergs are so hazardous to ships.
Using Pure Water as a Standard
One cubic centimetre holds 1 ml of water and 1 ml of pure water has a mass of 1 g. The density of water is 1 g/mL or 1 g/cm 3 . This value is used to compare the densities of all other fluids and solids.
Comparing densities allows us to determine which substances will
float on top of other substances. For example, carbon dioxide gas, a byproduct of respiration, is denser than air. As a result, air floats on top of carbon dioxide, but both air and carbon dioxide sink below helium gas, which has a very low density.