Density

 

Density

To understand density, it is important to first know the different states of matter: solids, liquids and gases. Let’s look at the arrangement of particles in these three states.

 

 

Arrangement of Particles

Solids

The particles in a solid are held together by strong forces that keep them in fixed places, in a regular pattern. The particles are very close and only shake around these fixed places. They cannot move around.

Solids have a fixed shape and do not flow like liquids or gases because the particles are in fixed places.

The image below shows the arrangement of particles in a solid.

 

A particle model of a solid (Note: Particles are all touching)

 

 

Liquids

The particles in a liquid are still close together, but they are randomly arranged. They can vibrate and also move around.

Liquids do not have a fixed shape and can flow because the particles can move and vibrate. This means that a liquid will fit the shape of the bottom of its container. Like a solid, liquids cannot usually be squeezed because there is hardly any space between the particles.

 

The particles in a liquid should be drawn as close together with very few gaps, in a random pattern.

 

A particle model of a liquid (Note: The particles in a liquid are closely spaced and randomly arranged.)

Gases

The particles in a gas are so energetic that they can break free from the forces that try to keep them together. This makes the particles in a gas spread out and move in any direction. The particles in a gas do not shake - they move fast in straight lines, randomly. This means they can hit each other and the walls of any container they are in.

 

Gases do not have a fixed shape because the particles can move freely in all directions. They can fill up their container and flow to take its shape. Gases can also be squeezed easily because there is a lot of space between the particles.

 

The particles in a gas should be drawn as far apart and in a random pattern.

 

A particle model of a gas (Note: The particles in a gas are widely spaced and randomly arranged.)

 

Understanding Density

Density refers to the mass of an object per unit of volume. For example, let’s compare two blocks with the same volume.

The cube on the right is much heavier than the cube on the left, even though they are both the same size. This is because the cube on the right has a much higher density.

 

 

Calculating density

Objects with a higher density feel heavier for their size because more matter is packed into the same space.

To calculate the density of an object, we need:

 

• The mass of the object
• The volume of the object

 

Density can be measured using a variety of different units, but the most commonly used units are kilograms per cubic metre (kg/m³) and grams per cubic centimetre (g/cm³).
The equation to calculate density is:

 

Or

 

 

Where:

 

• ρ = Density in grams per cubic centimetre (g/cm³)
• M = Mass in grams (g)
• V = Volume in cubic centimetres (cm³)

 

For example, if the cube on the right has a volume of 10 cm³ and a mass of 50 g, therefore:

density = mass / volume
density = 50 / 10
density = 5 g/cm³

 

And the cube on the left has a volume of 10 cm³ and a mass of 5 g, therefore:

density = mass / volume
density = 5 / 10
density = 0.5 g/cm³

 

Summary:

 

• The density of an object or substance is its mass divided by its volume.
• The units of density are based on the units of mass and volume. The most common units are grams for every cubic centimetre (g/cm³) or kilograms for every cubic metre (kg/m³).
• The density of a substance depends on both the mass of the particles, and how close they are. This means that density changes when substances change state, and when they are heated or cooled.