### KS3 Physics

Energy
Motion and Forces
Waves
Electricity and Electromagnetism
Matter
Space Physics

# Density

When looking at density, it is important to first understand states of matter. The three common states of matter are solidsliquids and gases.

Let’s look at the arrangement of particles in these three states.

## Arrangement of Particles

### Solids

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

The particles are very close together and arranged in a regular lattice. Solids keep a fixed shape, meaning the particles are unable to move around. However, the particles can vibrate, especially when heated.

### Liquids

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

Particles in a liquid are arranged randomly. This is because the forces between them are weak. However, some of the particles are still close together.

Liquids take the shape of their containers due to their ability to flow. Particles in a liquid are in constant random motion. Similar to solids, when heated, the particles vibrate more.

### Gases

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

Gas particles have very weak forces between them, so they are able to move around freely. the particles are spaced far apart from each other.

Gases fill the shape of their containers because they have no fixed shape. When you increase the level of heat, gas particles move faster.

## 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 high-density block has a lot of mass packed into its volume
• The low-density block has a lower mass packed into the same volume

## Calculating Density

To calculate the density of an object, we need:

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

The mass of the object can be measured using a balance and a measuring cylinder can be used to measure the volume of a liquid.

To measure the volume of a solid, measure, submerge it in a graduated cylinder filled with a known amount of water and see how much the water level rises. This rise in water level indicates the volume of the solid. For solids with regular shapes like cubes or rectangles, simply multiply the length by the width by the height.

The equation to calculate density is:

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

## Example

A rock has a mass of 15 g and a volume of 5 cm³. Calculate the density.

To calculate the density of the rock, you can use the formula:

Density = Mass / Volume

Given:

• Mass is 15 g
• Volume is 5 cm³

Step 1: Plug these numbers into the formula. Density = 15 g / 5 cm³

Step 2: Perform the division. Density = 3 g/cm³

So, the density of the rock is 3 g/cm³.

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