The full lesson can be viewed by enrolling in the Year 8 Chemistry Online Course or by purchasing the Year 8 Chemistry Lesson Notes.

Learning Objective

In this lesson we will look more closely at density as a physical property, including what happens when substances with different densities are combined.

Learning Outcomes

By the end of this lesson you will be able to:

- Describe density.

- Express density as a mathematical relationship between mass and volume.

- Calculate the density of regular and irregular solids.

- Describe and explain the layering of substances with different densities.

- Explain how temperature and pressure can affect the density of a substance.

(Image: Baharlou, Adobe Stock)

## Lesson Summary

- Density refers to how much matter is in a given amount of space.

- It can be expressed mathematically as:

- where mass is the amount of matter in the substance and volume is the amount of space the substance occupies.

- Density is measured in units such as g/cm
^{3}or kg/m^{3}.

- The mass of a substance can be determined by weighing it on an electronic balance.

- The volume of a liquid can be determined by measuring it in a measuring cylinder.

- The volume of a regular solid can be determined using a mathematical formula.

- The volume of an irregular solid can be determined by measuring how much liquid it displaces in a measuring cylinder.

- Volumes measured in mL can be converted to cm
^{3}using the formula:

- Generally:

- Solids generally have high or very high densities.

- Liquids generally have high densities.

- Gases generally have very low densities.

- Mixtures containing substances with different densities will form vertical layers, where:

- The least dense substance will lie at the top.

- The most dense substance will lie at the bottom.

- Pure water has a density of 1 g/cm
^{3}.

- Therefore, any substance with a density less than 1g/cm
^{3}will float in water and any substance with a density greater than 1g/cm^{3}will sink in water.

- Most frozen solids will sink in a liquid of the same substance as substances are usually more dense as solids than liquids.

- Ice is unusual because it is less dense than water and therefore floats in it.

- Heating substances causes them to expand and therefore become less dense.

- Cooling substances causes them to contract and therefore become more dense.

- Pressurising gases causes them to contract and therefore become more dense.

(Image: terimakasih0, Pixabay)

(Header image: photoeverywhere, Adobe Stock)