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.

Lesson Topics

1 | Density

2 | Calculating Density

3 | Comparing Densities of Solids, Liquids and Gases

4 | Density Layering

5 | The Effect of Temperature on Density

6 | The Effect of Pressure on Density of Gases

## 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/cm3 or kg/m3.
• 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 cm3 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/cm3.
• Therefore, any substance with a density less than 1g/cm3 will float in water and any substance with a density greater than 1g/cm3 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)

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