Substances sink when placed in liquids that are less dense. Oil will float on top of a layer of water because it is less dense than water. This difference in density can be used to create toys such as those pictured on the right.

Click to see a 120kb movie of the toy.
Click to see how to make one

A simple lava toy can be constructed using differences in density between oil and water.

Click to see how to construct a simple lava toy.

A 10 gram salt crystal was dropped into 20cm³ of water. What is the density of the salty water? The density of pure water is 1g/cm³ .

The mass of the pure water is 20g. The mass of the salt crystal is 10g. Total mass of the solution is 30g. The total volume is 20cm³ .

Density = mass/volume = 30g/20cm³ = 1.5g/cm³

A 20 gram salt crystal was dropped into 20cm³ of water. What is the density of the salty water? The density of pure water is 1g/cm³ .

The mass of the pure water is 20g. The mass of the salt crystal is 20g. Total mass of the solution is 40g. The total volume is 25cm³ .

Density = mass/volume = 40g/25cm³ = 1.6g/cm³