Lesson plan

Density, buoyancy and pressure

Understand density in terms of molecules, and how varying densities determine how buoyant things are and what pressure is.
Science content
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Atoms, Molecules (3-7)
  • materials in each of the activities
  • a kitchen scale and graduated cylinder/beaker to measure density

Review density:
Density is how heavy something is compared to its size. Density depends on how close the molecules are together and what kinds of molecules make up an object.
In solids the molecules are closer together, so solids are generally more dense than liquids, which are generally more dense than gases i.e. a desk is heavier than the air, a rock is more dense than water. Explained in another way, the rock sinks in water because the mass of the rock is greater than the same volume of water that it replaces. Its weight (the force of gravity pulling on its mass) is greater than the upwards force from the water (force of buoyancy).
Sometimes a solid can float on water e.g. a piece of wood. This is because wood has tubes running through it (filled with sap when the tree is alive) which fill with air when wood is dried out. The overall density of a piece of wood is the combined densities of the wood and the air it contains. If this is less than the density of water, the wood floats. (Some kinds of very dense wood, even when dried out, sink.)

Students try the dancing raisins activity at their desk, in smaller tubes, and think about why they move up and down. Then do a larger demonstration in a cylinder, while discussing why they dance.

Sinking floating challenge 2. Once students have added just the right amount of nails/paperclips/modelling clay to their styrofoam piece to make it float half way down in the water, calculate its density, and compare to the density of water (1g/ml). The answers should be similar.

Discuss pressure:
When a gas (or liquid) is compressed to bring its molecules closer together, it exerts a pressure on the walls of its container.
Do an activity that exploits pressure:
Stomp rocket.
(Pressure in a bottle and popcorn also good pressure experiments to add here.)

Grades taught
Gr 5
Gr 6
Gr 7