Sulfur hexafluoride is denser than air. When a foil boat was placed in a tank of sulfur hexafluoride, the boat floated. That’s because the air in the boat is less dense than the sulfur hexafluoride gas in the fish tank.
When scientists use the word “matter” they are talking about solids, liquids, and gases.
Matter can be found on Earth in three main forms: solids, liquids, and gases. Solids are materials that have a defined shape and volume that stays the same. Rocks are a good example of a solid – they have a rigid shape that isn’t easily changed.
Liquids are a type of matter that changes shape depending on the shape of its container. For example, when you pour milk into a cup, it takes up the cup’s inner shape.
Matter that spreads out to take up all the space available in the container is called a gas. Air is a gas. So is helium, which is put inside birthday party balloons.