Heating and density
Hot air is lighter and rises.
A hot air balloon
A hot air balloon rises into the sky.
Why does hot air rise?
What this lesson covers
The idea
Generally density decreases on heating and increases on cooling: heating makes particles spread, increasing volume while mass stays the same; so hot air, being less dense than the cool air around it, rises.
A hot air balloon
A hot air balloon rises into the sky.
Why does hot air rise?
- It is less dense than the cool air around it
- It is heavier than cool air
- It has no mass
Heat the particles
Open both tabs.
Heat lowers density
Generally the density of a substance decreases on heating and increases on cooling. The particles move apart, so the volume increases while the mass does not change. As Density = Mass ÷ Volume, the density decreases.
Generally heating decreases density and cooling increases it: the volume increases, the mass is the same. Hot air is less dense than cool air, so it rises.
Notes
Heat → volume up, mass same → density down. Hot air rises.
Check yourself
Generally, when a substance is heated, its density …
On heating, the volume increases but the mass …
Hot air rises because it is …
On cooling, the density of a substance generally …
- increases. Heating makes the volume increase, so density decreases.
- stays the same. Density generally decreases on heating.
- decreases — correct. Yes!
- does not change — correct. Yes!
- increases. The mass stays the same.
- decreases. The mass stays the same.
- denser than the cool air around it. Hot air is less dense.
- less dense than the cool air around it — correct. Yes!
- heavier than the cool air around it. Hot air is less dense than cool air.
- decreases. Cooling generally increases the density.
- becomes zero. It generally increases.
- increases — correct. Yes!