Density and the particle model
Chemistry · Key Stage 3 · The School
Density is about how packed particles are
DENSITY is mass packed into a given volume. In the particle model, a densely packed substance has more particles, and therefore more mass, in the same amount of space. A solid's particles are packed close together. A gas's particles are spread far apart. That spacing difference is the whole reason densities differ so much between states.
Why gases are so much less dense
In a gas, particles are typically spaced roughly a thousand times further apart than in the same substance as a liquid. The same volume of space therefore holds vastly fewer gas particles than liquid particles, and vastly less mass. That is the particle-level reason a balloon full of air weighs almost nothing compared with the same balloon full of water.
Why heating usually lowers density
Heating a substance generally makes its particles spread slightly further apart, whether it is a solid expanding a little or a gas expanding a lot. The same number of particles now fills a bigger volume, so the same mass is spread more thinly — meaning density falls slightly. This is why hot air, being less dense, rises above cooler air around it.
A burner heats the air trapped inside a hot air balloon's envelope. The heated air particles move faster and spread further apart, so the SAME mass of air now fills a slightly larger volume — meaning the density of that air has fallen. The cooler, denser air surrounding the balloon has not changed. Denser fluid surrounding less dense fluid pushes the less dense fluid upward, the same basic idea that makes a cork float in water. That is why the balloon rises: not because the air inside vanished, but because the same particles, spread further apart by heat, now take up more room for the same mass.