How Temperature Changes Density

Density is mass divided by volume. When you heat a fluid, its molecules move faster and push farther apart, so the same mass now takes up more space. Volume goes up, mass stays put, so density goes down. A hot air balloon works on exactly this: the burner heats the air inside the envelope, that air expands and becomes less dense than the cooler air outside, and the surrounding denser air effectively pushes the balloon upward, basket and all. The same logic applies to any fluid, liquid or gas, as long as it is free to expand.

That freedom to expand is the catch. In a rigid, sealed container completely full of water, the walls will not budge, so volume cannot change no matter how much heat you add. Since density is mass over volume and both are locked in place, density stays exactly the same. The heat still raises the water's temperature and pressure, but density is untouched. This is the detail that trips people up: heating always changes density ONLY if the fluid can change volume.

Convection currents are density differences in motion. Heat water at the bottom of a pot and that layer expands, becomes less dense, and rises. Cooler, denser water sinks to take its place, gets heated in turn, and the cycle repeats, setting up a circulating current. This same rising-and-sinking pattern explains weather patterns and ocean currents, and it sets up the next idea: buoyancy, where an object floats or sinks based on how its density compares to the fluid around it.