Fill in the blank: Hot air ____________ because it expands and gets less dense.

What the sentence is describing

The clue is the reason given: heating makes air expand and become less dense. When a fluid becomes less dense than its surroundings, it tends to move upward.

Linking heating to density

When air is heated, its particles move faster and spread out. That increases its volume.

Density is $$\rho = \frac{m}{V}$$ If the mass $m$ stays about the same while the volume $V$ increases, then $\rho$ decreases.

Why lower density makes air move up

In a gravitational field, a less-dense parcel of air is buoyant compared with denser surrounding air. That buoyant force makes the warm air move upward, so the correct word is rises.

Quick check with a real example

A hot-air balloon rises because the heated air inside is less dense than the cooler outside air, creating an upward buoyant force.