Final answer:
The smell of ammonia spread across the room by the process of diffusion, where gas molecules move from an area of higher concentration to lower concentration until distributed evenly.
Step-by-step explanation:
When a student accidentally spilled a bottle of ammonia in the lab, the smell spread across the room due to the process of diffusion. This is explained by the kinetic particle theory, which states that the molecules of a gas are in constant and random motion. Ammonia is a gas at room temperature, and once released, its molecules spread out to move from an area of high concentration, near the spill, to areas of lower concentration, throughout the room. This movement is passive and occurs until the concentration of ammonia is uniform across the room. The process responsible for the smell of ammonia spreading across the room is diffusion, which is defined as the net movement of particles from an area of greater concentration to an area of lesser concentration.