Final answer:
When a pan of water is placed outdoors on a warm day, the water heats up and undergoes convection, where the heated water rises and the cooler water replaces it. This movement leads to the color spreading in the water.
Step-by-step explanation:
When a pan of water is placed outdoors on a warm day, several phenomena occur. One of the main processes is convection, which is the transfer of heat through the movement of fluid particles. As the pan and the water are exposed to the warm outdoor temperature, the water near the surface of the pan heats up. This hot water rises, creating convective loops, and the cooler water from the bottom of the pan moves to take its place.
This continuous movement of heated and cooled water leads to the color spreading and eventually reaching the top of the pan. This can be observed by adding a drop of food coloring to the water and watching it spread through these convective loops. This process continues until a thermal equilibrium is reached, meaning the water and the surrounding air are at the same temperature.