The melting of ice left out on a countertop on a hot day occurs primarily due to conduction and convection.
Conduction is the transfer of heat through direct contact between two objects. In this scenario, the cup of ice is in direct contact with the warmer air in the surroundings and the countertop. The air molecules in contact with the cup gain heat energy through conduction, causing the ice to melt gradually.
Convection, on the other hand, is the transfer of heat through fluid motion or the movement of particles. As the air around the cup of ice gets warmed, it becomes less dense and rises while cooler air rushes in to replace it. This creates a convection current where the warmer air continuously comes in contact with the ice and causes it to melt. It enhances the heat transfer process compared to conduction alone.
Radiation, the third mode of heat transfer, plays a smaller role in the melting of ice in this specific scenario. Radiation is the transfer of heat through electromagnetic waves, such as sunlight. While the sun's rays could contribute to warming the cup and the ice, its effect is minor compared to conduction and convection.
Therefore, conduction and convection are the primary mechanisms responsible for the melting of ice left out on a countertop on a hot day.