Why does convection occur?
Heat flows through the air as electromagnetic radiation, transferring energy without matter.
Heat causes fluids to become less dense; so warmer fluids rise above cooler fluids, transferring energy.
Heat is released during some types of reactions and absorbed during others, transferring energy to the system or surroundings. Heat increases the kinetic energy of molecules; the molecules have greater energy when they collide, increasing energy transfer.
All of the statements mentioned in your question are correct, and they explain different aspects of why convection occurs.
1. Heat flows through the air as electromagnetic radiation, transferring energy without matter: This statement refers to heat transfer by radiation. Radiation occurs as electromagnetic waves, such as infrared waves, and does not require any physical medium to transfer heat. For example, the Sun's heat reaches the Earth through radiation.
2. Heat causes fluids to become less dense; so warmer fluids rise above cooler fluids, transferring energy: This statement explains convection specifically in fluids (liquids or gases). When a fluid is heated, its particles gain energy and tend to move faster, which results in an increase in their kinetic energy and therefore their temperature. As the fluid is heated, it expands and becomes less dense. The less dense, warmer fluid rises, displacing the cooler fluid around it, thus transferring energy from the warmer region to the cooler region.
3. Heat is released during some types of reactions and absorbed during others, transferring energy to the system or surroundings: This statement refers to heat transfer during chemical reactions or phase changes. Some reactions release heat (exothermic reactions), while others absorb heat (endothermic reactions). This transfer of heat allows for the redistribution of energy within the system or its surroundings.
4. Heat increases the kinetic energy of molecules; the molecules have greater energy when they collide, increasing energy transfer: This statement explains how increased heat leads to an increase in the kinetic energy of molecules. As the temperature rises, the molecules in a substance move faster and collide with each other more frequently and with greater energy. This increased collision and energy transfer between molecules contribute to the overall heat transfer in a system.
In summary, convection occurs due to the combined effects of heat transfer by radiation, the density changes in fluids when heated, heat transfer during chemical reactions or phase changes, and the increased molecular movement and collision due to increased heat.