Final answer:
During the heating mode, the refrigerant in the indoor coil is under high pressure and high temperature. It absorbs heat from the outdoor air, evaporates and turns into a gas, and then releases heat to the indoor space, condensing back into a liquid.
Step-by-step explanation:
During the heating mode, the refrigerant in the indoor coil is under high pressure and high temperature. As the refrigerant flows through the evaporator, it absorbs heat from the outdoor air, causing it to evaporate and turn into a gas. The compressor then increases the pressure and temperature of the gas and forces it into the condenser coils, where it releases heat to the indoor space, allowing the gas to condense back into a liquid.