Refrigerant in the A/C system can be either liquid or gas, often referred to as vapor. In fact, every A/C system has what is known as a liquid side and a vapor side. Also understand that the refrigerant in any auto A/C system continues to change from liquid to vapor and back to liquid, depending on where it is in the refrigerant circuit. When the refrigerant changes from liquid to vapor or from a vapor back to a liquid, that change is known as the change of state.

The change of state of the refrigerant is effected by heat or the lack of heat. When the refrigerant enters the evaporator, it is liquid and enters the bottom of the evaporator. As the hot outside air passes over the evaporator, the refrigerant absorbs that heat. When doing that, the excess heat causes the refrigerant to boil. Just like water, when water boils it turns to steam or vapor. The same change of state occurs with the refrigerant in the A/C system. When the liquid refrigerant in the evaporator has absorbed enough heat to cause it to boil, it changes state to become vapor refrigerant.

On the other hand, when that refrigerant vapor enters the condenser, the cooling fans remove the heat that was absorbed in the evaporator. When doing that, the vapor refrigerant changes state back to a liquid. Once again, just like water, when we boil water and change it to steam, we know that cooling that steam will cause it to condense back to water. That same effect or change of state occurs with the refrigerant as it moves through the A/C system.

The vapor side of the A/C system starts at the evaporator where the liquid refrigerant is boiled and changes state to vapor. The vapor side continues to the accumulator if equipped and also includes the compressor and all hoses connecting the compressor to the condenser. The vapor side stops part way through the condenser where the vapor refrigerant changes state back to liquid.

The liquid side of the A/C system starts at that point in the condenser and continues all the way back to the inlet of the evaporator. If the A/C system includes a liquid line receiver drier, that too is part of the liquid side of the A/C system.