When describing energy transfers, chemists look at the flow of energy between the system (which we have defined as the refrigerant fluid) and the surroundings and how these energy transfers change the system’s internal energy. If energy flows from the system to the surroundings, the internal energy of the system falls; if energy flows from the surroundings to the system, the internal energy of the system increases. A system’s internal energy is closely related to its temperature; when a system’s internal energy increases, its temperature often (but not always) increases.

To determine whether or not an open refrigerator can cool down a house, let’s first consider how the refrigerant fluid (the system) cools the food inside a closed refrigerator (the food is part of the surroundings). Since temperature is a measure of the average kinetic energy of the particles of an object, to lower the temperature inside a refrigerator, the system must “remove” energy from the surroundings. But how does this happen?