Which of the following is true when heat is added to a system?
a.Temperature always increases
b.Temperature always decreases
c.Temperature may increase, or it may remain unchanged
d.Temperature may increase, or it may decrease
C. If heat is added to a system it makes the molecules of that system move faster (increases the average kinetic energy), then the temperature will increase (eliminate choice B). However, there are instances when adding heat to a system will not increase the temperature of the system. One such situation is during a phase change, when the added energy is used to break the intermolecular forces that hold molecules together. In this case, the temperature remains unchanged. Similarly, in an isothermal system, adding heat to a system (heat > 0) is counterbalanced by the system doing work against its environment (w < 0), so temperature remains the same because the internal energy of the system does not change. This eliminates choices A and D (adding heat can never make temperature decrease), and makes choice C the correct answer.