For each of the situations described below, the object considered is undergoing some changes. Among the possible changes you should consider are: (Q) The object is absorbing or giving off heat.
(T) The object's temperature is changing.
(E) The object's thermal energy is changing.
(W) The object is doing mechanical work or having work done on it.
(C) The objects chemical energy is changing.

For each of the two situations, identify which of the five changes is taking place, indicating the applicable letters Q T U W C (or none) as appropriate while providing your reasoning.

a. An ice cube sits in the open air and is melting.
b. A cylinder with a piston on top contains a compressed gas and is sitting on a thermal reservoir (a large iron block that can provide or take thermal energy as the system needs). After everything has come to thermal equilibrium, the piston is moved upward somewhat (very slowly). The object to be considered is the gas in the cylinder.

a. (Q), (T), (E)

              b. (Q), (T), (E), (W)

Explanation: Thermal Energy and Temperature are closely related: when the temperature rises causing atoms or molecules to move, thermal energy is produced. Thermal energy is the energy within the system.

Mechanical Work is the amount of energy transferred due to an applied force.

Chemical Energy is the energy contained in the bonds of chemical structures of the molecules released when a chemical reaction happens.

Given the explanations, let's analyse the situations:

a. For an ice cube to be melting, it has to be absorbing heat, which means its thermal energy is changing and, consequently, so does its temperature;

b. First, the gas inside the cylinder reaches a thermal equilibrium, which means its thermal energy and temperature changed. Since there were  exchange of heat to reach the equilibrium, the gas absorbed or gave off heat. After the equilibrium, when the piston starts to be moved, the energy of the pressure is transferred to the gas, so mechanical work had been done.