Which of the following is an example of a reversible process? A hand pump is rapidly compressed and the pressure and temperature in the pump rise. The pump is held in position as the temperature drops to the ambient temperature. A car engine burns fuel to produce motion of the car and exhausts the 140°C waste products of the combustion into the 20°C atmosphere. An ice cube at 0°C melts while in a water bath at 0°C. A stick of dynamite explodes. A high pressure air tank at room temperature has its valve opened and the gas in the tank rushes out until the pressure in the tank is equal to atmospheric pressure.

Among the examples provided, an ice cube at 0°C melting while in a water bath at 0°C is an example of a reversible process.

In thermodynamics, a reversible process is defined as a process that can be reversed without leaving any trace on the surroundings. In other words, the system and the environment can be returned to their initial states without any net change. The melting of an ice cube at 0°C in a water bath also at 0°C closely approximates a reversible process because if the surrounding temperature were to decrease slightly, the water at 0°C would freeze back into ice at 0°C, potentially returning the system back to its initial state without significant change to the surroundings.

When considering reversibility, it is crucial to remember that in reality, all natural processes are irreversible. However, the melting of ice in equilibrium with water at the same temperature is a classic textbook example of a process that can be considered reversible for theoretical and practical purposes within the scope of thermodynamics.

Extra: The concept of reversible processes is key in understanding entropy, one of the fundamental concepts in thermodynamics. Entropy can be thought of as a measure of disorder or randomness in a system, and it increases in the case of irreversible processes. In reversible processes, the entropy change can be made arbitrarily small.

The other scenarios described are examples of irreversible processes for the following reasons:

1. A hand pump rapidly compressed: When the pump is compressed and then allowed to equilibrate with the surroundings, there is a dissipation of energy (heat transfer to the surroundings) that cannot be completely recovered if we attempt to reverse the process.

2. A car engine burning fuel: The combustion process is highly irreversible as it involves chemical reactions and significant energy dissipation in the form of heat and work which cannot be fully converted back.

3. A stick of dynamite explodes: The explosion of dynamite is a rapid, highly exothermic chemical reaction that spreads out energy and matter, and there is no way to reverse this process.

4. A high-pressure air tank releasing gas: Once the gas has expanded into the atmosphere, work has been done by the gas as it expands, and in order to compress the gas back to its original state, additional work would have to be applied, thus it's not reversible.

For a process to be completely reversible, it must be carried out infinitely slowly to ensure the system remains in a state of near equilibrium with its surroundings throughout the process. This theoretical construct is useful when considering idealized scenarios, but in the physical world, all processes have some inherent irreversibility.