What is absolute zero?

Absolute zero is defined as the lowest possible temperature at which all classical motion of particles theoretically comes to a complete stop. This temperature is zero Kelvin, or -273.15 degrees Celsius. At absolute zero, a system reaches its minimal thermal energy state, meaning that atoms and molecules are in their most ordered state, minimizing their kinetic energy.

This concept is foundational in thermodynamics and underpins the behavior of gases and the laws of thermodynamics. As systems approach absolute zero, they exhibit quantum mechanical effects and behaviors, such as superconductivity and superfluidity. Thus, option A accurately captures the essence of absolute zero as it relates to particle motion and thermal energy.

The other choices do not accurately describe absolute zero. The boiling point of water is a specific temperature unrelated to absolute zero, and phase changes, while relevant to temperature, are not directly tied to the concept of absolute zero.