Absolute Zero is defined as which of the following?

• A. The lowest physically possible temperature on the Kelvin scale.
• B. The temperature at which water boils.
• C. The temperature at which air pressure is zero.
• D. The same as 0°C.

Answer: (A)

Absolute zero is the lowest limit of temperature on the Kelvin scale. Temperature reflects how much the particles in a substance move; as you remove heat, that motion slows, and at 0 K it would be at its minimum possible level. The Kelvin scale is defined so that this limit sits at zero, which corresponds to −273.15°C. The other statements don’t describe this limit: water boils at 373.15 K (100°C at 1 atm), zero air pressure describes a vacuum condition rather than a temperature, and 0°C is 273.15 K, not 0 K. In practice we can approach absolute zero but never reach it.