The concept of absolute zero, which is defined as the lowest possible temperature, was developed based on empirical observations and the understanding of gas behavior.
The relationship between temperature and the volume of a gas was investigated by several scientists, including Jacques Charles and Joseph Louis Gay-Lussac, in the late 18th and early 19th centuries. They discovered that as the temperature of a gas decreases, its volume decreases as well, and this relationship held true at all pressures.
In 1848, Lord Kelvin (William Thomson) proposed an absolute temperature scale, now known as the Kelvin scale, which is based on the behavior of gases. Kelvin's scale places absolute zero at the point where the volume of a gas theoretically reaches zero. This point corresponds to -273.15 degrees Celsius or 0 Kelvin (K). It is important to note that the Kelvin scale starts from absolute zero, whereas the Celsius scale starts from the freezing point of water.
The understanding that the volume of a gas approaches zero at this temperature was based on the extrapolation of the observed relationship between temperature and volume at higher temperatures. As gases cooled and approached -273 degrees Celsius, the volume reductions became more significant, leading to the inference that at -273 degrees Celsius, the volume of an ideal gas would be zero.
The concept of absolute zero has since been supported by further scientific research and experimentation, including investigations into the behavior of gases, the laws of thermodynamics, and the quantum mechanical understanding of particles and their energy states.
Therefore, it is through these empirical observations, theoretical models, and scientific understanding of gas behavior that we have come to define -273 degrees Celsius as the point of absolute zero on the Celsius scale, or 0 Kelvin on the Kelvin scale.