The Kelvin and Celsius temperature scales are both based on the properties of matter and the concept of thermal equilibrium.
The Kelvin scale, also known as the absolute temperature scale, is based on the properties of an ideal gas at constant volume. It is defined in relation to the behavior of gases as their temperature changes. The Kelvin scale uses the Kelvin (K) as its unit of measurement, and it starts at absolute zero, which is the lowest temperature theoretically possible. Absolute zero is defined as 0 Kelvin (0 K) and is equivalent to -273.15 degrees Celsius.
The Celsius scale, also called the centigrade scale, is based on the freezing and boiling points of water at standard atmospheric pressure. It was originally defined with 0 degrees Celsius (0 °C) as the freezing point of water and 100 degrees Celsius (100 °C) as the boiling point of water, both at standard atmospheric pressure. However, the Celsius scale has since been redefined in relation to the Kelvin scale to provide a more precise and consistent temperature measurement system.
The Celsius scale is now defined based on the Kelvin scale by using the same size unit, called the degree. The Celsius degree is the same size as the Kelvin degree. Therefore, the Celsius scale is offset from the Kelvin scale by 273.15 degrees. In other words, to convert between Kelvin and Celsius, you add or subtract 273.15.
To summarize, the Kelvin scale is based on the behavior of gases at constant volume, while the Celsius scale is based on the freezing and boiling points of water, with the modern definition being related to the Kelvin scale through a simple offset.