Q.A gas at 900°C is cooled until both its pressure and volume are halved. Calculate its final temperature.

Pressure-versus-temperature and volume-versus-temperature graphs for gases (at fixed volume or fixed pressure respectively) are straight lines when temperature is measured in °C, but with a non-zero intercept. Extrapolating these lines backwards (idealising away the fact that real gases actually liquefy first), every gas's line crosses the temperature axis at exactly the same universal point, $-273.15\,°\text{C}$ -- absolute zero, the theoretical lower limit of temperature. This anchors the Kelvin (absolute/thermodynamic) scale, whose zero coincides with absolute zero and whose degree size matches the Celsius degree; modern calibration uses a single fixed point, the triple point of water, defined as exactly 273.16 K. Absolute temperature also makes the classical gas laws take their simple …