5.11 understand why there is an absolute zero of temperature which is -273ºC

The coldest something can get is -273ºC (0K), this is because atoms have as little energy as they can possible have at this temperature (more heat = more energy, less heat = less energy). This temperature is known as absolute 0.

NOTE: to convert from ºC to K, just add 273. Alternatively, to convert from K to ºC just take away 273

NOTE NOTE: there is no º symbol when talking about degrees Kelvin, it is just a K