One mistake that I see time and time again is the use of degrees Celsius when performing calculations in chemistry class. The problem with this is that there are no properties are directly proportional to the Celsius temperature.

Let's look at gas volume as an example. Charles' Law states that the volume of a gas is directly proportional to the temperature of that gas. If you could use the Celsius temperature here, then what would happen in the winter, when water starts to freeze? We would have nothing to breathe, since gases would have zero volume at the freezing point of water (0 °C was arbitrarily chosen to correspond to the freezing point of water). Worse yet, consider what would happen at temperatures below zero. If the temperature is negative, the volume must be negative. I hope you can understand that a gas can't have a negative volume.

Contrary to what your professor may tell you, Kelvin is not the ONLY temperature scale that can be used. ANY absolute scale can be used here, as long as the units work out. When you reach absolute zero (regardless of the scale), the ideal gas volume will be zero. (Of course this is only true for a true ideal gas, which doesn't exist.) It just so happens that the Kelvin scale matches up very well with the Celsius scale. However, there is a corresponding scale for Fahrenheit, called the Rankine scale (Rankine = Fahrenheit - 459.67). While the Fahrenheit scale is nice for living conditions, it's not as user friendly for most scientific purposes (it's not metric), so the Rankine scale never really caught on.

The ONLY time you may use Celsius in a calculation (or Fahrenheit, or any relative scale) is when using or calculating temperature difference (delta T). The reason that this works is that the difference between Celsius and Kelvin (or Fahrenheit and Rankine) is a constant value. So, when the absolute temperature changes from 273 K to 274 K (a difference of 1 K), the relative temperature changes from 0 °C to 1 °C (a difference of 1 °C). Because of this relationship, delta T can be calculated with units of either K or °C and you'll get the same value.