How Do You Convert Celsius to Kelvin?

To convert a temperature from degrees Celsius to kelvin, add $273.15$:

$$T(\text{K}) = T(^\circ\text{C}) + 273.15$$

For most IB Chemistry calculations, $273$ may be used when the data or required precision makes this appropriate. This conversion is required at both SL and HL.

Why Is 273.15 Added?

The Kelvin scale is an absolute temperature scale. Its zero point, $0\text{ K}$, is absolute zero, corresponding to $-273.15\, ^\circ\text{C}$. Therefore, the same temperature has a Kelvin value that is 273.15 greater than its Celsius value.

A temperature interval of $1\text{ K}$ has the same size as an interval of $1\, ^\circ\text{C}$. Only the zero points differ, so the conversion requires addition rather than multiplication.

Celsius temperatures can be negative because zero degrees Celsius is defined relative to the freezing point of water, not the complete absence of particle motion. Kelvin temperatures cannot be negative in ordinary IB Chemistry calculations because the scale begins at absolute zero.

In Structure 1.1, temperature in kelvin is linked to the average kinetic energy of particles. This is why equations involving particle motion and gases require absolute temperature rather than Celsius temperature.

Worked Examples

For $25.0\, ^\circ\text{C}$:

$$T = 25.0 + 273.15 = 298.15\text{ K}$$

This would usually be reported as $298\text{ K}$, depending on the precision of the original data.

For $-20\, ^\circ\text{C}$:

$$T = -20 + 273 = 253\text{ K}$$

To reverse the conversion, subtract $273.15$. For example, $300\text{ K}$ corresponds to $300-273.15=26.85\, ^\circ\text{C}$, usually reported as $27$ when the Kelvin value has no decimal places.

The common misconception is to multiply by 273 or subtract 273. To convert from Celsius to Kelvin, always add 273.15; subtraction is used when converting from kelvin to Celsius.

IB Exam Technique

Use kelvin whenever temperature appears in equations such as the ideal gas equation, $PV=nRT$. Write the unit as $\text{K}$, not $^\circ\text{K}$, and retain sufficient digits during working before rounding the final answer appropriately. Check that the converted value is physically sensible: a Celsius temperature above absolute zero must produce a positive Kelvin value.