Temperature Scales

Temperature feels intuitive — you know when something is hot or cold. But the number you assign to that feeling depends entirely on the scale you’re using. The same room can be “20°,” “68°,” or “293°” — and all three are correct, in different scales.

For the MCAT, you need to move fluently between three temperature scales and know exactly when to use each one. There’s only one conversion you really have to memorize, and one rule about which scale to use when. Get those two things right and you’ll never lose a thermodynamics point to a temperature mistake.

Celsius - The Chemistry Default

The Celsius scale is built around water. Zero degrees Celsius (0 °C) is the freezing point of water at 1 atm, and 100 °C is its boiling point. Each degree represents $\frac{1}{100}$ of that range.

Most everyday science uses Celsius. When a passage describes a reaction “at 37 °C” (body temperature), it is using this scale.

Fahrenheit - The One You Can Ignore

Fahrenheit sets water’s freezing point at 32 °F and boiling at 212 °F. The conversion is:

°F = ($\frac{9}{5}$)°C + 32

The MCAT almost never tests Fahrenheit conversions, but you should know the formula exists. The important takeaway: Fahrenheit degrees are smaller than Celsius degrees (180 Fahrenheit degrees span the same range as 100 Celsius degrees).

Kelvin - The Physics Standard

The Kelvin scale is the absolute temperature scale. It starts at absolute zero - the lowest temperature theoretically possible, where molecular motion effectively stops.

Key Kelvin benchmarks:

• 0 K = -273 °C (absolute zero)
• 273 K = 0 °C (water freezes)
• 310 K = 37 °C (body temperature)
• 373 K = 100 °C (water boils)

Why Kelvin Matters for the MCAT

Every gas law and thermodynamic equation that includes temperature requires Kelvin. Using Celsius in PV = nRT or the Carnot efficiency formula will give you a wrong answer. The reason is simple: these equations involve ratios and products of temperature, and negative temperatures would produce nonsensical results.

Absolute Zero

At 0 K, particles have the minimum possible energy. You cannot go below this. Classical physics says all molecular motion stops; quantum mechanics says a tiny “zero-point energy” remains, but for the MCAT, the key idea is: no temperature below 0 K exists.

Quick Conversion Summary