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Which Temperature Scale Should You Be Using?

  • by Allison Chae
  • Nov 15, 2017
  • Chemical Processes, PCAT Blog

The existence of three widely-used temperature scales is a common source of confusion when solving chemistry or physics problems that involve temperature. Some content areas where this is likely to come up are thermodynamics and phases of matter (more specifically, phase transitions and the behavior of gases). These topics are regularly tested on the PCAT, and students often wonder which units have to be used with which equations.

On Test Day, you don’t want to waste any time wondering about which temperature scale to use, so in this blog post we’ll review some guidelines to help demystify this choice.


Let’s get the easiest one out of the way first. Fahrenheit is only appropriate for non-scientific communications. You should never use Fahrenheit for numerical problem-solving in equations. However, Fahrenheit is often the most familiar temperature scale for people who have grown up in the USA, meaning that pharmacists practicing in the USA need to be able to communicate with patients in Fahrenheit.

For the PCAT, you should be aware of some important values in Fahrenheit. The freezing point of water is 32°F, normal body temperature is 98.6°F (although some variation happens throughout the day), and the boiling point of water is 212°F (under standard conditions). You should also know how to convert between Fahrenheit and Celsius using the equations (1) °C = (°F – 32) × (5/9) and (2) °F = (°C × 9/5) + 32.

Kelvin vs Celsius

The Celsius and Kelvin scales use the same intervals, but peg a value of 0 to different physical definitions. In the Celsius scale, 0°C corresponds to the freezing point of water, while 100°C is the boiling point of water. Normal body temperature, a useful value to know for anyone in medicine, is 37°C. In contrast, zero in Kelvin is defined as absolute zero, which occurs at −273.15°C. Therefore, we can use the following simple conversion: K = °C +273 and °C = K – 273, recognizing that the extra 0.15 degree is unlikely to matter. Practically speaking, the Kelvin scale is essentially the Celsius scale without negative numbers. On one hand, it can be convenient to avoid negative numbers, but on the other hand, everyday temperatures are more intuitively described in terms of degrees Celsius than degrees Kelvin. With this insight in mind, we can figure out how to apply these temperature scales on the PCAT.

If a question asks about change in temperature (ΔT), you can use either Celsius or Kelvin.

In most cases, Celsius will be easier because it involves smaller numbers. However, since Celsius and Kelvin use the same intervals between degrees, ΔT in Celsius will always equal ΔT in Kelvin. A classic example of an equation using ΔT is the thermodynamics equation q = mcΔT, which relates the change in temperature of a substance to the amount of heat entering and exiting the system, as modified by mass and the substance’s specific heat.

If a question requires you to use an equation with a T term, you should use Kelvin.

The simplest reason for this is that negative values mess up equations like the gas laws. On a slightly deeper level, the Kelvin scale is also more appropriate for assessing proportional relationships involving temperature (as in the gas laws, for example). Consider Charles’ law, which states that V/T is constant for an ideal gas. This can be rewritten as V = kT, where k is some constant, which is a clearer way of showing that Charles’ law indicates that volume and temperature are directly proportional. So, what happens to the volume of a gas if the temperature is increased from 1°C to 10°C? If we use the Celsius numbers, we might erroneously conclude that the volume would increase 10-fold, because substituting these Celsius numbers in would give the impression that the temperature increased 10-fold. That clearly isn’t correct! Using Kelvin, which is an absolute temperature scale, we would see that this is an increase from 274 K to 283 K, which is much more modest percentage-wise.

Hopefully this discussion of the Fahrenheit, Celsius, and Kelvin scales has demystified the choice of temperature scales for you! Looking to put this knowledge to the test with some realistic PCAT practice? Try our PCAT practice tests, or our free PCAT practice bundle. Need a more guidance or looking for more in-depth PCAT prep? Check out our online PCAT course or our private, one-on-one PCAT tutoring.

We wish you the best of luck in your journey towards pharmacy school!

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