What is the Third Law of Thermodynamics?

In one line

The rule

The Third Law of Thermodynamics states: As the temperature of a perfect crystal approaches absolute zero (0 K), its entropy approaches zero.

The formula is: S → 0 as T → 0 K

• S = entropy (a measure of disorder or randomness)
• T = temperature (in kelvin, K)

Let's understand it simply

Imagine you are rolling a roti on a cold winter morning. As the dough gets colder, it becomes harder and stiffer — the molecules inside move less and less. Now, imagine if you could make it as cold as possible, down to absolute zero (−273.15°C). At this point, the molecules would stop moving completely. There would be only one way the dough could be arranged — perfectly still, perfectly ordered. This is what the Third Law says: at absolute zero, the disorder (entropy) of a perfect crystal is zero.

Think of a classroom full of students. When the bell rings, everyone runs around (high disorder, high entropy). But if the teacher says, "Freeze!" and everyone stands perfectly still, there is only one way to arrange them — this is zero disorder, or zero entropy.

How it works

1. All substances have some disorder (entropy) at normal temperatures — molecules move, vibrate, and can be arranged in many ways.
2. As temperature drops, molecular motion slows down — less movement means fewer ways to arrange the particles.
3. At absolute zero (0 K), a perfect crystal has only one possible arrangement — all particles are in their lowest energy state.
4. Entropy (S) measures the number of possible arrangements — at absolute zero, this number is one, so S = 0.
5. Real substances are never perfect crystals — so their entropy at 0 K is very close to zero, but may not be exactly zero.
6. No process can actually reach absolute zero — we can only get very close, but never fully reach it.

The story behind it

The Third Law was first stated by the German chemist Walther Nernst around 1906. He was studying chemical reactions at very low temperatures. Nernst noticed that as things got colder, the change in entropy during reactions became smaller and smaller. He wanted to explain why reactions seemed to "freeze" as temperature approached absolute zero. This law is sometimes called the "Nernst Heat Theorem" after him. Nernst won the Nobel Prize in Chemistry in 1920 for his work.

Where you see it in real life

• Making ice cream with liquid nitrogen: The colder you get, the slower molecules move, and the mixture becomes more ordered — showing entropy dropping.
• Cryogenics (super-cold science): Scientists cool gases to near absolute zero to study new states of matter, like Bose-Einstein condensates, where atoms act in perfect order.
• Superconductors: Some materials lose all electrical resistance at very low temperatures, a result of their highly ordered state.
• Metals at low temperature: The heat capacity (how much heat is needed to raise temperature) drops as you approach absolute zero, matching what the Third Law predicts.
• Chemical reactions in the lab: At very low temperatures, reactions slow down or stop, because there is almost no disorder left to drive change.
• Thermometers and temperature scales: The idea of absolute zero (0 K) comes from this law — it's the lowest possible temperature.

Why it matters

• Sets the zero point for entropy: Lets scientists measure entropy changes accurately in chemistry and physics.
• Explains why absolute zero can't be reached: No matter how much you cool something, you can only get closer, never actually reach 0 K.
• Helps design super-cold technology: Cryogenics, superconductors, and quantum computers all depend on understanding what happens near absolute zero.
• Predicts how materials behave at low temperatures: Important for space science, medicine (preserving organs), and making new materials.

Worked example

Let's say you have a perfect crystal of sodium chloride (common salt) at 0 K. What is its entropy?

Given: Perfect crystal at T = 0 K
Step 1: Write the Third Law: S → 0 as T → 0 K
Step 2: Since it is a perfect crystal and T = 0 K, S = 0
Step 3: Answer: The entropy is zero.

Common mistakes

• Thinking entropy can be negative: Entropy is always zero or positive, never negative.
• Assuming all substances reach zero entropy at 0 K: Only perfect crystals do; real materials may have a tiny bit of disorder left.
• Believing absolute zero can be reached: In practice, we can only get very close, never exactly to 0 K.
• Confusing the Third Law with the Second Law: The Second Law is about entropy increasing in natural processes; the Third Law is about what happens at absolute zero.

Remember it

At absolute zero, a perfect crystal has zero disorder — so entropy is zero.