third law of thermodynamics
As you cool something down, its molecules jiggle less and settle into ever quieter, more orderly arrangements. The third law of thermodynamics asks what happens at the very bottom — as the temperature creeps toward absolute zero. Its answer: the entropy of a perfect, flawless crystal slides toward zero. There is one tidiest possible arrangement, and the system funnels into it.
Stated carefully: the entropy of a perfectly ordered crystalline substance approaches zero as the temperature approaches absolute zero (0 kelvin). This fixes a natural, absolute origin for the entropy scale — not just a place where you happen to start measuring, but a true bottom. From it, the entropy of any substance at any temperature can be pinned to an absolute value.
Why it matters: this is what makes 'standard molar entropies' real numbers you can tabulate and add up in reactions, rather than mere differences. A second consequence is the unreachability of absolute zero itself — you can get fabulously close, but never all the way, because each step of cooling removes a smaller and smaller slice. The caveat: real materials often freeze with leftover disorder ('residual entropy'), so the perfect-crystal idealization is exactly that — an idealization.
Cooling labs have chilled atoms to billionths of a kelvin above absolute zero — colder than the deepest reaches of space — yet that final sliver of temperature remains forever out of reach. The third law is not a technology limit; it is built into the structure of thermodynamics.
Absolute zero can be approached without limit but never actually reached.
The third law gives entropy something energy never has: a true zero. We can speak of a substance's absolute entropy, but only of its energy changes — there is no comparable absolute internal energy.