Chemistry · Thermodynamics · NEET
Gas has the highest entropy. In a gas, particles move fast and freely in every direction, filling all the space. This gives the largest disorder (randomness). So gas > liquid > solid in entropy. This is asked directly in NEET, so remember it.
Entropy is a measure of disorder. In a solid, particles sit in fixed positions and only vibrate, so disorder is low. In a liquid, particles can slide past each other, so disorder is higher. In a gas, particles move freely and randomly, so disorder is highest. More freedom of movement = more entropy.
Yes. Melting (solid to liquid, called fusion) increases entropy because particles become freer to move. So delta S is positive for melting. The same is true for boiling (liquid to gas) and sublimation (solid to gas). All these changes make particles more disordered.
For the same substance at the same temperature, yes: the gas has more entropy than the liquid, which has more than the solid. This is because the gas state has the most random particle motion. NEET usually asks this comparison for the same substance.
Entropy decreases when disorder goes down. Examples: gas turning to liquid (condensation), liquid to solid (freezing), or fewer gas molecules forming from more gas molecules. For example, 2H(g) -> H2(g) reduces the number of gas particles, so delta S is negative. Cooling a crystal toward 0 K also lowers entropy.
Count gas moles. If gas moles increase (delta n_g > 0), entropy usually increases (delta S positive). If gas moles decrease, entropy usually decreases. Making gas from solid or liquid raises entropy a lot, because gas is the most disordered state. This trick answers most NEET entropy questions.
In which case is the change in entropy negative?
In which of the following processes does entropy increase? A) A liquid evaporates to vapour. B) Temperature of a crystalline solid is lowered from 130 K to 0 K. C) 2NaHCO3(s) -> Na2CO3(s) + CO2(g) + H2O(g). D) Cl2(g) -> 2Cl(g).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
S(solid) < S(liquid) < S(gas). Gas has the most disorder, so it has the highest entropy. Solid has the least disorder, so it has the lowest entropy.
In a gas, particles move fast and freely in all directions and spread out to fill the container. This maximum randomness gives the highest entropy among the three states.
Entropy (S) is a measure of disorder or randomness in a system. More freedom for particles to move means higher entropy. Its unit is J/K.
Negative. Freezing turns a liquid into a solid, which is more ordered, so disorder decreases and delta S is negative. This matters for NEET spontaneity questions.
NEET repeatedly asks you to find the sign of delta S for a process or reaction. Knowing solid < liquid < gas, plus counting gas moles, lets you answer these quickly (asked in 2019 and 2024).
