Chemistry · Thermodynamics · NEET
Entropy is a number that tells you how spread out or messy the particles in a system are. Think of a tidy room (low entropy) versus a messy room (high entropy). In chemistry, gas particles flying everywhere are 'messy' (high entropy), while atoms locked in a solid are 'tidy' (low entropy). NCERT says: 'entropy is a measure of the degree of randomness or disorder in the system.' The symbol is S.
Yes. For NEET, treat disorder and randomness as the same idea. Both mean how freely and unpredictably the particles can move or arrange themselves. When you see either word in a question, it points to entropy. High randomness = high disorder = high entropy.
Entropy INCREASES (ΔS positive) when disorder goes up: melting, boiling, evaporation, sublimation, dissolving a solid, or a reaction that makes MORE gas molecules. Entropy DECREASES (ΔS negative) when disorder goes down: freezing, condensing, cooling toward 0 K, or a reaction that makes FEWER gas molecules. Quick test: count moles of gas. If gas moles rise, ΔS is usually positive.
For a reversible process, ΔS = q_rev / T. Here q_rev is the heat added reversibly and T is the absolute temperature in kelvin. This is NCERT equation 5.18. The term q_rev/T does not depend on the path taken, which is why entropy is a state function. Higher temperature makes the same heat cause a smaller entropy change.
Entropy has units of joules per kelvin per mole: J K⁻¹ mol⁻¹ (sometimes written J/K/mol). This comes straight from the formula ΔS = q_rev/T, where heat q is in joules and temperature T is in kelvin. NEET often expects this unit, so remember it.
In a solid, particles sit in fixed positions and only vibrate a little, so there is little randomness. In a gas, particles move fast and freely in all directions, so randomness is very high. That is why entropy follows the order solid < liquid < gas. This is exactly why melting and boiling raise entropy.
An increase in total entropy (system plus surroundings) is what drives a spontaneous process. NCERT introduces entropy while explaining spontaneity: even when enthalpy does not change (ΔH = 0), gases still mix on their own because total disorder rises. So entropy, not just heat release, decides direction. You judge spontaneity by ΔS_total, not just the system.
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). Choose the correct option.
For a sample of perfect gas when its pressure is changed isothermally from p_i to p_f, the entropy change is given by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Entropy S is a state function, so ΔS depends only on the initial and final states, not on the path. NCERT notes that q_rev/T is independent of path, which is why entropy behaves as a state function.
Enthalpy (H) is about heat content and energy, while entropy (S) is about disorder or randomness. A process can be spontaneous even without releasing heat, as long as total entropy increases. Both together decide spontaneity through Gibbs energy.
Yes. From ΔS = q_rev/T, the same amount of heat causes a smaller entropy change at higher temperature. Also, heating a substance generally increases its entropy because particles move more.
Entropy explains why processes happen on their own and is the base for Gibbs energy (ΔG = ΔH - TΔS). NEET regularly asks whether ΔS is positive or negative for a given process, which you answer by checking disorder and gas moles.
