Phase Rule Questions and Answers

Answer: A

Cooling proceeds smoothly until the liquidus is reached, one component then deposits and drives the liquid along the liquidus, and the residue finally freezes as eutectic. The cooling curve records each stage as a break or an arrest.

Answer: B

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: gases mix in all proportions and form a single phase, and the reduced rule is exactly the constant-pressure case. Statement 2 is the standard misconception - a eutectic melts sharply but is a mechanical mixture of two solid phases, not a compound.

Answer: C

Correct answer: four phases coexisting at equilibrium

\( F = 1 - P + 2 \) would give \( F = -1 \), which is impossible, so at most three phases can coexist for one component. The other three all occur: areas for one phase, curves for two, and a triple point for three.

Answer: D

Ice and water are physically distinct and mechanically separable, so together they constitute two phases rather than one. A gas mixture, a true solution and a solid solution are each homogeneous throughout and count as a single phase.

Answer: A

Correct answer: of eutectic composition or a congruently melting compound

These are the two compositions in a binary system that behave thermally like pure substances, both being invariant on freezing. A pure element is not an alloy, a continuous solid solution freezes over a range at every composition except the two ends, and immiscible liquids would not give a single sharp melting point.

Answer: B

Correct answer: the observation is impossible for one component

\( P \le C + 2 \) gives a maximum of three phases for one component, so a quadruple point cannot exist for pure water however many forms of ice are involved - three of them together is still three phases. A dissolved impurity would make it a two-component system, which is a different system rather than a quadruple point in this one.

Answer: C

Melting reduces the volume, so by Le Chatelier's principle pressure favours the liquid and the melting point falls. The negative volume change is exactly the term that sets the sign of the slope in the Clausius-Clapeyron relation.

Answer: D

Correct answer: the assertion is false although the reason offered is a true statement in itself

It is broadly true that compounds melt sharply while mixtures melt over a range, but the conclusion drawn does not follow: the eutectic is the exception, melting sharply because it is invariant rather than because it is a compound. Microscopy settles it - the solid is two intergrown phases, not one.

Answer: A

Correct answer: melts to a liquid of its own composition

That is what congruent means. An incongruently melting compound decomposes at the peritectic temperature into a solid and a liquid whose compositions differ from its own, so it never exists as a liquid of its own composition at all. Both types appear as maxima or as arrested features on the liquidus, but only the congruent one can be treated as a component in its own right.

Answer: B

Correct answer: the triple point of a one-component system

Invariant means zero degrees of freedom, which requires the maximum number of phases. The triple point has three phases of one component and \( F = 0 \). A point in a single-phase area has two degrees of freedom, a point on the vaporisation curve one, and a point on a binary liquidus one under the reduced rule.