Phase Rule Questions and Answers

Answer: C

The liquid persists below its freezing point only for want of a nucleus, and the smallest disturbance or seed crystal converts it to ice at once. It appears on the diagram as a dotted extension of the vaporisation curve.

Answer: D

Correct answer: three

\[ P \le C + 2 = 1 + 2 = 3. \] The system has four possible phases but only three can coexist at once, so the sulphur diagram has several distinct triple points - rhombic with monoclinic and vapour, monoclinic with liquid and vapour, and rhombic with monoclinic and liquid - but no point at which all four meet.

Answer: A

Correct answer: polymorphic forms of the same substance

Rhombic and monoclinic sulphur are the same chemical substance in different crystal structures, so they count as two phases but only one component. This is what allows a one-component system to have four possible phases while still obeying the limit of three at equilibrium.

Answer: B

It quantifies what the phase rule only says qualitatively about a univariant curve, giving the gradient in terms of the enthalpy change and the volume change. The negative volume change on melting ice is what makes its fusion curve slope backwards.

Answer: C

Correct answer: \( F = C - P + 1 \)

Holding the pressure constant removes one variable, so the two in the full rule becomes one. The result is written \( F' \) and called the condensed or reduced phase rule, and it is what makes a binary solid-liquid diagram intelligible as a plot of temperature against composition alone.

Answer: D

Correct answer: almost unaffected by change of pressure

Solids and liquids are nearly incompressible and their equilibria shift very little with pressure, and the vapour phase is negligible. Pressure can therefore be treated as fixed, which reduces the variables by one and lets the diagram be drawn in two dimensions.

Answer: A

Correct answer: melts at the lowest temperature of any mixture of the pair

The eutectic is the minimum on the liquidus, the composition at which the two branches meet, and it melts lower than either pure component. This is the basis of solders and of low-melting alloys generally, and it is why a salted road stays liquid below zero.

Answer: B

Correct answer: zero

\[ F' = C - P + 1 = 2 - 3 + 1 = 0. \] Two solids and the liquid coexist at the eutectic, so nothing can be varied: the temperature and both compositions are fixed. That invariance is why a eutectic melt gives a horizontal arrest on a cooling curve, just as a pure substance does.

Answer: C

Correct answer: remain separate phases in the solid

A eutectic solid is an intimate mechanical mixture of two distinct crystalline phases, visible as separate constituents under a microscope, not a single phase of fixed stoichiometry. Its composition is fixed by the phase diagram rather than by valency, and it varies with pressure, which a compound’s would not.

Answer: D

Correct answer: completely immiscible in the solid state

A simple eutectic diagram, with no solid solubility at either end, implies that the two metals reject one another entirely on freezing. This is what makes the Pattinson process work: cooling argentiferous lead deposits pure lead crystals, leaving the silver concentrated in the remaining melt.