Pattinson's process for desilverising lead exploits the fact that cooling an argentiferous melt first deposits
Answer: A
Ore poor in silver lies on the lead side of the eutectic, so lead crystallises out and is skimmed away while the remaining liquid moves toward the eutectic composition. Repeating the operation concentrates the silver considerably.
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A cooling curve for a pure substance shows a
Answer: B
The latent heat released holds the temperature constant until the last of the liquid has frozen. A mixture other than the eutectic instead shows a break where crystallisation begins and an arrest only at the eutectic temperature.
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A cooling curve for a mixture of eutectic composition resembles that of a pure substance in showing
Answer: C
Correct answer: a single horizontal arrest
At the eutectic the system is invariant, so the temperature cannot fall while the latent heat is being given up, and the curve shows a flat arrest exactly as a pure substance does. The difference is microscopic rather than thermal: the solid formed is two phases intergrown, not one.
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Thermal analysis constructs a phase diagram by recording cooling curves for
Answer: D
The temperatures of the breaks give the liquidus and those of the arrests the solidus or eutectic, and plotting them against composition draws the diagram. It is the standard experimental route to a condensed-system diagram.
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In a compound-forming system the intermediate compound melts congruently if it
Answer: A
Correct answer: melts sharply to a liquid of its own composition
A congruently melting compound gives a maximum on the liquidus and behaves like a pure substance at that point - the diagram can be treated as two simple eutectic systems joined at the compound. Incongruent melting instead decomposes the compound into a solid and a liquid of different composition, which is the peritectic case.
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A peritectic reaction on cooling converts
Answer: B
Correct answer: a liquid and one solid into another solid
\[ L + \alpha \rightarrow \beta. \] The peritectic is the reaction that produces an incongruently melting compound, and it is characteristically sluggish: the new solid forms as a shell around the old one and further reaction requires diffusion through it, so peritectic alloys often solidify out of equilibrium.
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A system whose components form a continuous series of solid solutions shows a phase diagram with
Answer: C
The two metals dissolve in one another in all proportions in the solid, so there is no composition at which two distinct solids deposit together. Copper with nickel is the classic example, its liquidus and solidus enclosing a narrow lens.
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Phenol and water are partially miscible, and their mutual solubility
Answer: D
Above the upper critical solution temperature of about 66 degrees the two mix in all proportions and the two-layer region disappears. Triethylamine and water behave oppositely, becoming less miscible as they are warmed.
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The transition temperature of a hydrated salt is the temperature at which
Answer: A
Sodium sulphate decahydrate converts to the anhydrous salt at 32.4 degrees, and the sharpness of the change makes such systems useful as fixed points for calibrating thermometers. Three phases coexist there, so the point is invariant.
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For a system of two components present as two phases, with pressure not fixed, the degrees of freedom number
Answer: B
Substituting into the full rule gives 2 minus 2 plus 2, that is two. Temperature and one composition may be chosen independently, or temperature and pressure, but fixing any two of them settles the whole state of the system.
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