CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
Four unknown crystalline solids—labeled Q, R, S, and T—are studied over a temperature range of 25 °C to 1200 °C. Each solid is placed between two inert platinum electrodes, and its electrical conductivity is measured as temperature increases. The results are summarized below.
• Solid Q: Conductivity is high at 25 °C and increases only slightly with temperature.
• Solid R: Conductivity is negligible at 25 °C, rises sharply at the melting point (780 °C), and remains high in the liquid phase.
• Solid S: Conductivity is negligible at all temperatures tested, including above its melting point (m.p. = 230 °C).
• Solid T: Conductivity is moderate at 25 °C and increases dramatically along one crystallographic direction while remaining low along the other two directions.
Assume that no chemical decomposition occurs.
(a) Classify each solid (Q, R, S, and T) as metallic, ionic, molecular, or covalent network. Provide justification for each classification based solely on the conductivity behavior described.
(b) For Solid R, describe the charge carriers responsible for conduction in the solid state versus the liquid state and explain the abrupt change at 780 °C.
(c) Solid T displays highly anisotropic conductivity. Propose a structural feature that could account for this observation and relate it to the nature of bonding within the solid.
(d) Design an additional experimental test (other than conductivity measurements) that would help distinguish between Solids S and T if their conductivity data were unavailable. Explain how the results of your test would support your identification.