---
title: "A materials-science student examines purified samples labeled \\(P\\), \\(Q\\), and \\(R\\). The samples are \\(\\text{SiC}\\), \\(\\text{W}\\), and \\(\\text{MgF}_2\\), represented without repetition. The student records the following observations.  | Sample | Electrical conductivity as a solid at \\(25^\\circ\\text{C}\\) | Response to applied stress | Observation during strong heating at \\(1\\ \\text{atm}\\) | |—|—|—|—| | \\(P\\) | Very low | Fractures and is very difficult to scratch | Decomposes before a stable liquid is obtained | | \\(Q\\) | High | Can be rolled into a thin sheet after annealing | Forms an electrically conducting liquid | | \\(R\\) | Very low | Cleaves when struck | Forms an electrically conducting liquid |  Which assignment and particle-level explanation are most consistent with all the observations?"
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url: "https://nerd-notes.com/ubq/120088/"
date_modified: "2026-08-21T08:41:26+00:00"
---

# A materials-science student examines purified samples labeled \(P\), \(Q\), and \(R\). The samples are \(\text{SiC}\), \(\text{W}\), and \(\text{MgF}_2\), represented without repetition. The student records the following observations.

| Sample | Electrical conductivity as a solid at \(25^\circ\text{C}\) | Response to applied stress | Observation during strong heating at \(1\ \text{atm}\) |
|—|—|—|—|
| \(P\) | Very low | Fractures and is very difficult to scratch | Decomposes before a stable liquid is obtained |
| \(Q\) | High | Can be rolled into a thin sheet after annealing | Forms an electrically conducting liquid |
| \(R\) | Very low | Cleaves when struck | Forms an electrically conducting liquid |

Which assignment and particle-level explanation are most consistent with all the observations?

A materials-science student examines purified samples labeled \(P\), \(Q\), and \(R\). The samples are \(\text{SiC}\), \(\text{W}\), and \(\text{MgF}_2\), represented without repetition. The student records the following observations.

| Sample | Electrical conductivity as a solid at \(25^\circ\text{C}\) | Response to applied stress | Observation during strong heating at \(1\ \text{atm}\) |
|---|---|---|---|
| \(P\) | Very low | Fractures and is very difficult to scratch | Decomposes before a stable liquid is obtained |
| \(Q\) | High | Can be rolled into a thin sheet after annealing | Forms an electrically conducting liquid |
| \(R\) | Very low | Cleaves when struck | Forms an electrically conducting liquid |

Which assignment and particle-level explanation are most consistent with all the observations?

- **A.** \(P=\text{SiC}\), \(Q=\text{W}\), and \(R=\text{MgF}_2\); \(P\) consists of discrete polar \(\text{SiC}\) molecules held together by strong dipole-dipole attractions, \(Q\) contains delocalized electrons, and molten \(R\) contains mobile ions.
- **B.** \(P=\text{SiC}\), \(Q=\text{W}\), and \(R=\text{MgF}_2\); \(P\) has an extended network of directional \(\text{Si}-\text{C}\) covalent bonds, \(Q\) has metal atom cores attracted to delocalized electrons, and \(R\) contains ions that become mobile when the solid melts.
- **C.** \(P=\text{SiC}\), \(Q=\text{W}\), and \(R=\text{MgF}_2\); \(P\) has an extended covalent network, but electric current through solid \(Q\) is carried by tungsten ion cores that migrate while the valence electrons remain localized, and molten \(R\) contains mobile ions.
- **D.** \(P=\text{SiC}\), \(Q=\text{W}\), and \(R=\text{MgF}_2\); \(P\) has an extended covalent network, \(Q\) contains delocalized electrons, and solid \(R\) consists of neutral \(\text{MgF}_2\) units that become ionized into \(\text{Mg}^{2+}\) and \(\text{F}^-\) only when melting occurs.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120088/*