A student receives three unlabeled crystalline samples—P, Q, and R—and a limited set of laboratory resources that includes a hot plate capable of reaching 800 °C, a multimeter for testing conductivity, a steel file for scratch tests, and common laboratory glassware. The student’s goal is to determine the dominant type of solid each sample represents.
The following preliminary observations are recorded:
• All three samples appear as colorless, irregular crystals.
• None of the samples show any visible change when briefly placed on the hot plate at 200 °C.
Design and justify an experimental plan to classify the three solids.
(a) Propose a sequence of at least three distinct tests (using only the available equipment) that would allow the student to distinguish among ionic, metallic, covalent network, and molecular classifications for samples P, Q, and R.
(b) For EACH proposed test, describe the specific observations that would indicate each possible classification of a sample. Include the chemical or physical reasoning that connects the anticipated observation to the underlying structure of the solid.
(c) Suppose sample R is found to be electrically non-conductive as a solid, possesses extreme hardness when scratched with the steel file, and does not melt on the hot plate at its maximum temperature. Deduce the most likely classification of R and justify your answer using the results of all relevant tests.