An unknown substance is a white solid at room temperature and has a melting point of 78 °C. Which of the following substances is most likely to be the identity of the unknown sample?
A. Naphthalene, a molecular solid with the formula C10H8
B. Silica, a network solid held together by covalent bonds with the formula SiO2
C. Calcium chloride, an ionic compound with the formula CaCl2
D. Water, an molecular compound with the formula H2O

The unknown solid with melting point of 78 degrees Celsius has been the molecular solid Naphthalene with formula [tex]\rm C_1_0H_8[/tex]. Thus, option A is correct.

Melting point has been defined as the temperature that has been able to transfer the solid to the liquid state.

The melting point has been higher for the substances with strong intermolecular interactions, and lower for weak attractive compounds.

Identity of the unknown sample

The unknown sample has a melting point of 78 degree Celsius.

The molecular compounds have high melting point with strong intermolecular interactions, and ionic and covalent compounds have low melting point with weak intermolecular interactions.

The melting point of water has been zero degree Celsius.

Thus, the unknown solid with melting point of 78 degrees Celsius has been the molecular solid Naphthalene with formula [tex]\rm C_1_0H_8[/tex].

Thus, option A is correct.

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