The elements sodium to sulfur in Period 3 all react with oxygen to form oxides - AQA - A-Level Chemistry - Question 2 - 2018 - Paper 3

The equation for the reaction when phosphorus is heated in oxygen is:

$4P + 5O_2 \rightarrow P_4O_{10}$

Observation: A white flame is observed, and white smoke (phosphorus pentoxide) is produced.

The increase in melting point from sodium oxide (1548 K) to magnesium oxide (3125 K) can be attributed to the stronger ionic bonds present in magnesium oxide. Magnesium ions ( ext{Mg}^{2+}) have a higher charge density compared to sodium ions ( ext{Na}^{+}), resulting in greater electrostatic attraction between Mg and O atoms, leading to a higher melting point.

Silicon dioxide ( ext{SiO}_2) has a giant covalent structure, which involves strong covalent bonds extending throughout the structure. In contrast, phosphorus pentoxide ( ext{P}4O{10}) has discrete molecular structures with weaker intermolecular forces. This difference in bonding results in silicon dioxide having a much higher melting point than phosphorus pentoxide.

To determine the melting point of the highest oxide of phosphorus, one can use a melting point apparatus. Place a small sample of the oxide in a capillary tube and heat it gradually while monitoring the temperature. The melting point is recorded when the sample transitions from solid to liquid.

To evaluate purity, compare the observed melting point with the known melting point of the pure compound. A sharp melting point within a narrow range indicates high purity, while a broader range suggests impurities.