This question is about elements in Period 3 and their compounds - AQA - A-Level Chemistry - Question 3 - 2021 - Paper 1

It is dangerous to react a similar piece of sodium with 10 cm³ of water in a boiling tube because of the exothermic nature of the reaction. A small volume of water may rapidly evaporate, leading to a potential explosion as the generated hydrogen gas can ignite due to the heat from the reaction.

The balanced equation for the reaction of phosphorus(V) oxide with water is:

\text{P}_2\text{O}_5_{(s)} + 6 \text{H}_2\text{O}_{(l)} \rightarrow 2 \text{H}_3\text{PO}_4_{(aq)}

The pH of the solution formed from the reaction of phosphorus(V) oxide with water is expected to be acidic, typically around pH 1 to 2, as it forms phosphoric acid.

Silicon(IV) oxide has a higher melting point than phosphorus(V) oxide because of its giant covalent structure where each silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement, creating a strong network of covalent bonds that require a large amount of energy to break. In contrast, phosphorus(V) oxide has a molecular or simple covalent structure with weaker Van der Waals forces between molecules, allowing it to melt at a lower temperature.

The formula for the oxide that is insoluble in water is likely $ext{SiO}_2$ (silicon dioxide).

The reaction of silicon dioxide with sulfuric acid can be represented as:

\text{SiO}_2_{(s)} + 2 \text{H}_2\text{SO}_4_{(l)} \rightarrow \text{Si(SO}_4\text{)}_{(aq)} + 2 \text{H}_2\text{O}_{(l)}

The formula for the hydroxide of an element in Period 3 that is used in medicine is $ext{Al(OH)}_3$ (aluminum hydroxide).

The element in Period 3 that has the largest atomic radius is sodium (Na).