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 sodium with a smaller volume of water in a boiling tube because the reaction is highly exothermic and can potentially lead to an explosion. The smaller volume of water may not absorb the heat generated effectively, increasing the pressure inside the tube and pushing the sodium out violently, which could ignite the hydrogen gas produced.

The reaction of phosphorus(V) oxide with water is given by:

\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 resulting solution is likely to be acidic, around 1 to 2, due to the formation of phosphoric acid.

Silicon(IV) oxide has a higher melting point than phosphorus(V) oxide primarily due to its strong covalent network structure. In silicon(IV) oxide, each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement, forming a giant covalent structure that requires a large amount of energy to break. In contrast, phosphorus(V) oxide is molecular with simple covalent bonding, leading to weaker Van der Waals forces between molecules, making it easier to break apart and thus having a lower melting point.

The formula for the oxide is $\text{SiO}_2$. The reaction of silicon dioxide with sulfuric acid can be represented as:

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

A hydroxide of an element in Period 3 used in medicine is $\text{Al(OH)}_3$, also known as aluminum hydroxide.

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