IB Structure 3.1 - Period 3 Trends

Which acid is produced when P₄O₁₀ reacts with water?

• H₃PO₂ (hypophosphorous acid)

• H₃PO₃ (phosphorous acid)

• H₄P₂O₇ (hypodiphosphoric acid)

• H₃PO₄ (phosphoric acid)

Answer explanation

When P₄O₁₀ reacts with water, it forms phosphoric acid (H₃PO₄). This reaction involves the hydrolysis of the anhydride P₄O₁₀, leading to the production of H₃PO₄, which is the correct answer.

Which of the following is the correct balanced equation for the reaction of P₄O₁₀ with water?

• P₄O₁₀ + 3H₂O → 2H₃PO₄

• P₄O₁₀ + 6H₂O → 4H₃PO₄

• P₄O₁₀ + 10H₂O → 2H₃PO₃

• P₄O₁₀ + 12H₂O → 4H₃PO₃

Answer explanation

The reaction of P₄O₁₀ with water produces phosphoric acid. The correct balanced equation is P₄O₁₀ + 6H₂O → 4H₃PO₄, indicating that 6 water molecules react to form 4 molecules of H₃PO₄.

Which statement best describes how Na₂O (sodium oxide) behaves in water?

• Na₂O dissolves to form a strongly acidic solution

• Na₂O is insoluble and does not react

• Na₂O dissolves to form a strongly alkaline solution

• Na₂O reacts to release hydrogen gas

Answer explanation

Na₂O reacts with water to form sodium hydroxide (NaOH), which is a strong base. This results in a strongly alkaline solution, making the correct choice "Na₂O dissolves to form a strongly alkaline solution."

When sulfur trioxide (SO₃) is carefully added to water, which acid forms?

• H₂SO₄ (sulfuric acid)

• H₂SO₃ (sulfurous acid)

• H₂S (hydrogen sulfide)

• HSO₄⁻ (bisulfate ion)

Answer explanation

When sulfur trioxide (SO₃) is added to water, it reacts to form sulfuric acid (H₂SO₄). This is a well-known reaction where SO₃ combines with water, leading to the formation of the strong acid H₂SO₄.

Magnesium oxide (MgO) in water produces which of the following?

• A strongly acidic solution

• Magnesium hydroxide, Mg(OH)₂

• Hydrogen gas, H₂

• No reaction at all under any conditions

Answer explanation

When magnesium oxide (MgO) dissolves in water, it reacts to form magnesium hydroxide, Mg(OH)₂, which is a weak base. Therefore, the correct answer is magnesium hydroxide, Mg(OH)₂.

Which one of these oxides is amphoteric?

• P₄O₁₀

• SO₃

• Al₂O₃

• Na₂O

Answer explanation

Al₂O₃, or aluminum oxide, is amphoteric as it can react with both acids and bases. In contrast, P₄O₁₀ and SO₃ are acidic, while Na₂O is basic, making Al₂O₃ the only amphoteric oxide among the options.

Al₂O₃ is amphoteric because it:

• Reacts only with acids

• Reacts only with bases

• Reacts with both acids and bases

• Does not react with any acid or base

Answer explanation

Al₂O₃ is amphoteric because it can react with both acids and bases, demonstrating its ability to act as either a proton donor or acceptor, which is characteristic of amphoteric substances.

Which oxide does not react readily with water under normal conditions?

• Na₂O

• MgO

• SO₂

• SiO₂

Answer explanation

SiO₂ (silicon dioxide) does not react readily with water under normal conditions, unlike Na₂O, MgO, and SO₂, which form hydroxides when they react with water.

Why is silicon dioxide (SiO₂) generally insoluble in water?

• It forms Si⁴⁺ and O²⁻ ions easily

• It has a giant covalent network structure

• It is entirely ionic in nature

• It is hygroscopic and dissolves completely

Answer explanation

Silicon dioxide (SiO₂) has a giant covalent network structure, which means it consists of a vast array of atoms bonded together by strong covalent bonds. This structure makes it difficult for water molecules to break these bonds, resulting in its insolubility.

When Al₂O₃ reacts with a strong base (e.g., NaOH), which species can form in solution?

• Al²⁺

• [Al(OH)₄]⁻

• AlH

• AlCl₃

Answer explanation

When Al₂O₃ reacts with a strong base like NaOH, it forms the soluble complex [Al(OH)₄]⁻. This occurs as aluminum ions react with hydroxide ions, leading to the formation of this tetrahydroxoaluminate species.