2021 H2 Chem P1 Q9 — Periodic table Solution

The answer is D (solid remains in all three experiments).

\(\text{SiO}_2\) has a giant molecular (giant covalent) structure with strong Si-O covalent bonds throughout, and it is an acidic oxide.
• Experiment 1, water: \(\text{SiO}_2\) is insoluble in water and does not react with it, so the solid remains.
• Experiment 2, dilute NaOH(aq) at room temperature: although \(\text{SiO}_2\) is acidic, it reacts with NaOH only when heated with concentrated NaOH(aq) (to form \(\text{Na}_2\text{SiO}_3\)). With dilute NaOH(aq) at room temperature no reaction occurs, so the solid remains.
• Experiment 3, dilute HCl(aq): \(\text{SiO}_2\) is an acidic oxide and does not react with acids, so the solid remains.

• A is wrong: \(\text{SiO}_2\) does not dissolve in any of the three; it is insoluble in water, unreactive towards dilute acid and needs hot concentrated alkali to react.
• B is wrong: a solid remains in all three experiments, not just one.
• C is wrong: this would need \(\text{SiO}_2\) to dissolve in one of the reagents, for example dilute NaOH(aq), but it does not react with dilute NaOH(aq) at room temperature.