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AS · Practice questions · Period 3: oxides and chlorides

Test the pH.

Six original Cambridge-style questions on the oxides and chlorides of Period 3: equations, pH, bonding and amphoteric behaviour.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Ionic and basic to covalent and acidic.

01
Recall
[3 marks]

Write equations for the reactions of (a) sodium, (b) magnesium and (c) phosphorus with excess oxygen.

  • (a) 4Na + O2 → 2Na2O. ✓
  • (b) 2Mg + O2 → 2MgO. ✓
  • (c) P4 + 5O2 → P4O10. ✓
02
Explain
[3 marks]

Describe the reactions, if any, of MgO, SiO2 and SO3 with water, giving the approximate pH in each case.

  • MgO: slightly soluble, forming Mg(OH)2; pH about 9. ✓
  • SiO2: no reaction (insoluble giant covalent); pH 7. ✓
  • SO3 + H2O → H2SO4; pH about 1. ✓
03
Analysis
[2 marks]

Write two equations to show that aluminium oxide is amphoteric.

  • With acid: Al2O3 + 6HCl → 2AlCl3 + 3H2O. ✓
  • With alkali: Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4. ✓
04
Explain
[3 marks]

Compare the behaviour of NaCl and SiCl4 when added to water, including equations and pH.

  • NaCl is ionic; it dissolves to give Na+(aq) and Cl−(aq); pH 7. ✓
  • SiCl4 is covalent and is hydrolysed: SiCl4 + 2H2O → SiO2 + 4HCl. ✓
  • The HCl gives a strongly acidic solution, pH about 2, and white fumes. ✓
05
Analysis
[2 marks]

Give the oxidation number of the Period 3 element in (a) SO3, (b) PCl5 and (c) Al2Cl6, and state the connection with the group number.

  • (a) +6; (b) +5; (c) +3. ✓
  • In each case the maximum oxidation number equals the number of outer (valence) electrons of the element. ✓
06
Explain
[2 marks]

Explain, in terms of bonding, why Na2O is basic but SO2 is acidic.

  • Na2O is ionic; the O2− ion reacts with water to form OH− ions. ✓
  • SO2 is covalent; it reacts with water to form H2SO3, which releases H+ ions. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.