Drop the oxides of Period 3 into water and the pH slides from 14 down to 1. Their chlorides follow a similar pattern. Both trends come straight from the change in bonding, ionic on the left and covalent on the right.
Across Period 3 the bonding in the oxides and chlorides changes from ionic (Na, Mg) to covalent (Si, P, S). The oxides go from basic (Na2O, MgO) through amphoteric (Al2O3) to acidic (SiO2, P4O10, SO2, SO3). Ionic chlorides dissolve to give neutral or near-neutral solutions; covalent chlorides are hydrolysed by water, producing HCl and strongly acidic solutions. The highest oxidation number in the oxides and chlorides equals the number of outer electrons.
Burn each element in oxygen in the simulator, then add the oxide to water and test the pH. Watch the bonding change as you move across.
| Oxide | Ox. no. | Structure | With water | Nature |
|---|---|---|---|---|
| Na2O | +1 | giant ionic | Na2O + H2O → 2NaOH, pH 13 to 14 | basic |
| MgO | +2 | giant ionic | slightly soluble, Mg(OH)2, pH about 9 | basic |
| Al2O3 | +3 | giant ionic, some covalent character | insoluble | amphoteric |
| SiO2 | +4 | giant covalent | insoluble | acidic |
| P4O10 | +5 | simple molecular | P4O10 + 6H2O → 4H3PO4, pH about 2 | acidic |
| SO2 / SO3 | +4 / +6 | simple molecular | H2SO3 / H2SO4, pH about 1 | acidic |
Insoluble oxides cannot be tested with pH paper, so test them with acids and alkalis instead.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the approximate pH of the solution formed when sodium oxide reacts with water?
Which oxide is amphoteric?
What is formed when silicon tetrachloride is added to water?
What is the oxidation number of phosphorus in P4O10?
Add each chloride to water in the simulator. Ionic chlorides simply dissolve; covalent ones react with the water.
| Chloride | Ox. no. | Bonding | With water | pH |
|---|---|---|---|---|
| NaCl | +1 | ionic | dissolves | 7 |
| MgCl2 | +2 | ionic | dissolves, very slight hydrolysis | 6.5 |
| AlCl3 (Al2Cl6) | +3 | covalent | hydrolyses; white fumes of HCl | 3 |
| SiCl4 | +4 | covalent | SiCl4 + 2H2O → SiO2 + 4HCl | 2 |
| PCl5 | +5 | covalent | PCl5 + 4H2O → H3PO4 + 5HCl | 2 |
Aluminium oxide and silicon dioxide are insoluble in water, so they do not change its pH; their acid-base character is shown by their reactions with acids and alkalis. And AlCl3 is not neutral in water: the small, highly charged Al3+ ion polarises water molecules, releasing H+.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which equation shows the acidic nature of silicon dioxide?
Why does a solution of aluminium chloride in water have a pH of about 3?
Which describes the trend in the oxides from Na2O to SO3?
Which equation shows the amphoteric behaviour of aluminium oxide with sodium hydroxide?
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