Oxides and hydroxides
Metals react with oxygen to make oxides. Most metal oxides are basic: they react with acids to make a salt and water. The oxides of the very reactive metals (sodium, potassium, calcium) dissolve in water to make alkaline hydroxides. Some oxides, such as those of zinc and aluminium, react with both acids and alkalis. They are amphoteric.
| Oxide or hydroxide | Type | Reaction |
|---|---|---|
| Sodium oxide, Na₂O | Basic, dissolves to alkali | Na₂O + H₂O → 2NaOH |
| Calcium oxide (quicklime), CaO | Basic | CaO + H₂O → Ca(OH)₂ (slaked lime) |
| Magnesium oxide, MgO | Basic, little dissolves | MgO + 2HCl → MgCl₂ + H₂O |
| Zinc oxide, ZnO | Amphoteric | Reacts with acids and with NaOH |
| Aluminium oxide, Al₂O₃ | Amphoteric | Reacts with acids and with NaOH |
| Hydroxide | Solubility in water | Heated |
|---|---|---|
| Sodium and potassium hydroxide | Soluble | Do not decompose |
| Calcium hydroxide | Slightly soluble (limewater) | Decomposes to the oxide on strong heating |
| Copper(II), iron(II) and iron(III) hydroxide | Insoluble | Decompose to the oxide and water |
Carbonates and hydrogen carbonates
Metal carbonates react with acids to give a salt, water and carbon dioxide. Most carbonates decompose when heated to the metal oxide and carbon dioxide. Sodium and potassium carbonates are exceptions: they do not decompose in a Bunsen flame. The more reactive the metal, the more stable its carbonate. Hydrogen carbonates decompose more easily, giving the carbonate, water and carbon dioxide.
Choose the mass of calcium carbonate heated in a kiln. Find the quicklime and carbon dioxide produced. (Ca = 40, C = 12, O = 16)
Nitrates, chlorides and sulphates
Solubility rules help us predict which salts form precipitates. All nitrates and all sodium, potassium and ammonium salts are soluble. Most chlorides are soluble except silver and lead(II) chloride. Most sulphates are soluble except barium, calcium and lead(II) sulphate. Most carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium.
| Salt | Soluble? | Colour of the insoluble salt |
|---|---|---|
| Silver chloride, AgCl | No | White |
| Lead(II) chloride, PbCl₂ | Only in hot water | White |
| Barium sulphate, BaSO₄ | No | White |
| Calcium carbonate, CaCO₃ | No | White |
| Copper(II) carbonate, CuCO₃ | No | Green |
Nitrates decompose when heated. Potassium and sodium nitrates give the nitrite and oxygen. Most other nitrates (zinc, lead, copper) give the metal oxide, brown nitrogen dioxide and oxygen: 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂. Sulphates of most metals are stable to heat.
Key terms
- Amphoteric oxide
- An oxide that reacts with both acids and alkalis.
- Thermal decomposition
- Breaking down a compound by heating.
- Precipitate
- An insoluble solid formed when two solutions react.
- Quicklime
- Calcium oxide.
Summary
- Metal oxides are basic or amphoteric; hydroxides of sodium, potassium and calcium dissolve.
- Carbonates (except sodium and potassium) decompose on heating to give an oxide and carbon dioxide.
- Nitrates decompose on heating; all nitrates are soluble.
- Use solubility rules to predict precipitates: AgCl, BaSO₄ and CaCO₃ are white solids.