Chemical properties of metals
Metals react with oxygen to form basic oxides, with water and dilute acids to give hydrogen, and with salt solutions of less reactive metals.
Reaction with oxygen
| Metal | How it reacts | Equation or product |
|---|---|---|
| Sodium, potassium | Catch fire in open air, so they are stored under kerosene | 4Na(s) + O₂(g) → 2Na₂O(s) |
| Magnesium | Burns with a dazzling white flame | 2Mg(s) + O₂(g) → 2MgO(s) |
| Aluminium, zinc, lead | A thin oxide layer forms and protects the metal | 4Al(s) + 3O₂(g) → 2Al₂O₃(s) |
| Iron | A lump does not burn, but iron filings burn with sparks when sprinkled in a flame | Iron oxide |
| Copper | Does not burn, but gets a black coat on heating | 2Cu(s) + O₂(g) → 2CuO(s) |
| Silver, gold | Do not react with oxygen, even at high temperature | No reaction |
Amphoteric oxides
Most metal oxides are basic and react with acids. Some, like aluminium oxide and zinc oxide, react with both acids and bases to give salt and water. These are called amphoteric oxides. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O, where NaAlO₂ is sodium aluminate. Metal oxides that dissolve in water form alkalis: Na₂O(s) + H₂O(l) → 2NaOH(aq) and K₂O(s) + H₂O(l) → 2KOH(aq).
Reaction with water
| Metal | Reacts with | Equation |
|---|---|---|
| Potassium, sodium | Cold water, violently; the hydrogen catches fire | 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + heat energy |
| Calcium | Cold water, less violently; calcium floats, because bubbles of hydrogen stick to it | Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g) |
| Magnesium | Hot water; it also floats | Mg + 2H₂O → Mg(OH)₂ + H₂ |
| Aluminium, iron, zinc | Only steam; they form a metal oxide | 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g); 2Al(s) + 3H₂O(g) → Al₂O₃(s) + 3H₂(g) |
| Lead, copper, silver, gold | Do not react with water at all | No reaction |
Reaction with dilute acids and with salt solutions
- Metal + dilute acid → salt + hydrogen. Example: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
- With dilute HCl, the bubbling is fastest for magnesium, then aluminium, zinc and iron: Mg > Al > Zn > Fe.
- Copper does not react with dilute HCl.
- Hydrogen is not given off with nitric acid, because HNO₃ is a strong oxidising agent that turns hydrogen into water. Magnesium and manganese do give hydrogen with very dilute HNO₃.
- Aqua regia (3 parts concentrated HCl to 1 part concentrated HNO₃) dissolves even gold and platinum.
- Salt solutions: a more reactive metal displaces a less reactive metal from a solution of its salt. Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). This is how the reactivity series is built.
How to draw the action of steam on a metal
- 1Draw a boiling tube clamped on its side. Put glass wool soaked in water at the closed end and the metal sample in the middle.
- 2Draw a burner heating the tube below the metal.The heat turns the water in the glass wool into steam, which passes over the hot metal
- 3From the cork, draw a delivery tube leading into a trough of water.
- 4Draw an upside-down gas jar full of water over the end of the tube, collecting hydrogen gas.
- 5Label: glass wool soaked in water, metal sample, burner, delivery tube, trough of water, hydrogen gas.
Sodium in a school laboratory is kept under kerosene because it catches fire in air. Aluminium kadais and pressure cookers last long because a thin oxide layer protects the metal.
2 marks: “Why does calcium start floating when it reacts with water?” Bubbles of hydrogen gas formed in the reaction stick to the surface of the metal and make it float.
2 marks: “Why is hydrogen gas usually not given off when a metal reacts with nitric acid?” Nitric acid is a strong oxidising agent. It oxidises the hydrogen produced to water, and is itself reduced to oxides of nitrogen.
3 marks: “Aluminium oxide dissolves both in dilute hydrochloric acid and in sodium hydroxide solution. What is such an oxide called? Write both equations, and name one more such oxide.” An amphoteric oxide: it reacts with both acids and bases to form salt and water. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O; Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. Another example: zinc oxide, ZnO.
The water ladder: K and Na, cold water (they catch fire). Ca, cold water (it floats). Mg, hot water. Al, Zn and Fe, only steam. Pb, Cu, Ag and Au, never.