Metals with oxygen and water

Metals with oxygen

Most metals react with oxygen to make metal oxides.

• Sodium and potassium react so vigorously that they catch fire if they are left in the open. They are stored under kerosene oil. $4\,\text{Na} + \text{O}_2 \rightarrow 2\,\text{Na}_2\text{O}$.
• Magnesium burns with a dazzling white flame. $2\,\text{Mg} + \text{O}_2 \rightarrow 2\,\text{MgO}$.
• Iron burns as fine filings, with sparks.
• Aluminium forms a thin, strong layer of aluminium oxide on the surface, and this layer protects it from more reaction. $4\,\text{Al} + 3\,\text{O}_2 \rightarrow 2\,\text{Al}_2\text{O}_3$. This layer can be made thicker by anodising, to make the metal more resistant to corrosion.
• Copper, when heated, turns black, because black copper oxide forms. $2\,\text{Cu} + \text{O}_2 \rightarrow 2\,\text{CuO}$.
• Silver and gold do not react with oxygen, even when heated.

You should find that the ash always weighs $\dfrac{5}{3}$ of the ribbon, because the extra is oxygen from the air.

Metal oxides are basic

Most metal oxides are basic. They react with acids to give a salt and water. Those that dissolve in water make alkalis.

Amphoteric oxides

Some metal oxides, such as aluminium oxide and zinc oxide, react with both acids and bases. They are called amphoteric oxides.

Metals with water

The more reactive the metal, the more vigorous its reaction with water.

• Potassium and sodium react violently with cold water. The heat that is given out makes the hydrogen burn. $2\,\text{Na} + 2\,\text{H}_2\text{O} \rightarrow 2\,\text{NaOH} + \text{H}_2$.
• Calcium reacts with cold water less violently. The bubbles of hydrogen stick to it, and it floats. $\text{Ca} + 2\,\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2$.
• Magnesium does not react with cold water, but it reacts with hot water. $\text{Mg} + 2\,\text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 + \text{H}_2$.
• Aluminium, zinc and iron react only with steam, to make the metal oxide and hydrogen. $2\,\text{Al} + 3\,\text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 3\,\text{H}_2$ and $3\,\text{Fe} + 4\,\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\,\text{H}_2$.
• Lead, copper, silver and gold do not react with water.

Practise with new numbers

Try it in Python

This program builds the formula of the oxide of a metal by crossing the valencies.

Remember these

• Metal + oxygen gives a metal oxide. Sodium and potassium are kept in kerosene.
• Metal oxides are basic. Aluminium oxide and zinc oxide are amphoteric.
• Potassium and sodium react with cold water, calcium less so, magnesium with hot water, aluminium, zinc and iron with steam.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What forms on aluminium that stops it from reacting further with air?

2. Copper turns black when it is heated in air. Why?

3. Which metals do not react with oxygen even when they are heated?

4. Magnesium needs hot water to react, but sodium reacts with cold water. Why?