Metals in air
Exposing two metals to the air and comparing them is enough to show the difference in reactivity.
Activity 16.3
- Expose a freshly cut piece of sodium and a 2 cm strip of magnesium whose surface has been cleaned to the air.
- Observe the cut surface of the sodium and the magnesium strip carefully.
The observation
The cut surface of the sodium is seen to lose its shine.
No appreciable change is seen in the magnesium strip.
The sodium loses its shine because it reacts rapidly with the constituents of the air. Magnesium gives no rapid reaction with them.
Three tiers of metals
React readily
Metals such as sodium (Na) and magnesium (Mg) react readily with oxygen when burnt in air and form the metal oxide.
4Na + O2 → 2Na2O
2Mg + O2 → 2MgO
React on heating
Metals such as zinc (Zn), iron (Fe) and copper (Cu) darken on the surface when heated in air. After long heating they turn into oxides.
2Zn + O2 → 2ZnO
Do not react even when heated
Metals such as silver (Ag), platinum (Pt) and gold (Au) do not turn into oxides even when heated strongly.
The conclusion
The readiness of different metals to react with oxygen varies from one to another — and that is the answer to why gold does not tarnish and iron does.