You have seen in Activity 3.8 that magnesium burns in air with a dazzling white flame. Do all metals react in the same manner? Let us check by performing the following Activity.

CAUTION: The following activity needs the teacher’s assistance. It would be better if students wear eye protection. n Hold any of the samples taken above with a pair of tongs and try burning over a flame. Repeat with the other metal samples. n Collect the product if formed. n Let the products and the metal surface cool down. n Which metals burn easily? n What flame colour did you observe when the metal burnt? n How does the metal surface appear after burning? n Arrange the metals in the decreasing order of their reactivity towards oxygen. n Are the products soluble in water?

Almost all metals combine with oxygen to form metal oxides. Metal + Oxygen → Metal oxide. For example, when copper is heated in air, it combines with oxygen to form copper(II) oxide, a black oxide. 2Cu + O → 2CuO (Copper) (Copper(II) oxide). Similarly, aluminium forms aluminium oxide. 4Al + 3O → 2Al2O3 (Aluminium) (Aluminium oxide).

Recall from Chapter 2, how copper oxide reacts with hydrochloric acid. We have learnt that metal oxides are basic in nature. But some metal oxides, such as aluminium oxide, zinc oxide show both acidic as well as basic behaviour. Such metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides.

We have observed in Activity 3.9 that all metals do not react with oxygen at the same rate. Different metals show different reactivities towards oxygen. Metals such as potassium and sodium react so vigorously that they catch fire if kept in the open. Hence, to protect them and to prevent accidental fires, they are kept immersed in kerosene oil.

At ordinary temperature, the surfaces of metals such as magnesium, aluminium, zinc, lead, etc., are covered with a thin layer of oxide. The protective oxide layer prevents the metal from further oxidation. Iron does not burn on heating but iron filings burn vigorously when sprinkled in the flame of the burner. Copper does not burn, but the hot metal is coated with a black coloured layer of copper(II) oxide. Silver and gold do not react with oxygen even at high temperatures.

Anodising is a process of forming a thick oxide layer of aluminium. Aluminium develops a thin oxide layer when exposed to air. This aluminium oxide coat makes it resistant to further corrosion. The resistance can be improved further by making the oxide layer thicker. During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode reacts with aluminium to make a thicker protective oxide layer.

After performing Activity 3.9, you must have observed that sodium is the most reactive of the samples of metals taken here. The reaction of magnesium is less vigorous implying that it is not as reactive as sodium. But burning in oxygen does not help us to decide about the reactivity of zinc, iron, copper or lead. Let us see some more reactions to arrive at a conclusion about the order of reactivity of these metals.