Activity . CAUTION : The following activity needs the teacher’s assistance. It would be better if students wear eye protection. Hold any of the samples taken above with a pair of tongs and try burning over a flame.
Repeat with the other metal samples. Collect the product if formed. Let the products and the metal surface cool down. Which metals burn easily?
What flame colour did you observe when the metal burnt? How does the metal surface appear after burning? Arrange the metals in the decreasing order of their reactivity towards oxygen. 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 → 2Al O (Aluminium) (Aluminium oxide) Recall from Chapter , 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. Aluminium oxide reacts in the following manner with acids and bases – Al O + 6HCl 2AlCl + 3H O Al O + 2NaOH 2NaAlO + H O (Sodium aluminate) Most metal oxides are insoluble in water but some of these dissolve in water to form alkalis.
Sodium oxide and potassium oxide dissolve in water to produce alkalis as follows – Na O(s) + H O(l) → 2NaOH(aq) K O(s) + H O(l) → 2KOH(aq) We have observed in Activity . 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.