Chemical Properties of metals
Metals are electropositive elements because they ionize by the loss of electrons and form positive ions. The electropositive character of metals gives rise to certain characteristic chemical properties. These are discussed below:
1. Reaction with Oxygen
Almost all metals combine with oxygen to form metal oxides. When a metal combines with oxygen, it loses its valence electrons and forms positively charged metal ions. Thus, oxidation of metal takes place. The atoms of oxygen accept the electrons lost by the metal and form negative oxide ions.
The metal oxides are basic in nature. Some metal oxides, such as aluminium oxide, zinc oxide, etc., show both acidic as well as basic behaviour. Such metal oxides are known as amphoteric oxides. Most of the metal oxides are insoluble in water. But some of the metal oxides dissolve in water to form alkali. Examples are sodium oxide and potassium oxide.
Na2O (s) + H2O (l) → 2NaOH (aq)
K2O (s) + H2O (l) → 2KOH (aq)
All metals do not react with oxygen with the same speed. Different metals show different reactivities towards oxygen. Metals, such as potassium and sodium, react so vigorously that they catch fire even if kept in the open air. Hence, to protect them, they are kept immersed in kerosene oil. At ordinary temperatures, the surface of metals like magnesium, aluminium, zinc, lead, etc., is covered with a thin layer of oxide. This layer prevents the metal from further oxidation. When magnesium is heated to its ignition temperature, it burns with a blinding white light to form the oxide. Iron does not burn on heating but glows brightly. Copper does not burn, but the hot metal is coated with a layer of black substance known as copper (II) oxide. Silver and gold do not react with oxygen, even at high temperatures.
2. Reaction with Water
Metals react with water and produce a metal hydroxide or oxide and hydrogen gas. But all metals do not react with water. Metals like potassium and sodium react violently with cold water. In case of sodium and potassium, the reaction is so violent that the evolved hydrogen immediately catches fire.
2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
The reaction of calcium is less violent with water.
Ca (s) + 2H2O (l) → Ca(OH)2 (aq) + H2 (g)
Magnesium does not react with cold water. It reacts with hot water to form magnesium hydroxide and hydrogen. Metals like aluminium, zinc and iron do not react either with cold or hot water. But they react with steam to form a metal oxide and hydrogen.
Metals like lead, copper, silver and gold do not react with water at all.
3. Reaction with Acids
All metals do not react with dilute hydrochloric and sulphuric acids. But when a metal reacts with any of these acids, a salt is formed and hydrogen gas is evolved. The metal replaces the hydrogen atoms in the acid to form a salt.
Hydrogen gas is not evolved when a metal reacts with nitric acid. It is because HNO3 is a strong oxidising agent. It oxidises the H2 produced to water and is itself reduced to any of oxides of nitrogen (N2O, NO, NO2). But Mg and Mn react with very dilute HNO3 to evolve H2 gas.