There are two types of elements – metals and non-metals. About 80 per cent of known elements are metals. Elements which are hard, ductile, brittle and malleable; possess luster and conduct heat and electricity are termed metals. All metals are solids, except mercury and gallium which are liquids at room temperature.
Metals usually have high melting and boiling points.
Chemical nature of metals –Usually metals have the tendency to lose electrons and while reacting with acids, usually replace hydrogen in dilute non-oxidising acids like hydrochloric acid (HCl) and sulphuric acid (H2SO4). Exceptions are copper, silver and gold. Chlorides of metals are true salts and oxides of metals are usually basic. Hydrides of metals are ionic, unstable and reactive.
Although all the metals are reactive, i.e. they are acted upon by common reagents like oxygen (air), hydrogen, halogens, sulphur, water, acids, etc. the extent of reaction is different in the case of each metal. Only gold, platinum and silver, are not affected by air and water under ordinary conditions, and are known as free metals. Various compounds of metals called minerals are found in nature and can be mined. The minerals from which metals can be extracted economically is called ore and the process of extraction of metals from their ores is called metallurgy, which involves:
Calcination – The process of heating the concentrated ore in the absence of air.
Roasting – The process of heating the ore in excess air.
Smelting – The process by which roasted ore is mixed with coke and heated in a furnace to obtain free metal.
Steel and iron –Steel is a form of iron. To make steel from iron, the carbon content is brought down from 5% to 0.5-1.5%.
Heat Treatment of Steel
Quenching – If steel is heated to bright redness and then suddenly cooled in water or oil it becomes extraordinarily hard and brittle.
Tempering – By controlled heating (250-325°C) of quenched steel, its brittleness can be removed without affecting its hardness.
Annealing – If steel is heated to a temperature well below red hot and then cooled turns it soft and the process is called annealing.
Rusting of iron – Majority of metals occur in nature in the combined form and are extracted from their ores. When these metals are exposed to atmospheric conditions they have a tendency to return to their original form. This change is called corrosion of metals and in the case of iron it is known as rusting.
Rusting consists of the formation of hydrated ferric oxide. For rusting water and oxygen are essential-in the absence of water or electrolyte rusting does not occur. The process involves addition of hydrogen and oxygen elements and it is found that mass of an iron rod increases by rusting. Rusting is prevented by coating the surface of iron with metals or non-metals, or by alloying. The coating of another metal is known as electroplating or hot dipping. In electroplating chromium or nickel is used. When a coat of zinc is applied on an iron surface by the hot-dipping process, it is known as galvanizing.
Non-metals – Non-metals are electro-negative elements which have a tendency to gain one or more electrons to form negative ions called anions. All non-metals generally exist as powders or gases, except bromine which is liquid under normal conditions. Non-metals are non-lustrous and are bad conductors of heat and electricity. They cannot be hammered into sheets or drawn into wires like metals. The melting point of non-metals is lower than that of metals.
Alloys – Alloys are homogeneous mixtures of two or more metals and non metals and have more commercial utility than some of the constituent elements. Some of the commercially important alloys are – Magnalium, Duralumin, Aluminium, Bronze, Brass, Tungsten steel.
Minerals – Minerals are naturally occurring chemical compounds of fixed composition and characteristics, physical form and properties. A few minerals consist of only one element. Example graphite and diamond (both forms of carbon), sulphur and gold.
Most minerals, however, are a combination of two or more elements as in halite or rock salt (NaCl). The most common group of minerals are: Silicates, oxides, sulphides, halides, carbonates.Minerals are of two types, viz: metallic or ore, and non-metallic, e.g. carbon, sulphur, etc.