CBSE Class 10 Chemistry Metals and Non-metals Notes Set 27

Physical Properties of Metals and Non-metals

Metals conduct because of free electrons. Heating a metal lowers its conductivity because vibrating ions hinder the electrons. Metalloids such as Si, Ge and As have properties in between.

Chemical Properties of Metals

1. With oxygen, metals form oxides which are generally basic: 4Na + O₂ → 2Na₂O. Some oxides (Al₂O₃, ZnO) are amphoteric and react with both acids and bases. Gold and platinum do not react with oxygen. Sodium and potassium are stored in kerosene.
2. With water: Na and K react with cold water (2Na + 2H₂O → 2NaOH + H₂), Ca less violently, Mg with hot water, and Al, Zn and Fe with steam to give oxides: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Pb, Cu, Ag and Au do not react.
3. With dilute acids: metals above hydrogen give a salt and hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂. Nitric acid is a strong oxidising agent and generally does not give hydrogen. Zn and Al also react with alkalis.
4. Displacement: a more reactive metal displaces a less reactive one from its salt solution: Fe + CuSO₄ → FeSO₄ + Cu.
5. Reactivity series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au (most to least reactive).

Chemical Properties of Non-metals

1. Non-metals form acidic or neutral oxides: SO₂ + H₂O → H₂SO₃, CO₂ + H₂O → H₂CO₃; CO and water are neutral oxides.
2. Non-metals do not displace hydrogen from dilute acids because they do not lose electrons. They react with chlorine to form covalent chlorides (PCl₃, PCl₅, CCl₄).
3. Sulphur exists as S₈ rings, dissolves in carbon disulphide, and burns to give SO₂. Concentrated sulphuric acid acts as a dehydrating agent (it chars sugar) and as an oxidising agent.

Ionic Compounds

1. A metal loses electrons and a non-metal gains them to reach a noble gas configuration: Na (2, 8, 1) gives Na+ and Cl (2, 8, 7) gives Cl–. The atomic number does not change when an ion forms.
2. Ionic compounds are hard crystalline solids with high melting and boiling points because of strong electrostatic attraction. They are soluble in water, and conduct electricity in the molten state and in solution but not as solids.

Occurrence and Extraction of Metals

1. A mineral is a naturally occurring compound of a metal. An ore is a mineral from which the metal can be extracted profitably. The earthy impurity in an ore is gangue.
2. Steps in metallurgy: concentration of the ore (hydraulic washing, magnetic separation, froth floatation for sulphide ores, leaching for bauxite), conversion to oxide, reduction to the metal, and refining.
3. Calcination (heating carbonate or hydrated ores in little air): ZnCO₃ → ZnO + CO₂. Roasting (heating sulphide ores in excess air): 2ZnS + 3O₂ → 2ZnO + 2SO₂.
4. Method of reduction depends on reactivity: metals low in the series (Hg, Cu) are obtained by heating or self reduction; metals in the middle (Zn, Fe, Pb) by reduction with carbon: ZnO + C → Zn + CO; very reactive metals (Na, Mg, Al) by electrolysis of the molten compound.
5. Thermite reaction: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, used for welding rails.
6. Electrolytic refining: impure metal as anode, pure metal as cathode and a soluble salt of the metal as electrolyte; impurities collect as anode mud. Zone refining gives ultra-pure semiconductors.

Corrosion and Alloys

1. Corrosion is the slow eating away of a metal by air and moisture. Iron rusts (Fe₂O₃·xH₂O) in the presence of both oxygen and water; silver turns black (Ag₂S) and copper gets a green coating of basic copper carbonate. Prevent it by painting, oiling, galvanising (coating with zinc), tinning and alloying.
2. Aluminium forms a thin protective layer of Al₂O₃, which is why it is used for utensils although it is reactive.
3. An alloy is a homogeneous mixture of a metal with other elements: brass (Cu, Zn), bronze (Cu, Sn), solder (Pb, Sn), stainless steel (Fe, Cr, Ni, C), duralumin (Al, Cu, Mg, Mn). An amalgam is an alloy with mercury. Alloys are usually harder and less conducting than the pure metal.
4. Pure gold (24 carat) is soft and is alloyed with copper or silver to make 22 carat jewellery; aqua regia dissolves gold.