Class 10 Science Notes Chapter 3 — Metals and Non-metals#

Physical properties#

Exceptions:

• Mercury is a liquid metal. Gallium and caesium melt on the palm.
• Sodium, potassium, lithium are soft enough to cut with a knife, and have low densities and melting points.
• Iodine is a non-metal but is lustrous.
• Diamond (carbon) is the hardest natural substance, with a very high melting point. Graphite (carbon) conducts electricity. Different forms of an element are allotropes.
• Lead and mercury are poor conductors of heat.

Chemical properties of metals#

1. With oxygen: metal + oxygen → metal oxide (basic).

• Amphoteric oxides react with both acids and bases to give salt and water: Al₂O₃, ZnO. $$\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$$ $$\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}$$
• Most metal oxides are insoluble in water. Soluble ones form alkalis: $\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}$.
• K and Na react so vigorously that they catch fire in air, so they are kept under kerosene.
• Mg, Al, Zn, Pb form a protective oxide layer. Fe burns only as filings. Cu does not burn but forms black CuO. Ag and Au do not react with oxygen even at high temperature.
• Anodising — making a thicker oxide layer on aluminium to protect it.

2. With water: metal + water → metal oxide + hydrogen; metal oxide + water → metal hydroxide.

3. With dilute acids: metal + acid → salt + hydrogen.

• Order of vigour: Mg > Al > Zn > Fe. Copper does not react.
• Nitric acid does not give H₂ with most metals, because it is a strong oxidising agent (it oxidises H₂ to water). Mg and Mn with very dilute HNO₃ do give H₂.
• Aqua regia = 3 parts conc. HCl + 1 part conc. HNO₃. It dissolves gold and platinum.

4. With salt solutions: a more reactive metal displaces a less reactive one.

Reactivity series#

K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au

(most reactive → least reactive)

• Metals above hydrogen displace hydrogen from dilute acids. Metals below it do not.

Ionic compounds#

• Metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions. Both reach a stable noble-gas configuration (octet).
• Ionic (electrovalent) compound — formed by the transfer of electrons from a metal to a non-metal.
  • Na (2,8,1) → Na⁺ (2,8) + e⁻; Cl (2,8,7) + e⁻ → Cl⁻ (2,8,8); so NaCl.
  • Mg (2,8,2) gives 2 electrons to two Cl atoms, so MgCl₂.

Occurrence and extraction of metals#

• Mineral — a naturally occurring compound of a metal. Ore — a mineral from which the metal can be extracted profitably. Gangue — impurities (soil, sand) in the ore.
• Steps: concentration of ore (removing gangue) → getting the metal → refining.

• Roasting — heating a sulphide ore strongly in excess air: $2\text{ZnS} + 3\text{O}_2 \rightarrow 2\text{ZnO} + 2\text{SO}_2$
• Calcination — heating a carbonate ore strongly in limited air: $\text{ZnCO}_3 \rightarrow \text{ZnO} + \text{CO}_2$
• Reduction: $\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$
• Thermit reaction — aluminium reduces iron(III) oxide with so much heat that the iron is molten. Used to join railway tracks and cracked machine parts. $$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3 + \text{heat}$$
• Electrolytic refining (Cu, Zn, Sn, Ni, Ag, Au): the impure metal is the anode, a thin strip of pure metal is the cathode, and a salt solution of the metal is the electrolyte. Pure metal deposits on the cathode; impurities settle below the anode as anode mud.

Corrosion#

• Silver turns black (silver sulphide). Copper gets a green coat (basic copper carbonate). Iron gets a brown flaky coat (rust).
• Iron rusts only when both air and water are present.
• Prevention: painting, oiling, greasing, galvanising, chrome plating, anodising, making alloys.
• Galvanising — coating iron or steel with zinc. It protects even if the coating is broken, because zinc is more reactive and corrodes first.

Alloys#

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. An alloy containing mercury is an amalgam.

• Alloys have lower electrical conductivity and melting point than the pure metals.

Common mistakes#

• Saying all metals are solids. Mercury is a liquid.
• Mixing up roasting (sulphide ore, excess air) and calcination (carbonate ore, limited air).
• Saying ionic compounds conduct as solids. They conduct only when molten or in solution.
• In electrolytic refining, putting the impure metal at the cathode. It is the anode.