Physical Properties of Metals
Elements are broadly classified on the basis of their physical and chemical properties-
Besides metals and non-metals, there is a third group of elements called metalloids which show properties intermediate to metals and non-metals.
E.g.- silicon (Si), germanium (Ge), arsenic (As), antimony (Sb) and tellurium (Te).
There are very few non-metals in comparison to metals (over 3/4 of periodic table is made up of metals).
Physical Properties of Metals
- Physical state –
Metals are generally solid at room temperature.
Exception – Mercury is only metal that exists in liquid state at room temperature.
- Metallic luster –
When metals are in pure state or freshly prepared, they show shining surface called metallic luster. However, metals lose their shine on exposure of atmosphere for a long time as they form a coating of oxide, sulphide or carbonate etc. on the surface. The brightness appears again by rubbing the surface with sand paper.
- Hardness –
Metals are generally hard. The hardness varies from metal to metal. Iron is used in building constructions, bridges etc. Copper is used for making utensils and statues etc.
Exceptions – Lithium, sodium and potassium (alkali metals) are soft metals which can be cut easily with a knife.
- Malleability –
Beat a iron nail and a coal piece with hammer. The shape of iron nail is changed on beating. If it is beaten harder, it could be changed into sheets without breaking. Whereas, coal pieces break into small pieces and becomes powdery.
Thus, malleability is a property by which some metals can be beaten into thin sheets. Gold and silver are the most malleable metals.
Exception- Zinc, antimony, bismuth and arsenic do not show malleability.
♦ Silver foil used for decorating sweets, aluminium foil used for wrapping food and medicines, show
malleable nature of silver and aluminium metal.
Beat 4-5 pieces made of iron, aluminium, lead and copper metals separately with a hammer with care.
(a) Observe the change in the shape of these metals.
Objective – Malleable property of metals.
Discussion – The shape of these metals become flat without breaking.
Conclusion – Metals are malleable in nature.
Metals can be drawn into thin wires without breaking. This ability of metals is known as ductility e.g. copper, aluminium wires.
(Mnemonic – Drawn into wires –Ductility)
Exception – Zinc, antimony, bismuth and arsenic are not very ductile.
♦ Gold is the most ductile metal as1 gm of gold can be drawn into a wire of about 2 km.
♦ Metals can be moulded into different shapes according to our needs due to their malleable and ductile nature.
- Good conductor of heat –
Metals are good conductor of heat. Due to this property, metals are used for making cooking utensils. Silver is the best conductor of heat followed by copper, gold and aluminium.
Exception – Lead, mercury, tungsten etc are poor conductors of heat.
- High melting point –
Metals generally possess high melting point. It means they do not melt easily.
Exception – Sodium, potassium, gallium and caesium melt at low temperature.
♦ The melting points of gallium (30°C) and caesium (28°C) are so low that they start melting by the heat of body if you keep them on your palm.
- Good conductor of electricity-
Metals are good conductor of electricity. That is why metal wires are used in the transmission of the electric current from one place to the other. Silver is the best conductor of electricity followed by copper, gold, aluminium and tungsten.
Exception – Iron and mercury possess lower conductivity of electricity.
♦ Why are electric wires coated with poly vinyl chloride (PVC) or a rubber like material?
This is because such coatings work as an insulator. They protect us from electric shock if touched
accidently during flowing of electric current through the wire.
- Sonorous (metallic sound) –
Metals produce a sound (sonorous) when hit with a hard object. This property(sonorous nature of metals) is called sonorousness or sonority.
- High densities–
metals are heavy substances. It means they possess high density, e.g. density of iron is 7.8g/cm³.
Exception- Alkali metals such as sodium(0.97g/cm³) potassium(0.86g/cm³) etc. have low densities. They are light metals.
♦ Sonorous property of metals is used for making bells and strings of musical instruments such as sitar and violin.
♦ Different metals [or metal and carbon (non-metal)] dissolve in each other in the molten state, in right proportion, form a homogeneous mixture. Such mixtures are known as alloys which are very useful. Example- Copper + tin = Bronze ; 70% Cu + 30% Zn = Brass ; 73% Fe + 18% Cr + 8% Ni + 1% C = Stainless steel ; lead + tin = Solder. If one of the metals is mercury, then the alloy is known as amalgam.
Four reasons for making alloys –
1. Improve the strength and hardness of metals.
2. Improve the resistance against corrosion of metals.
3. Improve the appearance of the metals.
4. Lower the melting point and electrical conductivity of the metals.