Elements can be classified into the following two groups depending on their physical and chemical properties:
- The elements which are hard, shiny can be beaten into sheets, drawn into wires and are good conductor of heat and electricity are generally metals. For example: iron, copper, gold etc.
- In nature most metals occur in the combined state as minerals and they are reactive.
- Only a few unreactive metals like gold, silver, platinum are found as free metals in the earth’s crust.
- Minerals from which metals can be profitably extracted are called ores. For example: calcium occurs in limestone (calcium carbonate) or iron in the ore haematite.
Physical properties of Metals:
- Metals are solids at room temperature, except mercury, which is liquid at room temperature.
- They are generally hard and strong but sodium and potassium are soft solids and can be cut with a knife.
- They are good conductor of heat and electricity. Copper is the best conductor of electricity followed by gold and aluminium.
- Metals such as gold, silver and copper all have lustre, that is they have an ability to shine and reflect light. Therefore they are lustrous.
- The property by virtue of which metal can be beaten into sheets is called malleability. We use aluminium foil to pack food.
- The property by which metals can be drawn into wires is called ductility. Metals like copper, silver and aluminium can be drawn into wires.
- Metals are sonorous. They produce sound when struck.
- With the exceptions of sodium, potassium and mercury most of the metals have high melting and boiling point.
- Metals have high densities.
- Most metals have high tensile strength.
- The elements which are brittle, dull cannot be beaten into sheets or drawn into wires and are poor conductors of heat and electricity are generally non-metals.
- For example: oxygen and nitrogen occur in free state in air and in combined state in earth’s crust. Sulphur occurs both in free and the combined state in earth’s crust.
- The noble gases, helium, neon, argon, krypton, xenon occurs only in Free State.
Physical properties of Non-metals:
- Non-metals are gases or solid at room temperature, except bromine which is liquid at room temperature.
- Non-metals do not have lustre except iodine and graphite.
- They are bad conductor of heat and electricity.
- Non-metals are brittle that is they are neither malleable nor ductile.
- They usually have low densities.
- Non-metals have low melting point and boiling point.
- Non-metals are non-sonorous.
- The elements which possess the properties of metals and non-metals are called metalloids.
- For example: boron, silicon, germanium, tellurium, arsenic and antimony are metalloids.
Chemical properties of Metals and Non-metals:
Metals along with hydrogen (a non-metal) are arranged in order to their activity in a series, called the activity series.
Reaction with oxygen:
Reaction of metals with oxygen:
Metals react with oxygen under different conditions to form basic oxides. These basic oxides react with water to form bases which turn red litmus into blue.
Reaction of non-metals with oxygen:
Sodium and potassium react with oxygen vigorously at room temperature.
To prevent this oxidation, sodium and potassium are stored under kerosene.
Magnesium reacted with oxygen only if ignited. It gives a bright dazzling flame and forms a white powder of magnesium oxide.
Iron metal does not burn in dry air even on strong heating. In moist air, iron get oxidise to form iron oxide (rust)
$4Fe +3O_2 \rightarrow 2Fe_2O_3$
$Fe2O_3+ nH_2 O \rightarrow Fe_2O_3.nH_2O$
Non-metals react with oxygen to form acidic or neutral oxides.
$C + O_2 \rightarrow CO_2$
$S + O_2 \rightarrow SO_2$
Other oxides like carbon monoxide $(CO)$ and nitrous oxide $(N_2O)$ are neutral.
Reaction with water:
Reaction of metal with water:
Metal react with water to form oxides or hydroxides along with hydrogen. Different metals react at different temperature.
Reaction of non-metal with water:
Sodium, potassium and calcium react vigorously with cold water to form metal hydroxides.
$2Na + 2H_2O \rightarrow 2NaOH + H_2 $
Metal from magnesium to iron react with steam (but not water) to form metal oxide and hydrogen gas.
$2Mg + H_2O \rightarrow 2MgO + H_2$
Tin, lead, copper, silver and gold do not react with water or steam.
Non-metals do not generally react with water.
Reaction with dilute acids:
Reaction of metals with dilute acids:
Metals react with dilute acids to form their salt and liberate hydrogen gas. The evolution of hydrogen gas can be tested by bringing a burning splinter near the mouth of the test tube. Hydrogen gas will put off the splinter with a pop sound.
Reaction of non-metals with dilute acids:
$2Na + 2HCl \rightarrow 2NaCl + H_2$
$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$
Metals below hydrogen (copper, silver, gold and platinum) do not react with dilute acids as they are less reactive than hydrogen. So they cannot displace hydrogen from dilute acids.
Non-metals do not react with dilute acids.
A more reactive metal displaces a less reactive metal from its compound in aqueous solution. Such a reaction is known as displacement reaction.
For example: When iron nail put in copper sulphate solution then colour of solution changes from blue to light green and a reddish brown coating of copper is seen on iron nail.
\left( blue \right)\\
\left( green \right)\\
\qquad Iron\,\,Sulphate\,\,+\,\,Copper\,\,\rightarrow \,\,no\,\,reaction
Because copper is less reactive than iron so cannot displace it.
Uses of metals and non-metals:
Uses of metals:
- Copper and aluminium are used to make wires for carrying electric currents.
- Zinc is used for galvanising iron to protect iron from rusting.
- Silver, gold and platinum are used make jewellery.
- Iron, copper and aluminium metals are used to make utensils for cooking and for manufacturing machines parts.
- Mercury is used in thermometer.
- Lead is mainly used for making electrodes for automobile batteries and for making alloys.
Uses of non-metals:
- Hydrogen is used in manufacture of ammonia, industrial chemicals.
- Liquid nitrogen is used in refrigerant, in storing human organs at low temperature.
- Sulphur is used in manufacture of sulphuric acid, sulphur dioxide gas, sulphur drugs etc.
- Sulphur is used for vulcanisation of rubber and making gunpowder.
- Phosphorus is used in the manufacture of fertilisers.
- Silicon is used for making semiconductors foe which microchips are made.
- Chlorine is used for disinfecting drinking water.
- Oxygen is used in breathing support system for patients.
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1. __________________ is the best conductor of electricity followed by gold and aluminium.
2. The property by virtue of which metal can be beaten into sheets is called _____.
3. _______ is used in foil to pack food.
4. The property by which metals can be drawn into wires is called _____.
5. The noble gases, helium, neon, argon, krypton, xenon occurs only in __________________ State.
6._____ is the Non-metals which is liquid at room temperature.
7._____ and ____ are the Non-metals which has lustre
8. The only liquid metal is _____.
9. boron and silicon are example of __________.
10. __________________ and _____ react with oxygen vigorously at room temperature.
11. __________________ reacted with oxygen only if ignited.
12. Metalic oxides are _____ in nature.
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Class 8 Maths
Class 8 Science