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Chapter 4 - Materials : Metals and Non-Metals Class 8 notes




Elements can be classified into the following two groups depending on their physical and chemical properties:
  1. Metals
  2. Non-metals

Metals

  • 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:

  1. Metals are solids at room temperature, except mercury, which is liquid at room temperature.
  2. They are generally hard and strong but sodium and potassium are soft solids and can be cut with a knife.
  3. They are good conductor of heat and electricity. Copper is the best conductor of electricity followed by gold and aluminium.
  4. 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.
  5. The property by virtue of which metal can be beaten into sheets is called malleability. We use aluminium foil to pack food.
  6. The property by which metals can be drawn into wires is called ductility. Metals like copper, silver and aluminium can be drawn into wires.
  7. Metals are sonorous. They produce sound when struck.
  8. With the exceptions of sodium, potassium and mercury most of the metals have high melting and boiling point.
  9. Metals have high densities.
  10. Most metals have high tensile strength.

Non-metals

  • 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:

  1. Non-metals are gases or solid at room temperature, except bromine which is liquid at room temperature.
  2. Non-metals do not have lustre except iodine and graphite.
  3. They are bad conductor of heat and electricity.
  4. Non-metals are brittle that is they are neither malleable nor ductile.
  5. They usually have low densities.
  6. Non-metals have low melting point and boiling point.
  7. Non-metals are non-sonorous.


Metalloids:

  • 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.
Sodium and potassium react with oxygen vigorously at room temperature.
$4Na+O_2\rightarrow 2Na2O$
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.
$2Mg+O_2\rightarrow 2MgO$
Iron metal does not burn in dry air even on strong heating. In moist air, iron get oxidise to form iron oxide (rust)
$4Fe +3O_2 \rightarrow 2Fe_2O_3$
$Fe2O_3+ nH_2 O \rightarrow Fe_2O_3.nH_2O$

Reaction of non-metals with oxygen:

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.
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.

Reaction of non-metal with water:

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.
$2Na + 2HCl \rightarrow 2NaCl + H_2$
$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$
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.

Reaction of non-metals with dilute acids:

Non-metals do not react with dilute acids.

Displacement reactions:

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.
\[ \underset{Iron}{Fe}+\underset{\begin{array}{c} Copper\,\,Sulphate\\ \left( blue \right)\\ \end{array}}{CuSO_4}\rightarrow \underset{\begin{array}{c} Iron\,\,Sulphate\\ \left( green \right)\\ \end{array}}{FeSO_4}+\underset{copper}{Cu} \\ \qquad Iron\,\,Sulphate\,\,+\,\,Copper\,\,\rightarrow \,\,no\,\,reaction \] Because copper is less reactive than iron so cannot displace it.

Uses of metals and non-metals:

Uses of metals:

  1. Copper and aluminium are used to make wires for carrying electric currents.
  2. Zinc is used for galvanising iron to protect iron from rusting.
  3. Silver, gold and platinum are used make jewellery.
  4. Iron, copper and aluminium metals are used to make utensils for cooking and for manufacturing machines parts.
  5. Mercury is used in thermometer.
  6. Lead is mainly used for making electrodes for automobile batteries and for making alloys.

Uses of non-metals:

  1. Hydrogen is used in manufacture of ammonia, industrial chemicals.
  2. Liquid nitrogen is used in refrigerant, in storing human organs at low temperature.
  3. Sulphur is used in manufacture of sulphuric acid, sulphur dioxide gas, sulphur drugs etc.
  4. Sulphur is used for vulcanisation of rubber and making gunpowder.
  5. Phosphorus is used in the manufacture of fertilisers.
  6. Silicon is used for making semiconductors foe which microchips are made.
  7. Chlorine is used for disinfecting drinking water.
  8. Oxygen is used in breathing support system for patients.


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Class 8 Maths Class 8 Science
Reference Books for class 8 science

Given below are the links of some of the reference books for class 8 science.

  1. Oswaal NCERT & CBSE Pullout Worksheets Class 8 Science These worksheets can help student evaluate whatever he/she has studied from the text books.
  2. Science for Class 8 Paperback by Lakhmir Singh Detailed science book to clear basics and concepts. I would say it is a must have book for class 8 student.
  3. Pearson Foundation Series (IIT -JEE / NEET) Physics, Chemistry, Maths & Biology for Class 8 (Main Books) | PCMB Combo : These set of books could help your child if he aims to get extra knowledge of science and maths. These would be helpful if child wants to prepare for competitive exams like JEE/NEET. Only buy if you can provide help to the child while studying.
  4. Reasoning Olympiad Workbook - Class 8 :- Reasoning helps sharpen the mind of child. I would recommend students practicing reasoning even though they are not appearing for Olympiad.

You can use above books for extra knowledge and practicing different questions.







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