LECTURE -2 : CLASS VIII : SCIENCE : CHAPTER 4 : MATERIALS : METALS & NON-METALS

CLASS VIII   |    SCIENCE    |    CHAPTER 4

     Notes prepared by Subhankar Karmakar 

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CHEMICAL PROPERTIES OF METALS & NON METALS:

REACTION OF METALS:

 a. Reactions of metal with Oxygen (O₂):

 Metal reacts with oxygen to form metal oxides. Metal oxides are basic in nature. 

The basic metal oxides turn red litmus to blue. 

*(Metals and R & B)

∴ Metal + Oxygen (from air) = Metal Oxide (basic oxide) 

Magnesium burning in air: 

I. When Magnesium (Mg) burns in air, it combines with the oxygen (O₂) of air to form magnesium oxide. 

• Mg + O₂ = MgO (a basic oxide)

II. Magnesium oxide dissolves partially in water to form magnesium hydroxide Mg(OH)₂  solution:

• MgO + H₂O = Mg(OH)₂ (a base)

Sodium (Na) reacts with Oxygen in air and produces Sodium Oxide (Na₂O)

• Na + O₂ = Na₂O (a basic oxide) 

Water solution of Sodium Oxide forms Sodium Hydroxide (NaOH)

• Na₂O +  H₂O  = NaOH

 Reaction of iron with oxygen of air:

During the rusting of iron, iron (Fe) metal combines slowly with the oxygen (O₂) of air in the presence of water or moisture to form a compound called iron oxide (Fe₂O₃). This iron oxide is called rust. Damp air contains Oxygen (O₂) + water (H₂O). 

• Iron (Fe) + Oxygen (O₂) + water (H₂O) ⟹ Iron Oxide or rust (Fe₂O₃) (basic oxide)

• Reaction of copper metal with moist air:

When a copper object is exposed to moist air for a long time, then copper (Cu) reacts with water (H₂O), carbon dioxide (CO₂) and oxygen (O₂) present in moist air to form a green coating on the copper object. The green coating is a mixture of copper hydroxide [Cu(OH)₂] and copper carbonate (CuCO₃) which is formed by the action of moist air on copper object.

• 2Cu + H₂O + CO₂ + O₂ = Cu(OH)₂ + CuCO₃ 

• Corrosion of copper: The formation of green coating of basic copper carbonate on the surface of copper objects on exposure to moist air is called corrosion of copper. 

 

b. Reactions of metal with water:

 When a metal reacts with water, then a metal hydroxide and hydrogen gas are formed. 

Metal + water = Metal hydroxide + Hydrogen

Not all metals react with water. Some of the metals reacts with cold water, whereas some metals reacts with hot water and steam. It depends upon reactivity of metals.

Sodium and potassium very quickly reacts with cold water. 

·        Magnesium reacts slowly with cold water and quickly with hot water and zinc and iron slowly react with steam. 

·        Sodium (Na) + water (H₂O) → Sodium Hydroxide (NaOH) + Hydrogen (H₂)

·       Sodium (Na) is a very reactive metal. It reacts with moisture, oxygen and other gases present in air. So, if sodium metal is kept exposed to air, it will react with the various components of air and get spoiled. In order to prevent its reaction with the moisture and other gases of air, sodium metal is always told under kerosene. Potassium metal is also very reactive and also kept in kerosene. 

c. Reactions of metals with acids:

Most of the metals react with dilute acids to form salts and hydrogen gas. 

Metal + Acid → Salt + Hydrogen gas.

Only less reactive metals like Copper, silver and gold do not react with dilute acids. 

• Magnesium reacts with dilute hydrochloric acid to form magnesium chloride (salt) and hydrogen gas.

Magnesium + hydrochloric acid → magnesium chloride + hydrogen gas

Mg + HCl → MgCl₂ + H₂

 When foodstuffs containing acids like orange juice, pickles, and curds are kept in iron, aluminium or copper containers, the acids present in them react with the metal of the container slowly to form toxic salts. That's why acidic foodstuffs should not be kept in metal containers.

d. Reactions of metal with bases:

Only some metals react with bases to form salts and hydrogen gas. Like aluminium is a metal and Sodium hydroxide is a base. When aluminium is heated with sodium hydroxide solution, then sodium aluminate which is a salt and hydrogen gas is formed. 

Sodium hydroxide + aluminium → sodium aluminate + hydrogen

NaOH + Al → NaAlO₂ + H₂

Zinc also reacts with bases like sodium hydroxide to produce hydrogen gas. 

REACTION OF NON METALS:

a. Reaction of nonmetals with oxygen:

 Non metals react with oxygen to form non metal oxides. Non metal oxides are acidic in nature. Non metal oxides water solution turn blue litmus into red. 

Non metal + oxygen → non metal oxide

 1. When sulphur burns in air, it combines with the oxygen of air to form sulphur dioxide. Sulphur dioxide is a acidic oxide. 

Sulphur + oxygen → sulphur dioxide

S + O₂ → SO₂

Sulphur dioxide dissolves in water to form sulphurous acid solution

SO₂ + H₂O → H₂SO₃

b. Reactions of nonmetals with water:

 Non metals do not react with water. Therefore, highly reactive nonmetals like phosphorus cannot be kept open in the air as it reacts with oxygen of air and catches fire. So, in order to protect phosphorus from atmospheric air, it is stored in a bottle containing water.

 c. Reactions of nonmetals with acids:

 Non metals do not react with dilute acids. 

 d. Reactions of nonmetals with bases:

 Some of the nonmetals react with bases but no hydrogen gas is produced.

 Difference between metal oxides and non metal oxides:

 Metal oxides are basic in nature and turn red litmus to blue. 

Non metal oxides are acidic in nature and turn blue litmus to red. 

 REACTIVITY SERIES OF METALS:

The arrangement of metals in a vertical column in the order of decreasing reactivities is called the reactivity series of metals.  

In reactivity series, the most reactive metal is placed at the top whereas the least reactive metal is placed at the bottom.

Potassium is the most reactive metal, so it has been placed at the top of the reactivity series. Gold is the least reactive metal so it has been placed at the bottom of the reactivity series.

 

Potassium (K) (most reactive)

Sodium (Na)

Calcium (Ca)

Magnesium (Mg)

Aluminium (Al)

Zinc (Zn)

Iron (Fe)

Lead (Pb)

Copper (Cu)

Silver (Ag)

Gold (Au) (least reactive)

Reactivity of the metals decreases as we go down in the above series. 

 

LECTURE -1 : CLASS VIII : SCIENCE : CHAPTER 4 : MATERIALS : METALS & NON-METALS

CLASS VIII   |    SCIENCE    |    CHAPTER 4

      notes prepared by subhankar Karmakar

                                                                         

• Element:

A substance which cannot be broken down into two or more simpler substances by chemical reactions is is called an element. 

Some of the common elements are:

Hydrogen, helium, carbon, nitrogen, oxygen, sulphur, phosphorus, silicon, chlorine, bromine, iodine, sodium, potassium, magnesium, calcium, zinc, iron, copper, silver, gold and mercury. 

Every element is represented by a symbol. No two elements can have the same symbol.

Symbols of common elements:

1. Hydrogen - H

2. Helium - He

3. Carbon - C

4. Nitrogen - N

5. Oxygen - O

6. Sulphur - S

7. Phosphorus- P

8. Silicon - Si

9. Chlorine - Cl

10. Bromine - Br

11. Iodine - I

12. Sodium - Na

13. Potassium - K

14. Magnesium - Mg

15. Calcium - Ca

16. Zinc - Zn

17. Iron - Fe

18. Copper - Cu

19. Silver - Ag

20. Gold - Au

21. Mercury - Hg

• Atom:

The smallest particle of an element is called atom. An element is a substance which is made up of only one kind of atoms. 

There are as many type of atoms as are elements. So different elements are made up of different kinds of atoms. For example, sulphur element is made up of only sulphur atoms. This means an amount of oxygen is totally made of atoms of oxygen only. 

There are only 92 naturally occurring elements known to us at present. Other elements in the periodic table are synthesized elements. 

Properties of elements:

Different elements have different properties. Some of the most important properties of elements are malleability, ductility, brittleness, lustre, Sonorousness, conductivity, strength, hardness, toughness etc. 

On the basis of their properties, all the elements can be divided into two main groups: 

Metals and nonmetals.

Characteristics of metals:

Metals are malleable and ductile elements. They are good conductors of heat and electricity. Metal are lustrous or shiny. Metals are usually hard and strong. All the metals are solids except Mercury which is a liquid metal. Metals have high densities which means they are heavy. Metals have high melting points and boiling points. Metals are sonorous which means that metals make a ringing sound when we strike them with a hard object. 

Some of the examples of metals are: iron, copper, aluminium, zinc, silver, gold, Platinum, chromium, sodium, potassium, calcium, magnesium, nickel, Cobalt, tin, Mercury, tungsten, manganese, uranium etc. Out of 92 naturally occurring elements, 70 elements are metal.

Characteristics of nonmetals:

Non metals are the elements which are neither malleable nor ductile, they are brittle. Non metals do not conduct heat and electricity. Non metals are not lustrous or shiny. Non metals can be solid, liquid or gases at the room temperature. Non metals have usually low melting points and boiling points. Non metals have low densities which means they are light. Non metals are not sonorous, which means non metals do not make ringing sound when we strike them with a hard object. 

Some of the examples of non metals are: Carbon, sulphur, phosphorus, hydrogen, oxygen, nitrogen, chlorine, fluorine, bromine, iodine, helium, neon, argon, Krypton and xenon. Out of of 92 naturally occurring elements, 22 elements are non metals. Out of these, 10 non metals are solids, 1 non metal is a liquid (bromine), and 11 non metals are gases. 

Metalloids:

There are some elements which show some properties of metals and the other properties of nonmetals. The elements whose properties are intermediate between those of metals and nonmetals are known as metalloids. 

The example of metalloids are: silicon, germanium, arsenic and tellurium.

Physical properties of metals and nonmetals:

a. Malleability:

The property which allows the metals to be hammered into thin sheets is called malleability. Most of the metals are malleable. Gold and silver are the best malleable metals and can be hammered into very fine sheets or foils. Aluminium and copper are also highly malleable. It is due to the property of malleability that metals can be bent to form objects of different shapes by beating with a hammer. 

Non metals are not malleable.

b. Brittleness:

The property due to which non-metals  break on hammering is called brittleness. This means non metals can not be hammered into a thin sheets, it break into small pieces when hammered. All the non metals are brittle.

c. Ductility:

The property which allows the metals to be drawn into wires is called ductility. Ductility is another characteristics property of metals. Generally all the metals are malleable and ductile. 

Gold and silver are among the best ductile metals. Copper and aluminium metals are also very ductile and can be drawn into to tin Copper and aluminium wires.

Non metals are not ductile. 

d. Conductivity:

Heat and electricity can easily flow through metals. Therefore metals are good conductors of heat and electricity. ( as they allow heat and electricity to pass through them easily).

Silver metal is a best conductor of heat. Copper, gold, aluminium and iron metals are good conductors of heat.

Except graphite and diamond all the non metals are bad conductors of heat and electricity. Therefore, they are called insulators. Diamond is a good conductor of heat whereas graphite is a good conductor of electricity. 

e. Lustre:

All the metals have a shiny appearance. This property of metal is known as Lustre of metal. 

All the non metals are not lustrous. Only iodine has a lustre. 

f. Strength:

Metals are usually strong, they have high tensile strength. Metals can hold large weights without snapping. Iron is one of the most strongest material, hence iron is used in construction purposes. 

Non metals are not strong. They have low tensile strength.

g. Sonorousness:

All the metals make a ringing sound when we strike them. This property of metal is known as Sonorousness. Sonorous means capable of producing a ringing sound. 

Non metals are not sonorous. They do not produce ringing sounds when we strike them. 

h. Hardness:

Metals cannot be cut very easily. This property is known as hardness of metal. Only sodium and potassium metals are soft and can be easily cut with a knife. 

Most of the solid non metals are quite soft.

Differences in physical properties of metals and nonmetals

1. Metals: metals are malleable and ductile. 

    Non metals: non metals are neither malleable nor ductile. They are brittle.

2. Metals: metals are good conductors of heat and electricity.

Non metals: nonmetals are poor conductors of heat and electricity except graphite is a good conductor of electricity and diamond is a good conductor of heat.

3. Metals: metals are lustrous. 

Non metals: non metals are not lustrous. They are dull. Only iodine has shiny appearance.

4. Metals: Metals are strong and have a high tension strength. Only sodium and potassium are not strong and have low tensile strength.

Non metals: non metals are not strong. They have a low tensile strength.

5. Metals: metals of sonorous. They make a ringing sound when struck.

Non metals: non metals are not Sonorous. They do not make a ringing sound when struck.

6. Metals: Metals are generally hard. Only sodium and potassium are soft metals.

Non metals: solid nonmetals are quite soft. Only diamond are very hard, in fact it is the hardest material.