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

CLASS VIII   |    SCIENCE    |    CHAPTER 4

     Notes prepared by Subhankar Karmakar 

---

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.