NCERT science class 8- metals and non metals, chapter-4, notes

METALS AND NON-METALS CHAPTER-4 CLASS-8  

NCERT science class 8- metals and non-metals, chapter-4, notes

We are familiar with a variety of materials which are used in our daily life. In lower grades, we have classified materials based on its appearance, hardness, transparency and texture.

In this session, we will deal with a major classification of materials. 
Based on its physical and chemical properties we can classify materials like metals and non-metals.

Out of 118 elements discovered, the majority of them are metals and these elements are arranged in the periodic table to make our studies easier. Gold, silver, iron, copper and aluminium are some of the popular metals.

copper metal

PHYSICAL PROPERTIES OF METALS

• Metals are generally hard. But there are few soft metals like sodium and potassium which can be cut with a knife.
• Metals are generally solid. But mercury is a liquid metal at room temperature.
• Metals are lustrous- having a shining surface.
• Metals are malleable- It can be beaten into thin sheets. We are using aluminium foil and silver foil as thin sheets. 

• Malleability-It is the property of metals by which it can be beaten into thin sheets.
• Metals are ductile- It can be drawn into wires. Aluminium and copper wires are normally used in electrical cables and wires.           Ductility- It is the property of metals by which it can be drawn into wires.
• Metals are good conductors of heat and electricity.
• Metals are sonorous- makes a ringing sound when struck hard.
• Melting point and boiling points of metals are generally high.

aluminium foil

conductors of electricity

PHYSICAL PROPERTIES OF NON-METALS

• Non-metals are not hard and they exist as solid, liquid and gas. Solid- carbon, sulphur, phosphorus, liquid- bromine, gas- hydrogen, oxygen, nitrogen
• They are not malleable.
• They are not ductile.
• They are poor conductors of electricity and heat.
• They are generally dull except iodine.
• They are non- sonorous.
• Non- metals have a comparatively low boiling point and melting point.

CHEMICAL PROPERTIES OF METALS 

1.Reaction of metals with oxygen

Metals will react with oxygen to form their corresponding oxides.

Metal + Oxygen → Metal oxide

Magnesium burns with oxygen to form Magnesium oxide (white powder)

On dissolving magnesium oxide with water, it forms magnesium hydroxide which turns red litmus paper to blue.

Mg + O2 → MgO

MgO + H2O → Mg(OH)2

Similarly iron reacts with oxygen and water in the air to form an iron oxide which we commonly call as rust.

Fe + O2 + H2O → Fe2O3 .xH2O

RUSTED GATE

Copper reacts with air to form basic copper carbonate which is commonly called as verdigris(green coating).

Cu + O2 + H2O + CO2 → Cu(OH)2 + CuCO3

VERDIGRIS GREEN COATING

Metal oxides formed are generally basic in nature.

2. Reaction of metals with water

Different metals are reacting with water in different manners. Some react very slowly and some of them do not react at all. But metals like sodium and potassium react very violently with water. Hence they are stored in kerosene to prevent its reaction with water and other components in the air. 

Iron reacts very slowly with water. Noble metals like gold and silver do not react with water.

3. Reaction  of metals with acids

Metals react with acids to form metal salts and hydrogen.

Mg + HCl → MgCl2 + H2

The presence of hydrogen can be tested using a burning match stick. It burns with a pop sound when bought near the mouth of the test tube.
The reaction condition is different for different metals. Some react at room temperature, but some of them will react when warmed.

Copper reacts with sulphuric acid, but it does not react with hydrochloric acid even on heating.
Less reactive metals like silver and gold do not react with dilute acids.

4. Reaction with bases

Metals react with bases like sodium hydroxide to form hydrogen.

On dropping a strip of aluminium into sodium hydroxide solution, hydrogen gas is evolved which makes a pop sound on a burning match stick.

5. Displacement reaction

A more reactive metal can displace a less reactive metal from its compound in aqueous solution.

On placing an iron nail in blue coloured copper sulphate solution, it forms a green coloured iron sulphate solution and brown deposits of copper. Here iron being more reactive can replace copper from copper sulphate.

Fe + CuSO4 → FeSO4 + Cu

But copper is not able to replace iron as it is less reactive than iron.

copper sulphate

iron sulphate

Displacement reaction happens based on the reactivity series. It is the arrangement of metals in a vertical column in order of decreasing reactivity.

Reactivity series:

Potassium     (most reactive)

Sodium

Calcium

Magnesium

Aluminium 

Zinc

Iron 

Lead

Hydrogen

Copper

Mercury

Silver

Gold  (least reactive)

(mnemonics to learn reactivity series- please stop calling me a zebra, I like her calling me a smart goat- the first letter of metals)

CHEMICAL PROPERTIES OF NON-METALS

1. Reaction with oxygen

Certain non- metals with react with oxygen to form oxides and on dissolving with water, it forms acids. Hence non-metal oxides are acidic in nature.

For example, sulphur on burning with oxygen forms sulphur dioxide(SO2).On reaction with water, it forms sulphurous acid(H2SO3).

S + O2 → SO2

SO2 + H2O → H2SO3

2. Reaction with water

Non- metals are not reactive with water. Hence non-metals like phosphorus is stored in water to avoid the reaction with oxygen in the air. It is a very reactive non-metal and catches fire on exposure to air.

3.Reaction with acids

Non-metals are not reactive with acids generally.

4.Reaction with bases

The reaction of non-metals with bases are normally complex.

USES OF METALS

automobile

gate

rails

utensils

USES OF NON-METALS

fertilizer

for breathing

tincture iodine antiseptic