Class 8 Science


Flame is the visible and gaseous part of fire. When vapour of a substance undergoes combustion, it results in flame. Only those substances which vaporize on burning produce flame. If a substance does not get vaporized, it does not produce flame.

Table of Contents

Zones of Flame:

There are three zones in flame, viz. outer zone, middle zone and inner zone.

Outer Zone

This zone is blue in colour. It is the hottest part of the flame, because complete combustion takes place in this zone.

Middle Zone

This zone is yellow and orange in colour. This zone is moderately hot because partial combustion takes place in this zone.

zones of flame

Inner Zone

This zone is dark in colour. This is the coolest part of the flame because no combustion takes place in this zone.

Characteristics of a Good Fuel:

There is probably no fuel which can be considered as ideal fuel.

Fuel Efficiency

Ccalorific value gives the measure of fuel efficiency. The amount of heat energy produced on combustion of 1 kg of a fuel is called calorific value of that fuel. It is expressed as kilo Joule per kg (kJ/kg).

Calorific values of fuels
FuelCalorific value (kJ/kg)
Cowdung cake6000 – 8000
Wood17000 – 22000
Coal25000 – 33000
Biogas35000 – 40000

Harms of Burning of Fuel