A flame is a hot, luminous gas or vapor. Gaseous substances burn with a flame. Those solid and liquid fuels which can vaporize on being heated also burn with a flame.
Depending on the amount of oxygen available for burning the flames can be of two types:
- Non-luminous or blue flame
- Luminous flame
1. Non-luminous or blue flame:
When the supply of oxygen is sufficiently large, the combustion is complete and fuel burns with a blue flame. For example, the flame in a pressure stove. This type of flame does not give much light and is called non-luminous flame.
When the supply the oxygen is insufficient, the combustion is not complete and in the flame some unburnt carbon particles are formed. These carbon particles become hot and glow in flame. As a result, the flame emits yellow light. This type of flame is, therefore, called luminous flame. For example, the flame of a kerosene lamp. In kerosene lamp the fuel does not undergo complete combustion due to the insufficient supply of oxygen.
Now let us study structure of a paraffin wax candle. Paraffin wax consists of a number of hydrocarbons. Light a candle and observe the flame. You can notice three zones of the flame. These are:
- The inner darker zone round the wick.
- A luminous central zone.
- Outer zone which is in the form of envelope and is light blue in color.
Now let us try to understand how these zones are formed. When a candle is lighted, the wax melts, rises up the wick and due to heat gets converted into vapors. These vapors burn in the air producing a flame. The inner dark zone is formed by unburnt vapors of the wax. It is black due to the presence of unburnt vapors of the wax in it. This zone is least hot.
The middle luminous zone contains free carbon particles which are formed due to incomplete combustion of paraffin wax. These carbon particles get heated up and become white hot. This accounts for the luminosity of the flame. The incomplete combustion in this region takes place due to insufficient supply of air. This zones forms major part of the candle flame and is relatively less hot than the outer zone.
The outer zone of the flame is blue in color and contains the products of paraffin combustion, namely water vapor and carbon dioxide. In this region, complete combustion takes place due to plenty of air present around it. This region has the highest temperature in the flame. As the carbon particles from the middle zone move to the outer zone is less luminous and has higher temperature than the middle zone.