Question

The blue zone in a LPG flame indicates
(A) Unburnt vapours
(B) Partial decomposition
(C) Moderately hot
(D) Hottest and complete combustion

Answer 1

Hint :First we must understand that flame colour depends mainly on three factors such as the nature of the substance that undergoes combustion, amount of oxygen and also the temperature. There are two types of flames which are non-luminous flame and luminous flame. A flame has three different zones which has different temperatures and a different colour.

Complete Step By Step Answer:
As we discussed above a flame has three different zones depicting different temperatures and colors. The three zones are;
-The first zone is called the Outer zone
-The second zone is called the Middle zone
-The third zone is called as Inner zone
Flame color determines the rate of combustion and thus there is a difference in temperature thereby producing a different color flame. In LPG the blue zone represents the zone of complete combustion of the fuel. So, the amount of soot decreases with increasing oxygen supply and the temperature is highest as high heat is released. This zone is said to be the hottest in temperature when compared to the other zones. This blue coloured zone is the non-luminous part of the flame. The reaction produces enough energy to excite and ionize gas molecules in the flame, leading to a blue appearance.
Hence we can say that option D is correct which states that the blue zone in a LPG flame indicates hottest and complete combustion.

Note :
The yellow colour region which is the bright part of the flame represents the middle zone. It is moderately hot and partial combustion of fuel takes place here. The least hot region of the flame is present at the innermost region. This innermost zone is black due to the presence of unburnt wax vapours.