Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A candle flame consists of three distinct zones: the outermost zone, the middle zone, and the innermost zone.
The Outermost Zone is the zone of complete combustion. It is blue in color and is the hottest part of the flame because it has an ample supply of oxygen ().
The Middle Zone is the zone of incomplete combustion. It is luminous and yellow in color due to the presence of glowing unburnt carbon () particles. It is moderately hot.
The Innermost Zone is the zone of no combustion. It contains unburnt wax vapors and appears black. It is the least hot part of the flame.
The chemical process of combustion in a candle involves the reaction of wax (hydrocarbons) with oxygen to produce carbon dioxide (), water vapor (), heat, and light.
Luminosity in the middle zone is caused by the heating of unburnt carbon particles until they glow, which can be represented by the incomplete combustion reaction: .
📐Formulae
💡Examples
Problem 1:
If you hold a glass slide in the middle (luminous) zone of a candle flame for a few seconds, what do you observe and why?
Solution:
A circular blackish ring of soot is formed on the glass slide.
Explanation:
The middle zone is the zone of incomplete combustion. The blackish ring is caused by the deposition of unburnt carbon () particles present in this zone. These particles are produced because there is insufficient oxygen () for total combustion.
Problem 2:
Why do goldsmiths use the outermost zone of a flame for melting gold and silver?
Solution:
They use a metallic blowpipe to direct the outermost zone of the flame onto the metal.
Explanation:
The outermost zone is the hottest part of the flame where complete combustion occurs. Since gold has a high melting point of approximately , the high temperature of the blue zone (which can exceed ) is necessary to melt the metal efficiently.