Combustion of a wax candle

Candles are usually made of paraffin wax, which is obtained from petroleum When a candle is burnt, following observations can be made -

• The wick burns and it stands in a pool of liquid wax.
• There is a small portion of unburnt wick between the flame and the liquid wax.
• The liquid wax is trapped in a cup of solid wax.
• The liquid or solid wax never catches fire. Also the flame never travels down to burn either liquid wax or the solid wax.

WHAT ACTUALLY HAPPENS IN THE PROCESS OF BURNING OF A CANDLE?

• It is only the wax vapours that burn. Neither liquid wax nor solid wax burn.  When a candle wick is lit, the heat produced from the flame melts the wax. 
• The wick soaks or absorbs the molten wax.
• The heat of the flame vapourizes the molten wax in the wick.
• The gaseous wax burns in the flame, which keeps the flame alive. This process goes on till the entire wax gets burnt.

(a) Zones of a Candle Flame :

The candle flame can be divided into three zones depending upon the amount of air it receives. These zones can be distinguished by their colours.

• The outer zone
• The middle zone
• The inner zone

• The outer zone : The outer zone is the hottest part of the flame. In this zone, the wax vapours have enough oxygen to burn completely and produce carbon dioxide and water. The flame appears blue and the temperature is very high.
• The middle zone : The middle zone is the zone where the wax vapours start burning. They emit a yellowish flame due to glowing of carbon particles, which are formed along with carbon monoxide owing to their partial combustion in insufficient oxygen. The flame is luminous but not very hot.
• The inner zone : The inner zone appears black as it lacks oxygen and hence no combustion takes place. It contains unburned wax vapours and is the coolest region of the candle flame.NCERT solutions for class 8 Science prepared by Physics Wallah will help you to solve your NCERT text book exercise.