Deposition of unburnt carbon particles.
Explanation
A candle flame is divided into three distinct zones based on the availability of oxygen and the extent of combustion: the innermost zone (dark zone), the middle zone (luminous zone), and the outer zone (non-luminous zone). The chemical process involves the combustion of hydrocarbons (wax).
Detailed Analysis:
- Luminous Zone (Middle Zone): In this zone, the supply of oxygen is limited. Consequently, the wax vapours undergo incomplete combustion.
- Formation of Carbon Particles: Due to incomplete combustion, the hydrocarbons break down into minute carbon particles. These particles heat up and glow, giving the flame its characteristic yellow light.
- Deposition on Glass Plate: When a glass plate is introduced into this zone, these hot, unburnt carbon particles come into contact with the cooler surface of the glass. They condense and deposit, forming a circular blackish ring known as soot.
- Comparison with Other Options:
- Unburnt wax vapours are primarily found in the innermost dark zone (closest to the wick). If deposited, they would appear as a whitish film, not a black ring.
- Carbon monoxide is a colourless gas and would not leave a visible black mark.
Key Takeaway:
The presence of a blackish ring on a glass slide placed in the luminous zone of a flame serves as physical evidence of incomplete combustion, resulting in the deposition of unburnt carbon particles (soot).