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The science of candles

The science of candles

Candles have been used for lighting since ancient times.

Physics

Keywords

candle, combustion, burning with flame, plasma, combustion product, thermodynamics, fast, wick, wax, flame front, flame, heat release, oxygen, hydrogen, coal, carbon dioxide, water vapour, reaction, physics

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Scenes

Candle

Structure

  • solid wax - It is solid at room temperature and only burns when vaporised. It is melted by the flame.
  • molten wax - It is kept in place by the rim of solid wax. It moves upwards through the wick via capillary action.
  • wick - Molten wax moves upwards through it, via capillary action. The wax then evaporates due to the heat and burns.

Wick

  • molten wax - It is kept in place by the rim of solid wax. It moves upwards through the wick via capillary action.
  • wick - Molten wax moves upwards through it, via capillary action. The wax then evaporates due to the heat and burns.
  • flame front - The surface of the flame. Combustion takes place around it, that is, soot particles created in the centre of the flame react with oxygen here and produce CO₂ and water.
  • inner part of the flame - Oxygen cannot enter this region of the flame, as it is used up on the flame front. However, the heat generated by the combustion enters this region.

How candles burn

  • heat release
  • oxygen
  • evaporating wax - The resulting decomposition products (hydrogen, carbon) in the flame of the candle.
  • hydrogen
  • carbon
  • carbon dioxide
  • water vapour

Animation

  • solid wax - It is solid at room temperature and only burns when vaporised. It is melted by the flame.
  • molten wax - It is kept in place by the rim of solid wax. It moves upwards through the wick via capillary action.
  • wick - Molten wax moves upwards through it, via capillary action. The wax then evaporates due to the heat and burns.
  • molten wax - It is kept in place by the rim of solid wax. It moves upwards through the wick via capillary action.
  • wick - Molten wax moves upwards through it, via capillary action. The wax then evaporates due to the heat and burns.
  • flame front - The surface of the flame. Combustion takes place around it, that is, soot particles created in the centre of the flame react with oxygen here and produce CO₂ and water.
  • inner part of the flame - Oxygen cannot enter this region of the flame, as it is used up on the flame front. However, the heat generated by the combustion enters this region.
  • heat release
  • oxygen
  • evaporating wax - The resulting decomposition products (hydrogen, carbon) in the flame of the candle.
  • hydrogen
  • carbon
  • carbon dioxide
  • water vapour

Narration

Candles have been used since ancient times. Although the importance of candles has decreased in the developed world, they can still be found in many households. In the past, candles were made either of beeswax or tallow, while today they are usually made of paraffin wax, which does not burn when solid.

When we light a candle, the external heat source starts the process by melting and burning a small amount of wax. When a candle burns, the wax near the wick melts. Molten wax is kept in place by the rim of solid wax.

The wick absorbs the molten wax by capillary action. Then the molten wax vaporises in the wick due to the heat and burns away. As molten wax is continuously consumed, the flame moves downwards and melts new layers of wax.

The surface of the flame is called the flame front. Combustion takes place around it, which means that it is here that soot particles formed inside the flame react with the oxygen in the air, producing water and carbon dioxide. Oxygen does not reach the inner part of the flame, as it is used up in the flame front during combustion; however, the heat generated by combustion does manage to reach it.

The wax evaporating from the wick decomposes due to the heat. Then decomposition products generate soot particles which burn; this is what causes the flame’s yellow colour. The soot particles pass through the flame front, reach the oxygen-rich gas and burn. If the wick releases too much wax into the flame, unburned soot particles are released into the air, producing a sooty flame.

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