How does a candle burn?

 

    A candle is one of the artificial lights made by humans. Before electricity was found oil lamps and tallow candles (candles made from animal fat) were used to light up the houses at the night. After petroleum was found, candle manufacturing becomes revolutionized. Those candles were made up of wax. Waxes are one of the products that were extracted from the different compositions of crude oil.  The candle made up of wax mostly contains hydrocarbons. Hydrocarbons are the combination of hydrogen and carbon atoms. The working of the candle is an impressive operation. Let’s look into it.

    A candle mainly consists of two parts; one is the wax which is the long solid part and the other one is the wick which is the thick thread-like structure placed inside the wax. The wick runs all along inside the wax stick and the little is made to sprout out at the top. The candle works by heating up the wax and combining it with the oxidizer (oxygen), to give fire. The main principle of this operation is capillary action.

    Capillary action is a type of action in which a liquid is drawn through a channel against gravity. For example, let’s take two beakers with water filled in one of those beakers. Let’s add some color agents in the beaker that contains water, for visibility. Take a thick thread and place it inside those beakers as shown in the figure. Leave it for some time. After some time you can notice that some amount of water is moved from one beaker to another beaker. This movement of liquid through the thread is called capillary action.

Capillary action

    Thus when we light up the candle, the heat melts up the wax near the wick and converts it into liquid. This liquid is then drawn up by the wick by the capillary principle and made as fuel to the fire. Again due to the heat of the fire, the wax again melts from the solid and is drawn up and made as a fuel. This cycle continues. But even though the wick burns by drawing up the liquid wax, the wick does not burn and fall off immediately because the liquid wax which is drawn up; cools the wick and makes it to hold the fire continuously.  This is similar to the water, boiling in a plastic bottle. Even though plastic bottles melt in the fire, the cool water in it makes it to escape from the fire. But as the wax melts continuously the height of the wick is also reduced since the liquid wax is drawn up only to a certain height.  So the remaining wick is burnt off in the fire.

Working principle of a candle

    So by using the capillary action, the liquid is drawn up and by combing with the oxidizer and the extra heat, the hydrocarbon atoms in the wax get split up, as hydrogen and carbon atoms. The hydrogen atoms combine with the oxygen and heat to give fire. This fire again melts the wax and burn. But there is a hunch. If the wax is the main fuel which produces the fire, then why don’t the wax burns all along completely similar to other fuel like petrol or coal ?.  Here, the important thing that to be considered is, a fuel needs to be combined with the air (oxidizer) to get fired. Petrol and coal can easily get combined with the air as the molecules or loosely bounded to them. Whereas diesel has a higher density and those molecules' bond strengths are higher. So the diesel cannot mix readily with air. So it cannot get fired. But if we spray the diesel, we break the atomic molecules which readily mix with air and get fired. In the situation of candles, we cannot spray the wax, where the wick serves the purpose. The heat melts the wax which is drawn up by the wick due to the capillary action. During this draw, the atomic molecules are broken down and mixed with the air to get fired. So some fuels will get fired, only if it is broken down into smaller particles to get mixed with the air. Also, to split the hydrocarbon atoms into hydrogen and carbon atoms it must be heated super hot. So the wax must be getting into the flame to get heated that much.

    It can be observed, that a candle flame glows in different colors. This is due to the different temperature ratings over the flame. These different ratings are categorized as different zones in the flame. At the inner part of the flame, there will be a presence of a darker region. As this region does not comes in contact with the air, it leads to non-combustion leaving a darker region. The temperature here will be very low. At the bottom level, and at the outer regions of the flame, more oxygen comes in contact with the flame which makes it to glow in blue color.  At the outer regions of the flame, the temperature will be very hot and the blue color will be non-luminous. In the middle part of the flame, there will be lesser oxygen coming in contact with the flame. So the flame gets converted into yellow color, as there will be incomplete combustion of the fuel. This makes it to have a moderate temperature. Also, the burning of carbon atoms produced during hydrocarbon separation produces a yellow color. As the warm air rises off less density, the flame always glows upwards and not downwards.

Different layers of a candlelight

    When a candle burns we can hear a popping or crackling sound. This is due to the moisture or any dampness present in the wick. Also when we blew the candle the fire get’s extinguished. This is due to the dilution process. When we blew, we are blowing the hot gases out. Thus the flame get’s weaker as the fuel gases went out.