|Why does a candle burn?|
|Question Date: 2020-01-14|
The wick of the candle is very adsorbent and is properly named, as its function is to “wick” liquid wax from its base to the tip. The wax of a candle is a hydrocarbon, meaning it contains large amount of the elements hydrogen and carbon. When hydrocarbons are exposed to oxygen (readily available in the air) and heat, they burn. Lighting the candle with a match, lighter, etc., provides the initial heat source, then the burning process releases enough heat as a byproduct to keep the reaction going- as long as there is enough fuel (wax in this case). Solid wax does not readily burn however- this is where the wick comes into play.
The heat from the flame melts the wax at the base of the wick. This liquid wax then travels up the wick into the flame, where it turns to vapor, and this vaporized was is what burns- releasing light and more heat to continue the cycle until all the wax is gone.
As explained here on Scienceline, burning of a candle is due to the reaction of the paraffin candle wax with oxygen in a combustion chemical reaction.
To expand on this answer, the reason that candle wax (or any other substance) combusts (i.e., burns*) is because the chemical reactions produce a rearrangement of electrons which lowers the total energy of the system. An indication that energy is lowered by burning the candle by the heat and light emitted.
A related question given that candles do burn is why they spend much of their time not burning. That is, why do candles need to be lit rather than starting on fire on their own? Essentially, the molecules in their initial states (separate wax molecules and oxygen) are neutral and relatively stable. To start the chemical reaction, some chemical bonds must be broken so that new bonds can be formed. In chemistry terms, this means that there is an activation energy which must be overcome to initiate the reaction. Breaking those bonds requires an input of energy, such as the heat from a lit match. Once the candle begins burning, the heat released by the reaction of the wax and oxygen is sufficient to start the reactions with nearby wax molecules.
*Using the term "burn" to describe the reaction typically implies that one of the reactants is molecular oxygen, O2.
A burning candle means that the wick is on fire and the wax is melting.
For that fire to exist, there needs to be three things: oxygen, fuel, and heat. This is called the fire triangle.
We can get oxygen from the air (our atmosphere is about 21% oxygen!), fuel from the candle itself (both the cotton wick and the wax act as fuel), and heat from a flame that already exists like a lit match or a lighter. Alternatively, we can get heat from different things as well, such as heating up the air around the candle until it reaches the candle's auto-ignition point. The auto-ignition point is the temperature that something can catch fire without being exposed to flame. That's usually not possible for the average person without special equipment, so we won't bother with that.
When we bring our heat source like a match to the candle, this heats the wick to its combustion point. The flame melts the wax around the wick and draws it up through capillary action. Capillary action is the ability for a liquid to flow into narrow spaces with or without (or even against!) the help of other forces like gravity. The flame turns the liquid wax into a hot gas and breaks down its molecules into water and carbon dioxide. The breakdown continually releases energy in the form of heat and light.
As the candle's flame keeps radiating heat, it continues to melt more wax and burn more of the wick, until it is gone.
The burning of a candle is a chemical reaction between the wick and the ambient air. When we light a candle, we heat the wick up with a flame that initiates a combustion reaction on the wick, what we commonly refer to as fire. That reaction requires something to burn (the wick), oxygen (in the air), and heat to initiate. When the heat provides enough energy to the wick, the wick wants to react with the oxygen to form carbon dioxide and water molecules, releasing energy in the process in the form of a flame. A candle, however, is more than just wick. The wax is there to slow down the burning of the wick and provide a rigid structure that prevents the flame from falling and spreading fire. The wax is more resilient to the flame so it doesn't catch fire; instead, it melts.
Tavin great question! There are two parts of a candle, the wick and wax. First the wick, made of rope or twine, is lit. The heat from flames melts the wax. As in paper towels soaking up water, the wick pulls melted wax upward. The flame turns the liquid wax to vapor, which burns. The wax also cools the wick so the wick burns slowly.
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