1. What Burns Is Vapor, Not Wax
When you light the wick of a candle, the wax around it melts into a liquid. This liquid wax travels up the wick through capillary action, which is when liquid moves up through narrow spaces [1]. As the wax reaches the top, the heat from the flame warms it further until it becomes a gas. The thing that is actually burning is this gas [1]. The solid wax does not burn directly. Instead, the vaporized wax combines with oxygen in the air to create fire [1]. Changes from solid to liquid, and liquid to gas, happen one after another right next to the flame [1].
2. Relighting the Smoke Proves It
When you blow out a candle, white smoke rises. If you bring a match or other flame close to this smoke, the fire can travel down the smoke and relight the wick [2]. This shows that the white smoke after extinguishing still contains components of wax that can burn [2]. This experiment is proof that the burning substance is vaporized wax, not the solid wax itself [2].
3. Three Layers Inside the Flame
If you look closely at a candle flame, you can see roughly three layers: a dark center, an orange area around it, and a nearly invisible outer edge [3]. The center is called the flame core. It is where unburned wax gas gathers. Its temperature is roughly 300 to 400 degrees Celsius (572 to 752 degrees Fahrenheit), the lowest of the three layers [3]. The layer outside that is the inner flame. It is orange and contains soot, which are tiny particles of carbon. Its temperature is about 500 to 600 degrees Celsius (932 to 1112 degrees Fahrenheit), and it is the brightest layer [3]. The outermost layer is the outer flame. Oxygen from the air reaches this area easily, so combustion takes place here, and it looks nearly transparent [3]. The temperature can rise to about 1400 degrees Celsius (2552 degrees Fahrenheit), making it the hottest layer [3]. However, the boundaries and temperatures of these layers change depending on the location in the flame and the conditions [3].
4. Why the Flame Glows Orange
The orange color comes from the soot particles in the inner flame. Soot consists of tiny carbon particles that emit light when they get very hot [3]. This is similar to how iron glows when heated until it is red, though the materials are different [3]. Gas stove flames look blue because the gas and air mix well, and plenty of oxygen is supplied, so almost no soot is created [3]. A candle flame is orange because oxygen does not reach the area around the wick easily, causing incomplete combustion and creating soot [3].
5. How the Wick Stays the Right Length
In the past, candle wicks would grow longer and stick out of the flame. This caused the wick to char black and produce smoke, so people had to cut the wick with scissors [4]. In 1825, a person named Cambaceres in France developed a braided wick [4]. As this wick burns, it naturally bends outward and burns out in the hottest part of the outer flame [4]. Because of this, the wick stays at the right length without needing to be cut [4].
6. Faraday’s Lessons for Children
From 1860 to 1861, the British scientist Michael Faraday gave six lessons about candles to children at the Royal Institution in London [5]. These were part of the Christmas Lectures [5]. The content was published as a book in 1861 titled "The Chemical History of a Candle," and it is still read around the world today [5]. Faraday discussed not just how a candle burns, but also how water and carbon dioxide are produced during burning [5]. He also noted that human breathing is very similar to candle combustion, as both use oxygen to produce carbon dioxide [5]. There is a lot of science in a single candle [5].
7. Safety When Investigating
If you want to try experiments, always do them with an adult in a place where there are no flammable things nearby .