The cone shape of a candle flame is due to the combustion of the mixture of gas and soot particles. The heat rises and creates an upward air current, drawing in fresh air from below, thus forming this characteristic cone.
When you look at a candle flame, you can observe several distinct areas. Close to the wick is a small very dark area called the dark zone: here, it is mostly vaporized wax, and combustion has not really started yet. Surrounding this part is the blue zone, which is a bit difficult to see, where combustion has truly begun, reaching fairly high temperatures. Finally, there is the typical yellow-orange outer part, the luminous zone, where the incomplete combustion of hot gases occurs and carbon particles glow as they heat up: this is what lights up your room when you light a candle. These three parts stack together to form the famous characteristic cone of the flame.
When a candle burns, the wax melts, becomes liquid, and then rises by capillarity in the wick. Up there, it vaporizes due to the heat. These vapors, once mixed with the oxygen in the air, trigger combustion, releasing heat and light. This heat creates an upward movement: the warm air expands, becomes less dense, and naturally rises. This upward flow is called a convection current. It pulls the gases produced by combustion upward, forming the sharp and conical tip of the flame. The wider base corresponds to the cooler area where fresh air constantly arrives from all sides to feed the combustion.
The conical shape of a flame exists largely because warm air rises while cold, heavier air descends: this is known as convection. The heat produced by the flame warms the surrounding air, making it less dense. This warm, lighter air quickly rises, carrying combustion products with it. At the same time, fresh, oxygen-rich air comes in to replace the warm air from the sides and below. This continuous movement gives rise to a flame that is narrower at the top and wider at the base, in short, a classic conical shape. Without gravity (like in weightlessness), there is no convection, and flames become almost spherical!
The chemical composition of the candle plays a significant role in the conical shape of the flame. A flame is primarily made up of carbon and hydrogen in the wax that burn thanks to the oxygen from the air. When the wax heats up and becomes gaseous, it rises to the top of the flame. There, it encounters more oxygen, which intensifies the combustion and sharpens the tip of the flame upward. The exact amount of carbon present in the wax also slightly alters the shape: the more carbon there is, the more the flame will tend to produce solid particles, called soot. This makes the flame more yellow and bright, but it can also affect its overall shape, making it longer and sharper. Conversely, with a fuel containing less carbon, like alcohol, the combustion produces a flame that is more blue, often shorter and less pointed.
Blowing out a candle works by dispersing the flammable wax vapor and rapidly cooling the wick, thereby effectively stopping the combustion reaction.
The absence of gravity radically changes the shape of a candle flame: in space, it becomes spherical instead of conical, due to the lack of upward convection currents.
The yellow-orange color that we see in a flame primarily comes from soot particles heated to high temperatures that emit visible light.
A complete combustion, such as that of a well-regulated gas torch, produces a blue flame, which is evidence of complete and efficient fuel combustion, with no significant emission of carbon particles.
Yes, the chemical composition of the fuel (paraffin, beeswax, vegetable wax, etc.) as well as the type of wick affect the combustion temperature, the amount of carbon particles released, and therefore can slightly alter the shape, height, and color of the flame.
Each color corresponds to a different temperature and a specific area of combustion: the blue part is usually the hottest, indicating complete combustion, while the yellow-orange comes from carbon particles heated to a lower temperature, resulting in partial combustion.
Sure! Here’s the translation: "Yes, absolutely. In the absence of gravity, natural convection disappears, the flame becomes spherical, and may even become invisible, as the hot gases no longer rise but remain around the wick, burning slowly only by molecular diffusion."
Even when the air seems still, there are often very slight air currents generated by tiny differences in temperature in the room. These variations are enough to slightly disturb the flame, causing it to sway or flicker gently.
The flame always points upwards due to the convection phenomenon, where the warm, less dense air rises and creates an elongated shape vertically. This upward movement gives the flame its characteristic cone shape that tapers towards the top.
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