A candle gives a yellow flame while kerosene gives a blue flame on burning. Now, saturated hydrocarbons give a blue flame but unsaturated ones give a yellow flame. I want to know why the colour of the flame is yellow and blue respectively?


It depends on the amount of oxygen available and on the ratio of hydrogen to carbon in the fuel.

Hydrogen burns with an almost invisible flame no matter how much oxygen is present, but solid carbon burns with a yellowish (black-body) flame. If the fuel combusts in a multi-step process where all reactants are gaseous, you get a bluish flame. For example, the combustion of methane produces methanol, then formaldehyde, formic acid, carbon monoxide etc.

The combustion of acetylene at lower temperature, with little oxygen, however, starts with cracking of the C-C triple bond, releasing carbon particles that have a luminous flame.

So the greater the hydrogen:carbon ratio, the easier it is to get a blue flame.


The colour of the flame does not necessarily have to do with the type of hydrocarbon (saturated or unsaturated). In most introductory lab courses and even at many schools, students learn how to use a Bunsen burner (or related burners) in one of the first practical exercises. These burners burn natural gas which is mostly methane and have inlets at the bottom to allow the addition of air. If the flame is lit when the inlets are closed, the flame is yellow but if the inlets are open and air is allowed to pass in, the flame becomes bluish and transparent. (Safety instructions always remind you to light the flame first and open the inlets afterwards; when putting out the flame, close the inlets first and then extinguish.)

Methane, the principal component of natural gas, is saturated; it only has one carbon atom thus it cannot be unsaturated. Yet we still get both flame colours. As you may have noticed from my description, the difference is the amount of air that is allowed to mix with the gas. And since we are dealing with combustion, the key difference boils down to oxygen content.

The bluish flame tells us that complete combustion is taking place. The actual mechanism is not important at this point, but ‘complete combustion’ means that the only products are carbon dioxide and water vapour. Thus, we can easily write a reaction equation as follows: $$\ce{CH4 + 2 O2 -> 2 H2O + CO2}\tag{1}$$ The difference between the two flames (mixing in air or not) tells us that additional air is the key to complete combustion. Probably, it is necessary for methane and oxygen to be well-mixed. Complete combustion also means that the reaction is most exothermic, i.e. this type of flame reaches the highest temperature because all possible half-reactions are strongly exothermic, and all atoms undergo these complete half-reactions.

What happens if we reduce the air content? Suddenly, the reaction becomes a lot less defined. In fact, practically anything can happen. Yet we have a strong indication that one of the products in a non-neglegible amount is elementary carbon, because it is that that gives the flame its yellow colour. We will still be burning some methane completely to carbon dioxide and we could have some incomplete burning to carbon monoxide or other compounds. Equation $(2)$ is one of the may possible equations and a particularly simplified one. $$\ce{2CH4 + 3O2 -> 4 H2O + CO2 + C}\tag{2}$$ This is the key difference between blue and yellow flames; whether it is a candle or kerosene or anything. In some cases (e.g. candle) we actually want incomplete combustion to occur because without it there would be hardly any light. In other cases (e.g. heating) we want complete combustion to occur because it liberates more energy and less harmful products.

Note that methane is not the only compound that can be burnt in two different ways by modifying the oxygen content. Acetylene is another. If it is simply burnt normally like in a Bunsen burner, the flame will be yellow and a lot of soot (i.e. elementary carbon) will be produced. But if oxygen gas is added, the flame will become white-hot (not bluish because of the heat) and can be used for welding.

  • $\begingroup$ Hmmm... But why does complete combustion give a blue flame? $\endgroup$ Jan 8 '18 at 9:11

why do you not answer his *actual question! about colour * - yellow is due to hot carbon-as-amorphous-graphite ( Cx ) while the blue-green colour is due to hot C-H ( carbogen) blue-.... and hot C2 (carbon dimer)-green ______ref >http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=4695221&fileOId=4695231<

2.2.3 CH Emission The excited carbogen (CH∗) emits radiation in a narrow band at around 431 nm, in theindigo region. CH∗ is formed by the following reaction mechanism [14]: C2 + OH → CO + CH∗ (R3) O + C2H → CH∗ + CO (R4) O2 + C2H → CH∗ + CO2 (R5) CH∗ is usually used as an indicator of temperature in ame studies. It is a radical thathighly depends on the temperature thus it has a strong presence at slightly rich flames(φ ' 1.1) [15], where the flame reaches the highest temperature. 2.2.4 C2 Emission The excited diatomic carbon (C*2) is a radical that is not present at lean flame conditions. It is detected at equivalence ratios Φ ≥ 1.0. The reaction mechanism that describes the formation of C∗2 is given bellow [14]: CH2 + C → C∗2 + H2 (R6)

C∗2 :emits in a narrow band at around 513 nm, in the cyan/green region. Another-characteristic of this radical is its strong second side band at around 465 nm (blue region).


Not the answer you're looking for? Browse other questions tagged or ask your own question.