LPG and Candle wax both contain saturated hydrocarbon, yet LPG gives a clean white flame but a candle gives yellow flame with lots of smoke. Why so?
I have read some answers on web which says that it is due to the insufficient supply of oxygen, so I tried burning candle beside LPG ( so there is no difference in supply of oxygen for both candle and LPG ) , but still candle gives a yellow flame. Why?
LPG is propane, butane or a mixture of both. Paraffin candle wax is $(CH_2)_n$
typically $C_{31}H_{64}$ (other, nonparaffin, waxes are also used in candles, like stearin, beeswax, etc.)
However, LPG is a gas from the start and can mix with air (i.e., oxygen) before it begins to burn, while candle wax has to be heated by the flame before it can begin to burn, so the outer periphery of candle wax vapor (the part of the vaporized candle not yet heated up to ignition) is very small - a dark zone surrounding the wick. The possibility of contact with oxygen is limited to this area.
You will notice a blue zone around the wick also - perhaps obscuring the dark zone closer to the wick. This is a hydrogen-rich flame; under ideal conditions, you can burn off all the hydrogen and leave the carbon (put a glass into the flame: it will get coated with black soot). But if you don't cool the flame with a glass, the carbon which would otherwise deposit will burn yellow.
So, "insufficient supply of oxygen" is an OK answer, but maybe a better one is "scanty mixing of candle wax vapor with oxygen". I suspect that if you boiled candle wax rapidly in a closed container with a vent, and ignited the gases coming out of the vent, they would burn much more like LPG and less yellow.
The colors are dependent not only on line spectra of the atoms, but also on black body radiation from solid carbon. In the candle flame, by the time the carbon get access to oxygen, its temperature is not as high as in a LPG flame, and the possibility of solid carbon particles burning increases the amount of yellowish light emitted. Kind of like charcoal burning, but on a much smaller scale.
I have read some answers on web which says that it is due to the insufficient supply of oxygen, so I tried burning candle beside LPG ( so there is no difference in supply of oxygen for both candle and LPG ) , but still candle gives a yellow flame. Why?
You cannot conduct this experiment by yourself without using a proper apparatus. Otherwise comparing the combustion of a solid wax which is slowly melting by the heat of the flame vs. LPG, is a comparison of apple and oranges.
What matters is the proper fuel to air ratio to determine the color of the flame. In atomic spectroscopy, one uses acetylene $\ce{C2H2}$, a small hydrocarbon. It burns with a beautiful blue flame when acetylene is pre-mixed with a proper ratio of air (i.e., air flow is higher). Sometimes we need a yellow flame, and all that is needed is to reduce the air flow. Most hydrocarbons should burn with a blue flame in an adequate amount of air.
A propane molecule $\ce{C3H8}$ in the gas phase must find $7$ molecules $\ce{O2}$ to burn totally. A wax molecule $\ce{C_{31}H_{64}}$ must find $63$ molecules $\ce{O2}$ to burn completely. During its trip through the flame (having maybe $10 - 20$ cm length), the propane molecule will have enough time to meet $7$ molecules $\ce{O2}$. The wax molecule may not be lucky enough to meet enough molecules $\ce{O2}$ and also simultaneously to get rid of the $32$ $\ce{H2O}$ molecules (and $31~\ce{CO2}$ ) formed in the reaction to burn completely during the trip through the flame. As a consequence, the first atoms of the wax molecule to react with $\ce{O2}$ are the outer atoms, which are mainly Hydrogen atoms. The probability of producing long chains of carbon atoms in the flame between the lower part and the upper part of the flame is high. These chains of carbon atoms are hot but solid and they emit light as every heated solid.
A flame light is emitted by two mechanisms:
You always get the first mechanism. Depending on the conditions, you may or may not have soot in the flame or you may have soot only in some parts of the flame. If you make a flame without soot, it is blue and transparent. Perfect if you want heat and no light. When you have both mechanisms in action, the second one usually gives way more light and you don't see the blue color.
How to make a flame without soot: mix the fuel well with enough air or oxygen before it gets in the flame. That's how a LPG-powered stoves work. It doesn't really matter what type of fuel you use, gasoline or diesel or even wax vapors can burn with a blue flame as well. Most modern internal combustion engines do just that, because light emission is pretty much unwanted in the combustion camera.
How to make a sooty flame: light a jet of a gaseous fuel and let the flame itself mix the fuel with air. It usually doesnt do the best job. In a candle, the jet is the vaporized wax around the wick. The flame forms zones rich in fuel that burns only partially. The unburnt carbon forms soot. Again, it doesn't really matter what type of fuel you use. Propane can burn with a candle-like flame pretty well, just block the air inlets below the stove.
How to form a perfect candle or kerosene lamp flame: you want soot only in a part of the flame, it should burn completely as it advances up in the flame. Too much exposed wick and some soot will leave the flame without burning completely. The flame will smoke. Too less exposed wick and you get too little soot and too little light. In candles, the wick tickness and material is choosen in a way that it burns off at near the right length. In a kerosene lamp, you have a wheel that regulates the exposed wick surface.
How some substances don't make yellow flame when burning (e.g. alcohol): a combination of a fast diffusion, comparatively low need of oxygen for a unit of fuel and a chemical mechanism of burning that doesn't favor free carbon and soot formation. But in specially aranged conditions it is still possible to make alcohol burning somewhat yellow. Other fuels (like, hydrogen gas) simply don't contain carbon at all so no soot there, no matter of the burning conditions.
The color that we see in a flame is due to transitions of electrons between different energy "states" in hot molecules. The nature of the molecule determines the energy states and the temperature determines which highly energetic states are occupied. Which molecules are present in a flame depends on the fuel molecules, the fuel/air ratio, and some other aspects of the environment, as mentioned by others. In many flames, Polycyclic Aromatic Hydrocarbons (PAH, think hydrogenated fragments of graphite or the benzene molecules dehydrogenated and stuck together: multiple planar rings of carbon atoms with hydrogens mostly only on the outer edge) happen to have transitions that produce yellow light. In air, a small molecule like propane is likely to completely burn up before it can produce a PAH. Of course, propane isn't big enough to produce a PAH by itself; they can add to one another to produce them; it is just not likely under the flame conditions considered. Something like a long-chain wax molecule is relatively more likely to react with itself to cyclize and produce a PAH (or benzene-like ring molecules which go on to produce PAH's which go on to produce soot...) as it is 'easy' for a reactive part of the molecule to "bite" some other part of the molecule to produce a ring. While 'smoke' can sometimes be due to condensing water vapor, PAH's go on to produce soot or soot precursors which can be visible as smoke. Yellow flames and smoke are due to PAHs and soot and these are present in candle flames because they are easily formed by large chain molecules like those in waxes.
