Where does the wax or paraffin go when a candle burns?

Whether it’s to decorate a birthday cake or to produce light during a blackout, candles are still a part of our lives. These paraffin sticks with a wick have the particularity that they are consumed as time goes by until there is not enough wick left to maintain the flame or until practically all the wax has been consumed. This simple observation raises several questions:

• What happens to the candle wax?
• Why does the candle burn until it disappears?
• Where does the candle wax go?

To answer these questions, we must first understand what candles are made of, that is, what actually is candle wax. Later, we will talk about the series of processes, both physical and chemical, that occur when we light and burn a candle.

What is candle wax?

Anyone who has shopped for candles will have noticed that not all candles are created equal. It is not just that they have different colors, as this is usually achieved by adding dyes, but rather that they have different physical and chemical properties. There are some waxes that are harder than others, some that are more translucent and some that are more opaque, and some that are even oilier to the touch than others. This is because not all sails are made from exactly the same material.

For starters, there are candles made from natural waxes such as tallow and beeswax, while others are made from refined waxes derived from petroleum. In both cases, one of the main components consists of one or more solid paraffins.

paraffin candles

The term paraffin is an old name by which alkanes were known, that is, the family of saturated hydrocarbons.

Paraffins present in candle wax are always very long-chain hydrocarbons (with 30 or more carbon atoms), almost always linear (ie, unbranched). For example, a paraffin present in both natural waxes and waxes derived from petroleum is the 31-carbon alkane called hentriacontane, with the molecular formula C ₃₁ H ₆₄ .

Natural wax candles

On the other hand, natural waxes, such as beeswax or animal tallow, in addition to paraffins, also contain a complex mixture of other long-chain organic compounds such as fatty acid esters and even alcohols with more than 20 carbons.

An example of one of these compounds that is present in beeswax is the ester triacontyl hexadecanoate, whose molecular formula is C ₄₆ H ₉₂ O ₂ . This ester is formed by the condensation (or esterification) reaction between hexadecanoic acid (a fatty acid with the formula CH ₃ (CH ₂ ) ₁₄ COOH) and triacontyl alcohol (a linear alcohol with 30 carbon atoms with the formula CH ₃ (CH ₂ ) ₂₉ OH).

In the case of animal tallow, it generally contains large amounts of palmitic and stearic acid esters. However, the particular composition of the wax varies greatly from one animal species to another.

What happens when we light a candle?

Now that we understand what wax is, we are better prepared to understand what happens to these substances when we light a candle. First, we must accept the fact that whatever happens must comply with the law of conservation of matter. In other words, the fact that we observe that the wax is consumed does not mean that the atoms and molecules that compose it are disappearing, but that they are transforming into something that we cannot see with the naked eye.

In general terms, we can say that, when lighting the wick, the heat of the fire that we apply with the flame produces the following changes:

• Phase changes occur as the wax goes from a solid to a liquid, and then to a gas.
• Both complete and incomplete combustion reactions occur depending on the composition of the wax and the conditions under which the combustion takes place.

Next, each of these processes will be described in detail in order to understand where the wax or paraffin in the candle goes when we burn it.

phase changes

When we light a candle, the first thing that happens is that the wick material begins to burn and this heat together with the heat of the flame with which we light it melts the solid wax. We can easily verify this as a small pool of molten wax forms at the top of the candle shortly after lighting it.

The liquid wax then soaks the wick and rises, due to the effect of capillarity, approaching the flame that is produced by the combustion of the wick. As it ascends and approaches the flame, it heats up enough to undergo a second phase change, going from a liquid state to a gaseous state.

complete combustion reactions

Once in a gaseous state, the different substances that make up the wax react with the oxygen in the air through a combustion reaction. If the temperature is high enough and if the oxygen supply is high enough, the reaction that occurs is complete combustion in which the compound is completely oxidized to carbon dioxide and water.

Each component of candle wax has its own particular combustion reaction. However, since paraffin is made up of saturated hydrocarbons, which all have the same general formula (C _n H _2n+2 ), we can write a generic equation for the combustion reaction of the different components of paraffin candles:

where n represents the number of carbon atoms in the paraffin or alkane. The following chemical equation represents an example of one of these complete combustion reactions, in particular, that of the major paraffin present in beeswax and many refined paraffins, hentriacontane.

These are the type of chemical reactions that occur to the different components of paraffin or candle wax when we see that the flame burns intensely, producing an almost white light and without producing any type of smoke. It is especially common in the case of veras made from refined paraffins, since these do not contain other components that burn less easily.

incomplete combustion reactions

When the amount of oxygen in the air is limited, it may happen that the combustion of paraffins and other components of candle wax is not complete. Unlike complete combustion, which is one, incomplete combustion reactions can vary depending on the availability of oxygen.

In some cases, instead of producing carbon dioxide, which is the most oxidized product possible for hydrocarbons and oxygenated organic compounds, carbon monoxide (CO) is produced. The reaction corresponding to the same paraffin above is:

Visually, it is not possible to distinguish between this partial combustion and complete combustion. Thus, both could be occurring at the same time and we would not notice it, since both carbon dioxide and carbon monoxide are colorless gases and the water that is produced in both cases is in a gaseous state, so we cannot see it either. In fact, unless the paraffin is burned in a very oxygen-rich atmosphere, it is common for both reactions to occur at the same time.

However, there is another incomplete combustion reaction that we can see with the naked eye. This is the one where smoke is produced. Among other things, the smoke contains carbon in the form of graphite. We can see smoke because it is made up of very small solid particles. It is not a gas at all. For this reason, when we can see a fine stream of black smoke emanating from the tip of the flame, we can be sure that incomplete combustion is taking place.

Even in those cases where a stream of smoke cannot be clearly seen, incomplete combustion is clearly manifested by black smudges on the surface of any object placed above the flame.

Conclusion

At this point we can already answer the question of where the wax goes when a candle burns. Once the combustion has started, the paraffin and the other components of the wax burn with the oxygen in the air to transform themselves into carbon dioxide, carbon monoxide, carbon or other products of incomplete combustion, as well as water vapor. . The first two products, like water vapor, are gases and disperse in the atmosphere.

On the other hand, the part of the candle wax that is transformed into elemental carbon or some other solid product of incomplete combustion initially rises, carried by the currents of hot air coming from the flame, but when it cools down, it ends up falling again. and depositing on the first surface they come across, since all these products are much denser than air.

It should be noted that a part of the paraffin can also be lost in the form of vapor that does not undergo any combustion reaction and, when cooled, this vapor quickly condenses, also depositing on any surface it finds. This is particularly noticeable at the moment when the flame goes out.

Just after the combustion reaction has stopped, the remaining heat from it continues to evaporate part of the paraffin, which rises as a vapor and quickly condenses to produce a slight white mist visible to the naked eye. This small stream of paraffin can be easily ignited with a match or lighter from a few inches above the wick, and the flame will travel downward to light the candle again, just like magic.

References

Carey, F. (2021). Organic Chemistry (9th ^ed .). MCGRAW HILL EDUCATION.

Chang, R. (2021). Chemistry (11th ^ed .). MCGRAW HILL EDUCATION.

del Fresno, JS (2016, September 27). OF WAXES AND CANDLES, A CHEMICAL VISION . Science in Common. https://cienciaencomun.wordpress.com/2016/03/14/quimica-ceras/

Parra, S. (2017, March 8). Where does all the wax from a burning candle end up? Xataka Science. https://www.xatakaciencia.com/sabias-que/donde-va-a-parar-toda-la-cera-de-una-vela-que-arde