Boom Advent #6 – Reaction of the Week

The Christmas pudding is the showpiece of the Christmas dinner. Some people love it, some people hate, but we are familiar with its shape, appearance and taste. We are also probably used to watching it being set on fire. Turn the lights off, strike a match and watch the wonderful mysterious blue aura dance around the pudding. It’s a Christmas themed reaction for this week – Combustion!

christmas pudding on fire with combustion of ethanol

What you are witnessing is a type of reaction known as combustion. It is not the pudding itself that is burning, but the ethanol inside the alcoholic spirit (usually brandy).

christmas pudding combustion flames
Please do be careful doing this at home. Always get a responsible adult to do it (it took us a while to find one last Christmas Day…)
Video version

So what is combustion? Why is the flame of burning alcohol blue, but the flame of a candle yellow?

Oh Oxygen

During a combustion reaction a substance reacts with oxygen in the air [1].

Combustion of elements such as magnesium or carbon produces molecules known as oxides. So, if you take a piece of magnesium and burn it, you produce a white powder of magnesium oxide. The reaction also produces a brilliant white flame.

The combustion of sulfur produces sulfur dioxide. You may remember this reaction as the first process in producing sulfuric acid.

How about carbon? Well carbon can react with oxygen to produce to different oxides: carbon dioxide or carbon monoxide. However, which molecule you get depends on the amount of oxygen available. This is a very important point!

Don’t panic, it’s organic

It’s not just carbon. Lots of organic molecules can be combusted. Alkanes and alkenes are molecules made of only hydrogen and carbon. You might have heard of some alkanes such as methane, butane or propane. Combustion of these molecules produces carbon dioxide and water.

Alcohols are also organic molecules. The most famous alcohol is ethanol, and this is the molecule found in alcoholic drinks such as beer, wine or spirits.

Alcohols contain hydrogen, carbon and oxygen. The combustion of alcohols also produces carbon dioxide and water. So that brandy burning over your Christmas pudding is producing carbon dioxide and water.

Complete or Incomplete Combustion?

When there is plenty of oxygen available, is it likely that complete combustion will occur. Complete combustion produces water and carbon dioxide. However, when there is not enough oxygen present, incomplete combustion occurs. Incomplete combustion is a big problem.

Incomplete combustion of molecules like butane or ethanol produces carbon monoxide or even just plain carbon. Water is still produced.

The chemical equation for the incomplete combustion of ethanol is:

C2H5OH + 2O2 → 2CO + 3H2O

an illustrated reaction for incomplete combustion with yellow flames

While a chemical equation for the complete combustion of ethanol is:

C2H5OH + 3O2 → 2CO2 + 3H2O

an illustrated reaction for complete comubstion with blue flames

And these flames can look really impressive – especially in the dark and if you get the alcohol to light really quickly. This means spreading the alcohol out as much as possible so that it can mix with more air – like turning it into a vapour! We did that in the XPeriMas video ‘Whoosh and a Cloud’:

A blue flame in a bottle
Lighting alcohol vapour inside a bottle – from X-Peri-Mas #14

Silent, but deadly

Carbon monoxide is an extremely dangerous gas to our health. Every year around 60 people are killed by carbon monoxide poisoning in England and Wales. Breathing in carbon monoxide allows it to enter our bloodstream. Once there, it can interact with the haemoglobin in our red blood cells. You may remember haemoglobin from our Halloween molecule of the month. Once carbon monoxide has mixed with the haemoglobin, is stops being able to carry oxygen around our bodies. This is not good. Lack of oxygen can lead to all kinds of health problems and eventually seizures, comas and death.

The other problem with carbon monoxide is that it is clear, colourless and odourless. This makes it very difficult to detect and why you should have a carbon monoxide alarm in your house .

Through the fire and flames

It is common for combustion to result in fire and therefore flames. The colour of the flame can tell us some interesting information. Many metals will produce coloured flames. For example lithium will produce a fantastic red colour, potassium a pink/purple colour and barium a pale green colour.

The colour of the flame can also give some indication about whether complete or incomplete combustion is happening. Usually, a blue flame indicates plenty of oxygen and complete combustion.

A yellow flame usually results from a reduced amount of oxygen and therefore incomplete combustion.

Bunsen Burner Learner

You may have noticed this with a Bunsen burner. Bunsen burners use a mixture of natural gas containing mostly methane. At the bottom of the burner is normally a moveable piece that can either open or close an air hole.

When the hole is fully open, the Bunsen burner roars and a much hotter blue flame is produced. The hole being open allows more oxygen into the burner and results in complete combustion. However, when the hole is closed, there is less oxygen present and incomplete combustion occurs producing the yellow flame.

Lighting alcohol and pouring it over your Christmas pudding is definitely very exciting. However, combustion is a much more important reaction than simply this.

Heating up the world

The combustion of fuels is important for the world with energy. For centuries the combustion of fuels such as coal, oil and natural gas been used to power our world. The heat produced from burning these fuels can be used to produce electricity.

However, as we have seen combustion reactions produce carbon dioxide, carbon monoxide and soot. The enormous amount of carbon dioxide being released into the atmosphere is a real concern. It is widely believed that the increase in carbon dioxide is largely responsible for global warming. To reduce the amount of CO2 being released into our atmosphere, different fuels are being looked for. These fuels do not produce carbon dioxide when combusted. In addition, the amount of fuels such as coal and oil are running out. We therefore also need fuels that are always available. These are known as renewable fuels.

Why we must combust

The combustion of fuels is an extremely important reaction for the world we live in. We encounter these reactions every day. The cars, buses and trains we travel on all run from the combustion of fuels. Gas cookers and hobs all use combustion to produce flames for cooking food and boiling water. The boilers that provide us with hot water and keep our radiators warm also work by the combustion of fuels.

Combustion reactions can be very dangerous. But, when done safely we cannot all enjoy the flickering, blue flame dancing around our Christmas pudding.

Just make sure the flame is out before trying to eat any dessert…

[1] Strictly speaking combustion requires a substance known as an oxidant. The most common oxidant is oxygen (there is a big hint in the name). However, this is not always the case. BACK TO POST


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