Why does alcohol burn?

Why something can burn

• Combustion is a reaction in which a substance combines with oxygen and heat is released. For something to burn, there must be a substance that can still oxidize. When something is completely oxidized, it can no longer burn. CO₂ is incombustible because it is completely oxidized, CO can burn, C too.
• In addition, the substance must be in a form in which a rapid and violent reaction is possible, i.e. it must be as finely divided as possible or be gaseous. An iron bar does not burn, but iron filings or iron wool burn well.
• The reasons why something burns or is incombustible are based on the circumstances mentioned.

Because of this, alcohol burns

• Alcohols consist of a hydrocarbon chain and at least one hydroxyl group (-O-H). So there are a lot of atoms that can bond with oxygen.
• The hydroxyl group ensures that an alcohol has a higher melting and boiling point than an alkane with the same chain length, as this group forms hydrogen bonds. Methane is gaseous at room temperature, but methanol is liquid.
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• The shorter the hydrocarbon chain in the alcohol, the better it burns. This is because the short-chain alcohol is highly volatile and therefore there are always some molecules in the air above the liquid. As soon as it burns, the heat of combustion provides a supply, methanol already boils at 64.7 ° C, ethanol at 78.37 ° C.
• If you mix alcohol with water, the flammability will decrease. The decisive factor for this are the molecules that escape from the liquid due to the vapor pressure. There must be enough flammable molecules above the liquid. If you dilute the alcohol with water, there will be less flammable molecules there. The higher the temperature, the more you can dilute alcohol without hindering its flammability, as more molecules will then escape from the liquid.

The substance burns because it contains unoxidized C and H atoms and these easily evaporate from the liquid. For example, water cannot burn because it has already been completely oxidized. That is why water is non-flammable and alcohol burns easily.