Sunday 20 August 2017

Why our Christmas fires burn so bright

Thinkstock Images
Thinkstock Images

SARAH HAYES

Humans have always been fascinated by fire. It is a central component in the legends and histories of cultures the world over: from the titan Prometheus, who allegedly stole the fire from the Greek gods of Olympus to give to man, to the Maori legend of Mãui who tricked Mahuika into giving her flames to the Kaikõmako trees.

Fire is a chemical reaction. It warms us at Christmas as we huddle up beside it and watch it brighten up the room.

In order for this particular chemical reaction to take place we need oxygen, which is present in the air we breathe. Next, we need some sort of fuel; wood or coal for example. But, fuel and oxygen do not make fire. For this reaction to initiate we need to heat the fuel to ignition temperature. Each material will have a different ignition temperature, and this temperature is the minimum at which a substance will burn on its own without the need for the continuous application of an external heat source.

Anthracite, the type of coal we usually use, has around 91.5pc carbon (C), 3.75pc hydrogen (H), 2.5pc oxygen (O) and less than 1pc sulphur (S). The ignition point for this type of coal is 600°C.

When the fuel reaches the ignition point then a chemical reaction occurs allowing the fuel to burn in oxygen. A side effect of this chemical reaction is that energy is given out in the form of heat and light. As the atoms heat up they emit heat and light. This is called incandescence.

We see different colour variations in the flames caused by uneven temperature throughout the flame. The cooler parts of a flame are orange and yellow. The hottest part of a flame glows blue. We often see that blue tinged flame in gas fires.

If we want to change the colour of the flame we can burn different substances, similar to the flame tests you see in school science. For example copper salts give a green flame (what we imagine a witch's fire looks like in fairy tales). Potassium salts give a lilac flame. Sodium salts burn bright orange, which we can see in our street lamps, which actually use sodium vapour. Lithium salts give a vivid pink colour when they burn and strontium salts burn bright red.

Dr Sarah Hayes is Education and Outreach  Officer at the Synthesis and Solid State Pharmaceutical Centre (SSPC).

Irish Independent

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