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Asked by anna12345 to Meeks, Pete, Stephen, Steve, Tom on 21 Jun 2010 in Categories: General.
Question: why is fire hot......?
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Steve Roser answered on 15 Jun 2010:
Hot means there is a lot of energy around. The energy comes from the chemical reaction of burning ( or electrical energy of heating an electric fire for example). Energy cannot be created or destroyed, just moved around, so the energy of a bonfire, the heat of a bonfire comes from energy stored in the wood that i’m burning, which ultimately comes from the sun….
The heat energy – the hotness – can be used to make steam for example to drive motors or make electricity. the problem is you always lose a little energy along the way, and so you get less electrical energy than the heat energy you put in…
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Stephen Curry answered on 15 Jun 2010:
Strictly, fire isn’t hot!
Fire is the light that you can see when some materials burn. It is the chemical reaction that occurs when things burn that releases the heat that you feel. When heat is released the molecules involved start to vibrate more rapidly or, if they are in liquid or gas form, they fly about much more quickly. They will collide with the air nearby, causing the air molecules to speed up and *that* is what you sense as heat.
The thing I don’t understand is how does our skin sense this increase movement of the air (or the increased vibration of atoms in a hot iron poker) as something that is hot? I know that there are molecules on the outside of our skin cells that somehow detect heat. If I am not mistaken, what is really cools (pardon the pun!) is that the same receptors in our mouths also detect spicy food which is why curry feels hot!
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Tom Hartley answered on 16 Jun 2010:
I expect the others will have explained the chemistry and physics of this, so I will talk about the biology and psychology. Fire is hot because if it wasn’t we would burn ourselves badly; some of our ancestors perished because they failed to move away from fire, so there was an advantage to animals which sensors allowing them to detect and move away from fire. Those creatures survived longer, and eventually we came along.
I am not taking the mickey in answering this way – I like it that we can think about lots of different ways to answer this kind of question.
It leaves lots of unanswered questions such as “why does heat feel the way it does”. I don’t know the answer, but I tried to answer a related question about hunger earlier on this site:
/imagingj10-zone/2010/06/what-is-hunger-its-not-painful-but-we-dont-like-it
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Pete Edwards answered on 17 Jun 2010:
Hi anna12345
For something to burn it must have energy stored in it that it wants to release. When it burns, the energy is quickly released as heat and light, which we see as fire.
One example is the burning of wood which has energy from sunlight stored inside. As a tree grows, it absorbs sunlight and uses that energy to break apart molecules of water and carbon dioxide turning them into larger molecules that make up the wood of the tree.
The molecules in the wood would really prefer to exist as carbon dioxide and water and we can start the conversion of the wood of the tree back into these molecules by heating dry wood. As the wood is heated some of the large molecules break free from the wood and rise into the air as a vapour. These molecules react with oxygen in the air and break down into water and carbon dioxide again with a release of heat and light – this is the sunlight that the tree absorbed to grow in the first place! This is a neat example of something called the conservation of energy – the idea that energy cannot be created or destroyed it just changes form.
So a fire is hot because it’s a fast release of energy that was stored in the thing that’s burning. -
Marieke Navin answered on 21 Jun 2010:
Hi Anna, sorry for the delay in my answer!
Fire comes from a chemical reaction and like all reactions, like those you have done at school, it is the movement of electrons that is important. The heat and light released is a result of the reaction, i found quite a nice website on this you can check out at :http://science.howstuffworks.com/fire1.htm
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