Bite-size answers to commonly asked questions from inquiring minds
Q. Why is fire hot?
Did you know?
When the Lord Mayor was informed of the start of the Great Fire of London in September
1666, his response is reported to have been “Pish! A woman could piss it out”. The
fire went on to destroy 5/6ths of the city.
A. Fire is a chemical process involving the rearrangement of molecules. You may or
may not remember from your schooldays that molecules comprise several atoms bonded
together. Whenever molecules rearrange their atoms during such a chemical change,
energy is either released or absorbed. With fire, the release of energy takes the
form of heat (and light), which is basically why fire is hot. But let’s look at things
in a little more detail...
Three things are needed to trigger the aforementioned chemical process and start
a fire - oxygen, fuel and heat/an ignition source (e.g. a match, focused sunlight,
etc). The chemical process is called oxidation. To give a familiar example of oxidation,
when iron rusts, oxygen in the air combines and reacts with iron atoms (without having
the scope to go into too much detail, electrons basically pass from the iron to the
oxygen). This chemical reaction between the oxygen and iron releases energy. In the
case of corroding iron, the oxidation rate and release of energy is extremely slow
and so the rusting area increases only very slightly in temperature. But in the
case of fire, the oxidation rate (which in the case of a wood fire for example occurs
between oxygen, hydrogen and carbon atoms) and resulting release of energy is very
fast and thus a lot more heat is produced.
Of course, a pile of wood won’t just catch fire because it is surrounded by oxygen!
For the combustion (i.e. burning) to take place you need the third element mentioned
above - heat - to make the fuel reach its ‘ignition temperature’. If heat cannot
be released faster than it is created then combustion happens. So, for example, sliding
a match fast enough across a coarse surface will generate sufficient heat (due to
friction) to produce a temperature which is at least as high as the chemicals within
the match head’s relatively low ignition temperature, and it will ignite. The resulting
combustion provides even more heat and the match will often continue to burn until
the fuel (stick) runs out.
So, once something is ignited, enough heat needs to be maintained/produced to keep
the fuel and oxygen at or above its ignition temperature and keep the fire going.
It is a chain reaction of sorts in that the fire must sustain its own heat and have
an ongoing supply of oxygen and fuel to keep going.
Phew, that’s a lot to try to take in! But in summary, fire is a chemical process
called oxidation and requires oxygen, fuel and a heat source. During this process,
the chemical bonds in oxygen and fuel are rapidly broken and new bonds are formed.
Although some energy is used up when chemical bonds are broken, more is released
during the creation of the new ones. That extra energy is released as heat.
Source(s): Cornell Center For Material Research, Pbs.org, Physicsforums.com, Energyquest.ca.gov