Forensic Fire Investigation - Unit 25 Assignment 1
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Course
Unit 25 - Forensic Fire Investigation
Institution
PEARSON (PEARSON)
Practical investigation into the chemistry of combustion, extinction, and heat transfer
The fire tetrahedron and different sources of ignition
Combustion and incomplete combustion
Convection, conduction, and radiation
Different methods for extinguishing a fire
Describing how fires can be cause...
Unit 25 Assignment 1 – Forensic Fire Investigation
Practical investigation into the chemistry of combustion, extinction, and heat
transfer
Fires are caused by a combination of heat, fuel, and oxygen. When investigating
a fire, it is important to look for signs of these factors as they may have helped
to cause the fire to start.
Below are three examples of experiments that can be used for fire analysis:
Candles and beakers:
The method is to light candles, place different sized beakers on top of them, and
then assess which candle extinguishes first.
Chemistry of combustion – the candle flame burns due to the combustion of wax
in the presence of oxygen from the air. As the wax melts and vaporises, it
combines with oxygen to produce carbon dioxide and water vapour. This
chemical reaction releases heat and light, sustaining the flame.
Heat transfer – the flame of the candle generates heat energy, which is
transferred to the surrounding air and objects. Placing a beaker over the candle
traps the heat, creating a closed environment where heat transfer occurs
through conduction, convection, and radiation. The beaker absorbs some of the
heat from the flame, causing its temperature to rise.
Fire extinction – as the beaker absorbs heat from the flame, it affects the balance
of the combustion reaction. If the beaker completely covers the flame, it can
deprive the flame of oxygen, leading to extinguishment. This demonstrates how
heat transfer and oxygen availability are crucial factors in sustaining or
extinguishing a fire.
From the practical performed, the smallest beaker would burn for the longest as
the fuel from the heat along with the levels of oxygen and carbon dioxide present
inside would allow for the flame to burn longer.
(ACS, 2019)
,Unit 25 Assignment 1 – Forensic Fire Investigation
Identifying the products of combustion:
The method is to light a candle
within a jar, and test for carbon dioxide and water by using blue cobalt chloride
paper for water, and limewater for carbon dioxide.
In this test, the cobalt chloride paper turned pink to indicate water, and the
limewater was poured into the glass jar and turned cloudy to indicate carbon
dioxide.
Chemistry of combustion – when the candle begins to burn, it undergoes
combustion, where as the wax reacts with oxygen in the air, carbon dioxide and
water vapour is created. These elements can be tested for with the use if cobalt
chloride paper which turns pink in the presence of water, and limewater which
turns cloudy in the presence of carbon dioxide.
Heat transfer – the flame of the candle generates heats, which warms the air
inside the jar. As the air heat up, it expands, creating a convection current that
circulates the warm air around the jar. The warm air rises and can also begin to
heat the surroundings of the jar too.
Fire extinction – once the tests have been completed, the jar can be sealed. The
flame will continue to burn until it eventually extinguishes itself due to the
depletion of oxygen. The lack of oxygen interrupts the combustion process,
leading to the flame going out.
(Science In School, 2024)
Chip pan fire investigation:
, Unit 25 Assignment 1 – Forensic Fire Investigation
The method for this is to heat a crucible that contains cooking oil at a high
temperature until the oil vaporises and catches alight.
Chemistry of combustion – in the experiment, the cooking oil undergoes
combustion when exposed to a heat source. The oil vaporises and combines with
oxygen from the air, producing carbon dioxide and water vapour. This chemical
reaction releases heat and light which sustains the fire.
Heat transfer – the heat from the Bunsen burner transfers to the cooking oil,
causing it to reach its ignition temperature. Ince the oil ignites, the fire generates
more heat, which can spread to surrounding surfaces.
Fire extinction – to extinguish the fire, the flame can be covered which cuts off
the oxygen supply, thereby smothering the flames. In a real cooking fire, a fire
extinguisher or fire blanket can be used to help suppress the fire by removing the
oxygen source.
(Education In Chemistry, 2017)
The fire tetrahedron and different sources of ignition
The fire tetrahedron consists of three different factors: heat, oxygen, and fuel. If
one of these factors are missing, then a fire is not able to start. The fire
tetrahedron looks like this:
As you can see from the image above, heat, fuel, and oxygen all work together
to produce a chemical chain reaction which causes a fire to occur. Due to these
factors, there are many different sources of ignition that can cause a fire to
alight. There are four main types of ignition: thermal, electrical, mechanical, and
chemical. Within these types of ignition, there are different examples of sources
that can be used as an ignition factor for a fire. Some of these include:
- Electrical sparks
- Static electricity
- Naked flames
- Hot surfaces
- Impact
- Friction
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