TOPIC 3: HEAT SOURCES AND FLAMES | CHEMISTRY FORM 1
TOPIC 3: HEAT SOURCES AND FLAMES | CHEMISTRY FORM 1
Bunsen burner is the best of all burners because it is convenient to
handle. Another advantage of the Bunsen burner is that it produces a hot
flame whose temperature is approximately 1000°C. The temperature can be adjusted easily to produce a non-luminous flame, which does not produce much soot.
hot enough to effect (produce) some chemical reactions. Apart from that,
the burner is filled with spirit, a substance that is highly flammable.
candle can only be used where a chemical reaction does not require much
heat. Its disadvantage is that it produces a lot of soot. The other
burners, though not commonly used, are an electric heater and a gas
burner.
electric heater uses electricity. The gas burner uses a liquefied gas.
The disadvantage of an electric burner is that it cannot be used in
rural areas where there is no electricity.
in Swahili, if well adjusted can produce a flame hot enough to heat
many substances in the laboratory. It is fulled with kerosene, a fuel
that is convenient to carry and store. This fuel does not catch fire
easily as compared to spirit and it is affordable
can conveniently be used by schools in the most remote areas where
there is no electricity. If too much heating is required, wire gauze
should be placed on top of the burner. This will enable reduce soot and
increase the heating temperatures to about 1000°C or more.
charcoal burner can also be used in remove areas. In case the kerosene
burner is not available, for one reason or another, a charcoal burner
can be the best alternative.
all the burners we have discussed so far, a Bunsen burner is the mostly
used. Therefore, we are going to discuss about the functioning of the
Bunsen burner in more detail. As the name suggests, this burner was
invented by a German scientist called Robert Bunsen, so it was named
after his name as a Bunsen burner. The burner uses coal gas, which burns
with a hot and non-luminous flame when the air holes are open. This is a
kind of flame we normally use in the laboratory.
Regulates the amount of air entering the burner. It has air holes that
can be turned open or closed depending on the kind of flame, and hence
amount of heating required.
knowing the different parts of the Bunsen burner, it is important that
you also learn how to light it. This is because careless use of the
burner may lead to accident or wastage of the gas. The following is a
correct sequence of steps on how to light the Bunsen burner:
- Connect the Bunsen burner by a rubber tube to the gas supply.
- Close the air holes.
- Turn the gas tap on to let in sufficient gas.
- Quickly bring a flame at the top of the barrel. You may use a matchstick, a lighter or wooden splint as a source of flame.
- Turn the collar to adjust the air holes until you get the type of flame you want. You may have the holes completely open.
- Adjust the gas tap until the gas supply is enough to produce a non-luminous flame.
put off the flame of the burner after you finish heating a substance,
turn the gas tap off in order to cut off the gas supply to the burner.
Disconnect the burner from the gas mains by removing the rubber tube
connecting the two. Then close the air holes. Pay attention not to touch
the hot collar with your fingers or else wait until it is cool enough.
Take the Bunsen burner and keep it at the appropriate place
are formed by burning gases or vapours. During burning, heat and light
are given out. For any solid or liquid to burn with a flame, it must
first turn into inflammable vapours (gaseous state).
flame can be luminous or non-luminous. Flames of a candle and any oil
are usually smoky and luminous. Flames of such kind are normally of
little laboratory use. This is because they are not hot enough and would
deposit soot on laboratory apparatus. Coal gas also burns with a smoky
and luminous flame. With a Bunsen burner, one can produce two types of
flames namely, the luminous and non-luminous flames.
is a type of flame produced when the air holes of a Bunsen burner are
closed. When the air holes are closed very little air enters the barrel
of the burner. In this case, the flame will be large, unsteady and
bright
- The inner dark zone – This is dark, cool and contains unburnt gas
- Luminous yellow zone –
The gas burns in this zone but because the air is not enough the
burning is incomplete. This leads to formation of tiny carbon particles
from the gas. When these particles are white-hot, they result in
formation of light (the yellow colour we see). If a cold evaporating
dish, porcelain crucible, or glass is placed in this zone, it will
blacken due to deposition of carbon particles (soot) on it. - Outer zone –
This is a non-luminous zone where the burning of the gas is complete
due to presence of enough air. Because of the absence of carbon
particles, this zone does not give out light. Consequently, the zone
cannot be seen easily. - Blue zone – Due to rising convectional current, there is sufficient supply of air for complete burning at this zone.
air holes are fully opened, sufficient air enters the Bunsen burner
barrel and mixes well with the coal gas. Hence, the burning of the gas
is much quicker and complete. The flame is smaller and hotter.
to absence of white-hot carbon, no light appears. The flame is
therefore non-luminous. The flame has three district zones each with a
different colour.
- Cool inner zone – this is a zone of unburnt gas.
- Green/blue zone –
part of the gas burns in this zone because there is not enough air to
burn all the gas completely. However, no carbon is formed. The hottest
part of the flame is at the tip of this zone. - Outer purple zone – Burning of the gas in this zone is complete.
Non luminous flame | Luminous flame | |
1. | Formed when air holes are open | Formed when air holes are closed |
2. | Very noisy | Silent or calm |
3. | Comprises of three zones | Comprises of four zones |
4. | Forms no smoke or soot on apparatus | Forms a lot of smoke or soot on apparatus |
5. | Blue and almost invisible | Bright yellow and clearly visible |
6. | Very hot flame | Not a hot flame |
7. | Not bright | Very bright |
8. | Triangular flame | Wave-like flame |
- (a) When a piece of cardboard is held horizontally over a non-luminous flame, we notice a burn mark as shown below:
held vertically over the flame, the burn mark is as shown in above.
Note that when performing this experiment, the cardboard should be
withdrawn from the flame just before it catches fire. We find that the
middle part of the cardboard does not get burned. This is the part in
the zone containing unburnt gas.
can prove the presence of unburnt gas in the Bunsen flame. This can be
done by inserting a glass tube into the flame as shown in figure bellow
unburnt gas can be shown to have risen up the tube by putting a light
at the top of the tube. The flame will form. This indicates the escape
of unburnt gas through the tube.
Production of heat for heating substances in the laboratory:
In this case, a non-luminous flame, which produces much heat, is used.
However, for reactions that require little heat, a luminous flame, which
is not very hot, can be used.
Flame tests for elements:
In chemical analysis of some elements, a flame test is one of the
preliminary tests normally used to identify an element. When some
elements are strongly heated, they produce characteristic flame colours
that distinguish them from one another. A non-luminous flame is often
used.
Production of light: Flames produce light that
can be used to light a dark room. Therefore, an experiment that involves
heating can even be conducted in the dark. The same flame is used to
give heat as well as light. Here, a luminous flame is used. Examples of
heat sources, which produce flames that may be used for lighting, are
hurricane lamp, tin lamp, spirit lamp and candle.
Cooking:
Since it gives a hot flame and produces no soot, a non-luminous flame
can be used for cooking food. Gas cookers, gas stoves and kerosene
stoves usually produce such flames.
Welding: A
non-luminous flame is suitable for welding because it is very hot. In
most welding operations, an oxyacetylene gas, a mixture of oxygen and
ethyne, is used. When burned, the gas produces a flame hot enough to cut
or melt the metal.
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