TOPIC 3: FIRE, FIRE FIGHTING AND FLAMES – Chemistry Notes Form One
Fire is a state at which ignited material combines with oxygen and gives off light, heat and flame and combustion products.
When controlled, fire can be beneficial and used for cooking, heating or industrial processes.
COMPONENTS NEEDED TO START A FIRE (FIRE TRIANGLE).
Three things are needed to start fire.
(i) Fuel
(ii) Oxygen
(iii) Heat energy
These three components are known as fire triangle as seen in diagram below.
If any of the three components is missing no fire will start or continue burning.
For a fire to start a combination of heat, oxygen and fuel is needed in the suitable proportions
Most firefighting equipment work by cutting off the oxygen supply to the fire to effectively extinguish it.
NB:
(i) Fuel: is a substance that releases energy when burning eg petrol, diesel, kerosene, charcoal, fire wood, coal, natural gas etc.
(ii) Oxygen (air): Oxygen supports combustion. A fuel will only burn if there is sufficient supply of oxygen.
(iii) Heat: The temperature should be at the kindling point of that fuel or above it. Every fuel has its own kindling point. Below the kindling point, the fuel will not catch fire.
FIRE FIGHTING
Uncontrolled fires can be dangerous and destructive leading to injuries, property damage and environmental harm.
Firefighting is the act of extinguishing of harmful fires and mitigating (reducing) their effects. Materials which are used to put out fires are called Fire extinguishers. Fire fighting involves eliminating at least one of the three components in the fire triangle. Most firefighting equipment work by cutting off the oxygen supply to the fire to extinguish it effectively.
CLASSIFICATION OF FIRE
To extinguish fire, one must know which class of fire is it. Fires are classified according to the burning materials. Use of wrong extinguisher may result in the spreading of fire.
Fires are classified based on the type of burning materials. There are five classes of fire which are
(i) Class A fires
(ii) Class B fires
(iii) Class C fires
(iv) Class D fires
(v) Class F fires
CLASS | BURNING MATERIAL | APPROPRIATE EXTINGUISHERS |
| A | Ordinary SOLID combustible material eg wood, paper, rubber, clothing, | (i) use water (ii) Any type of portable extinguishers except carbon dioxide. |
| B | Flammable liquids, such as petrol, diesel, alcohol(spirit), kerosene, oil-based paints | (i)If fire is small use fire blanket or sand (ii) If fire is large use dry powder, foam or carbon dioxide extinguishers NB: Water should not be used as it will make the oil to float and keep on burning |
| C | Flammable gases such as butane, propane, biogas, methane etc | (i) Use Dry powder extinguisher (ii) Use Carbon dioxide extinguisher |
| D | Combustible /burning metals, such as magnesium, sodium, potassium, lithium especially in powder form. | (i) Dry powder extinguisher (ii) Foam extinguisher |
| F | Cooking oils and fats | Use wet chemical extinguishers |
NB: Fires caused by electricity are not given their own class because they can fall to any other classes. This is because it is not electricity that burns but the surrounding material which has been set a light by electric current.
Electricity can be source of any of the fires class A, B, C, D or F. Therefore, the fires caused by electrical fault are extinguished based on the burning materials.
Thus, dry powder, dry sand and carbon dioxide gas can be used to extinguish this fire but before starting to extinguish the fire MAIN SWITCH should be put (turned) off.
SUMMARY ON THE CLASSES OF FIRES
Class A: Is fire caused by burning solids such as paper, wood, clothing, rubber, etc
Class B: Flammable liquids such as paraffin, petrol, oil, spirit, alcohol, etc.
Class C: Is fire caused by burning flammable gases such as propane, butane, methane, hydrogen, etc
Class D: Is fire caused by burning metals such as aluminium, magnesium, titanium, sodium, potassium
Class F: Is fire in which burning material is Cooking oil or fat in cooking pans etc
PORTABLE FIRE EXTINGUISHERS
Portable fire extinguisher is an equipment used to put out fires and can be easily moved from one place to another.
It is usually hung in an upright position in automobiles and on walls of building such as schools and public buildings.
A portable fire extinguisher consists of a metal container that contains the extinguishing agent (substance) stored at high pressure.
Portable fire extinguishers have ratings or codes on them. This means the extinguisher has good firefighting capacity for specific classes of fires.
The following are different types of portable fire extinguishers are:
(i) Air pressurized water extinguishers (APW) (ii) Dry chemical extinguisher (DC)
(iii) Carbon dioxide extinguisher (iv) Halon extinguisher
(v) Foam extinguisher (vi) Wet chemical extinguishers
(vii) ABC extinguisher (viii) Blanket extinguisher
(ix) Sand bucket extinguisher
THE TABLE BELOW SHOWS CHEMICAL COMPOSITION OF THE EXTINGUISHING AGENTS OF DIFFERENT PORTABLE FIRE EXTINGUISHERS.
| Type | Colour code | Chemical composition of agent | Suitable for | Unsuitable for |
| APW (Air pressurized water | Red | Ordinary tap water pressurized by air | Class A fires (wood, paper, rubber, cloth etc | Fires Class B, C and D (will spread the flame) |
| Dry chemical (DC) | Blue | Fine sodium bicarbonate powder Pressurized by nitrogen | Classes A, B and C fires | Air craft and electronics fires, Class D fires (corrosive to metals such as aluminium) , NB: Although it is safe to use indoors it can obscure vision. |
| Carbon dioxide | Black | Carbon dioxide gas under extreme pressure | Class B, C and fires caused by electrical faults | Class A fires (Material can re-ignite) |
| Halon | Green | Bromochloro-difluo-methane | Class A and fires caused by electrical faults | Class B and C fires (least suitable) |
| Foam | Cream | Proteins and fluoro-protein | Class A and B fires | Fires caused by electrical faults |
| Wet chemical | Yellow | Potassium acetate | Class F | Fires caused by electrical faults |
| ABC | Blue | Mono –ammonium phosphate with a nitrogen carrier | Class A, B and C fires | Fires on Electronic equipment |
| Blanket extinguisher | Red | Made by heat resistant fibrous mineral called asbestors | Small fires Class A, B and F, also used to rescue an individual by stopping fires on burning clothes | Big fires |
| Sand extinguisher | Red | Composed of sand collected in bucket | Small fires class A and B | Big fires |
NB: (i) For small class A fire (of combustible solids), water can be used to easily put out the flames.
Small fires that involves flammable liquids should be put out using sand or fire blanket. These cut off the oxygen supply.
(ii) Water should never be used to put out class B (flammable liquids) fires since it would spread the flame because it is denser than flammable liquids and the flammable liquids float above it.
HOW TO USE A PORTABLE FIRE EXTINGUISHER
Fire extinguishers should be used in the right way and on the right type of fire. All fire extinguishers operate in a similar manner. The acronym ‘PASS’ can be used for quick reference. The acronym stands for Pull, Aim, Squeeze and Sweep. The correct procedure is summarized below:
1. Pull (remove) the pin at the top of the extinguisher to make the extinguisher ready for use.
2. Aim at the base and not at the middle or top of the fire otherwise the fire only spread
3. Squeeze the top lever slowly to release the extinguishing agent in the extinguisher.
4. Sweep from side to side until the fire is extinguished.
PRECAUTIONS ON USING FIRE EXTINGUISHERS
The following are some safety precautions you have to keep in mind when using fire extinguishers:
1. Keep a reasonable distance (at least 1.5M) from the fire as it may suddenly change direction.
2. Never use a portable extinguisher for person (people) on fire, instead use a fire blanket.
3. Do not test a portable fire extinguisher at your school without permission.
4. Do not return a used portable extinguisher to its place.
5. Do not hold the discharge horn (nozzle) when using carbon dioxide type fire extinguisher as it Becomes extremely cold that can lead to severe frost burns.
5. When a fire gets out of control, notify the nearest fire brigade.
EXERCISE
1. A track carrying kerosene was involved in an accident and unfortunately fire broke out. With explanation, suggest the fire extinguishers not suitable to use.
2. A form one student reads a statement from a certain book, “ If the clothes worn by your friend catch fire, cover them with a fire blanket”. Why a fire blanket is to be used in that case?
3. With a support of vivid examples, explain how you will educate the community to avoid the causes of fire accidents.
FLAMES
Definition: Flame is a zone or region of burning gases that produces heat and light.
It is the visible glowing part of a fire
TYPES OF FLAMES
There are two types of flames
(i) LUMINOUS FLAME (ii) NON-LUMINOUS FLAME
LUMINOUS FLAME
Definition: Luminous flame is a flame which is formed when air holes of a Bunsen burner are closed.
It is yellow and bright in colour.
It is produced by burning candle, spirit lamps, fire wood, piece of paper, tin lamp (kibatari) and hurricane lamp. The luminous flame is called a safety flame because it is easily seen, therefore, is less likely to cause accidents.
Why luminous flame produces soot and less heat?
It produces soot and less heat because oxygen supplied is not enough to complete the burning up of the fuel.
NON-LUMINOUS FLAME
Definition: Non-luminous flame is a flame which is formed when air holes of a Bunsen burner are opened.
It is blue in colur. The flame can be produced by gas stove, Bunsen burner.
Non- luminous flame can easily cause accidents in the laboratory because cannot be seen easily therefore, one may not be aware that the burner is on.
NB: Gas stove, kerosene stove and Bunsen burner can be regulated to form both Luminous and Non-luminous flames.
THE BUNSEN BURNER
Definition: A Bunsen burner is a laboratory heat source which consists of a vertical metal tube connected to a gas source.
The Bunsen burner is named after the German chemist Robert Bunsen who invented it 1855. The Bunsen burner is able to produce a very hot flame of temperature up to 1000oC from a mixture of gas and air.
PARTS OF THE BUNSEN BURNER
These include base, gas inlet, jet, collar (metal ring), air hole, barrel(chimney).
FUNCTIONS OF DIFFERENT PARTS OF THE BUNSEN BURNER
Base: It supports the Bunsen burner. It makes the burner stable,
Gas inlet: Allows the gas to enter in the Bunsen burner from the gas supply.
Jet: Directs the gas to the barrel / chimney
Collar: Controls (Regulates) the amount of air entering the burner.
Air holes: Allow air to enter in the burner.
Barrel: It is where mixture of air and Gas burns.
HOW THE BUNSEN BURNER WORKS
- Gas enters the burner through gas inlet.
- Air enters the burner through the air holes.
- The amount of air entering the burner is controlled by turning the collar.
- At the top of the barrel, the mixture of air and gas burns to produce a flame.
- When air holes are closed the amount of oxygen is less and luminous flame is produced.
- When air holes are partially opened, there is more oxygen and flame produced is medium.
- When the air holes are fully opened, there is enough oxygen and Non-luminous flame is produced.
HOW TO LIGHT A BUNSEN BURNER
The following are correct steps (procedures) on how to light the Bunsen burner:
1. Close the air holes of the burner.
2. Connect the Bunsen burner to the gas tubing (gas supply).
3. Light a match or wooden splint and hold it on top of the barrel
4. Slowly turn on the gas until the gas light up
5. Open the air holes halfway and then fully (Turn the collar to adjust the air holes)
6. Adjust the gas tap until the gas supply is enough to produce a non-luminous flame.
BURNING BACK (STRICKING BACK)
Burning back occurs when too much air enters the burner so that the flame burns back into the burner tube.
The following can be the causes of burning back of the Bunsen burner
(i) Insufficient gas pressure (ii) Excessive air flow
(iii) Blockage of the jet or gas inlet (iv) Damaged burner
WHAT TO DO AFTER BURNING BACK?
1. Turn off the gas tap
2. Close the air holes
3. Turn on the gas tap
4. Re- lights the Bunsen burner.
HOW TO PREVENT BURNING BACK
1. Ensure gas supply is sufficient
2. keep the burner Nozzle (jet) and air inlet clean
3. Avoid over heating or prolonged use of the burner.
PARTS OF FLAMES
A: PARTS OF LUMINOUS FLAME
Luminous – flame has four parts (zone) which are
(i) Thin outer zone
(ii) Yellow middle zone
(iii) Inner unburnt zone
(iv) Blue outer zone
1. Thin Outer zone – This is a region of complete combustion of gas which gives out
light. It is almost a colourless zone
2. Yellow middle zone – This is a region of incomplete combustion of gas and gives out soot.
3. Inner unburnt zone – This is dark, cool and contains unburnt gas
4. Blue outer zone– Due to rising convectional current, there is sufficient supply of air for complete burning at this zone.
B: NON-LUMINOUS FLAME
PARTS OF NON – LUMINOUS FLAME
Non – luminous flame has three parts (zones) which are
(i) Colourless inner zone (ii) Blue – green middle zone (iii) Pale purple – blue outer zone
1. Pale purple – blue outer zone – It is a zone (region) where complete burning of the gas occurs.
2. Blue-Green middle zone – Part of the gas burns in this zone because there is no enough air
to burn all the gas completely. The hottest part of the flame is at
the tip of this blue- green zone.
3. Colourless (Cool) inner zone – It is a zone(region) of unburnt gas.
CHARACTERISTICS OF FLAMES
Heat sources produce either a luminous flame or non-luminous flames. These flames have different characteristics.
The table below shows the characteristics differences between luminous flame and non-luminous flame
| LUMINOUS FLAME | NON-LUMINOUS FLAME |
| 1. It has four (4) zones | It has three (3) zones |
| 2.It forms soot | Does not form soot |
| 3. It is yellow in colour | It is blue in colour |
| 4. It is not very hot | It is very hot |
| 5. It is not noisy (Burns quietly) | It is noisy (Burns with roaring noise) |
| 6. It does not burn back | Sometimes it burns back |
| 7. It is tall and shapeless (Has a wavy flame) Has a triangular flame | It is short and cone like (Has a triangular flame) |
USES OF DIFFERENT TYPES OF FLAMES
LUMINOUS FLAME– It is mainly used for lighting because
(i) It is bright yellow in colour
(ii) It is also not very hot;
Therefore, it is safer for lighting than the non – luminous flame. It is not suitable for heating /cooling because it is not very hot and produces soot. Eg Candle flame, tin lamp flame, hurricane lamp flame etc
NON-LUMINOUS FLAME
1. It is used for heating because it gives a lot of heat. (Bunsen burner and liquefied petroleum gas burners can be used for heating)
(2) It is used for cooking because it gives enough heat and it does not produce soot
(3) It is used for welding because it is very hot (Welding is the joining together of metal pieces or parts)
(4) It is used in the flame test of some chemical substances (A flame test is the introduction of a sample of the desired substance to non-luminous flame and observing the colour change).
NB: Heat sources can be natural or artificial. Natural sources of heat is the sun. artificial heat sources are man made sources such as heat from the spirit burner, Kerosene stove, Bunsen burner and gas stove.
Heat sources used in the laboratory include
(i) Spirit burner
(ii) Kerosene stove
(iii) Bunsen burner
(iv) Gas stove
(v) Electric burner