Contents
Physical properties, such as boiling point, melting point, and density, are important aspects of Chemistry Topics as they describe the characteristics of substances.
Explain the Structure of Flame With the help of a Labelled Diagram and Activities
If we heat one end of a magnesium ribbon over a burner, we find that the magnesium ribbon burns by producing a brilliant white flame. We have also seen a candle flame, a kerosene lamp flame and a Bunsen burner flame (see Figure). Actually, flame is the ‘blaze’ of a fire. It is called ‘jwala’ or ‘lapat’ in Hindi.
A flame is a region where combustion (or burning) of gaseous substances takes place. All the gases which undergo combustion (or burn) produce flame. But only those solid and liquid fuels which vaporise on being heated, so kerosene oil and mustard oil also burn by producing flames.
Thus, some of the substances which burn by producing flames are : LPG, biogas, wax (in the form of candle), camphor, magnesium, kerosene oil and mustard oil. Wax candle and kerosene oil lamp have wicks. Molten wax and kerosene oil rise through the wick, get vaporised during burning and form flames.
On the other hand, charcoal is a solid fuel which does not vaporise on heating. So, charcoal does not burn by producing a flame. Charcoal only glows on combustion. Similarly, coal is a solid fuel which does not vaporise on heating. So, coal also does not burn by producing a flame. Coal just glows red on combustion (see Figure).
When fuels burn, the type of flame produced depends on the proportion of oxygen (of air) which is available for burning the fuel (or for combustion of fuel).
(i) When the oxygen supply (or air supply) is insufficient, then the fuels burn incompletely producing mainly a yellow flame. The yellow flame is caused by the glow of hot unburnt carbon particles produced due to incomplete combustion of fuel.
This yellow flame produces light, so it is said to be a luminous flame (or light-giving flame). When wax burns in the form of a candle, it burns with a yellow, luminous flame (see Figure). Thus, the colour of candle flame is mainly yellow. When kerosene is burned in a lamp, it also burns with a yellow, luminous flame.
(ii) When the oxygen supply (or air supply) is sufficient, then the fuels burn completely producing mainly a blue flame. This blue flame does not produce much light, so it is said to be a non-luminous flame (or non light-giving flame). In LPG stove (or kitchen stove), the LPG burns with a blue flame (which is a non-luminous flame) (see Figure).
The blue flame is produced when the complete combustion of a fuel takes place. Thus, complete combustion of LPG takes place in the kitchen gas stove. The design of the burner of kitchen gas stove is such that it provides sufficient air for the complete combustion of LPG .
Structure of a Flame
A flame consists of three zones (or three parts). These are : innermost zone, middle zone and outer zone.
The three zones of a flame have different colours and different temperatures. We will now describe the three zones of a flame in detail by taking the example of candle flame.
(i) The innermost zone of a flame is dark (or black) (see Figure). The innermost zone of a flame consists of hot, unburnt vapours of the combustible material (say, wax vapours). The innermost zone is the least hot part of the flame. In other words, we can also say that the innermost zone (or dark zone) is the coldest part of the flame.
(ii) The middle zone of a flame is yellow. It is bright and luminous (light giving) (see Figure). The fuel vapours burn partially in the middle zone because there is not enough air for burning in this zone. The partial (or incomplete) burning of fuel in the middle zone produces carbon particles.
These carbon particles become white hot and emit light. So, it is the glow of hot carbon particles which makes the middle zone of a flame luminous (or light-giving). These carbon particles then leave the flame as smoke and soot. The middle zone (or luminous zone) of a flame produces a moderate temperature. This zone is the major part of a candle flame.
(iii) The outer zone of a flame is blue. It is non- luminous zone (which does not produce much light) (see Figure). In the outer zone of a flame, complete combustion of the fuel takes place because there is plenty of air around it. The outermost zone (or non- luminous zone) has the highest temperature in the flame.
In other words, the outermost zone (or non-luminous zone) is the hottest part of the flame. The outermost zone of a flame is quite thin as compared to the middle zone.
Activity 3
To Show That the Innermost Zone of a Candle Flame Consists of Unburnt Wax Vapours
Take a wax candle, fix it on a table and light it with a matchstick. Hold a thin glass tube with a pair of tongs and introduce one end of this glass tube in the innermost zone (dark zone or black zone) of the candle flame (see Figure). Now bring a lighted matchstick near the other end of the glass tube. We will see a flame at this end of the glass tube (see Figure).
This can be explained as follows :
The innermost zone (or dark zone) of candle flame near the heated wick consists of unburnt wax vapours. Some of these wax vapours enter the glass tube and come out from its other end. When we bring a lighted matchstick near this end of glass tube, the wax vapours coming out of it start burning, producing a flame. This activity shows that the innermost zone (dark zone or black zone) of a candle flame consists of unburnt wax vapours.
Activity 4
To Show That the Middle Zone of Candle Flame Consists of Unburnt Carbon Particles
Light a candle. Hold a clean glass plate with the help of a pair of tongs and introduce it in the middle zone (or luminous zone) of the candle flame (see Figure). Hold the glass plate in this position for about 10 seconds. Then remove the glass plate from candle flame and observe it carefully. We will find that a blackish ring is formed on the glass plate (see Figure).
This blackish ring is produced due to the deposition of unburnt carbon particles present in the luminous zone of the candle flame. This activity shows that the partial combustion of wax vapours in the middle zone produces unburnt carbon particles.
To Show That the Outermost Zone (Non-Luminous Zone) of a Flame is the Hottest
Take a long copper wire and hold its one end with a pair of tongs. Introduce the other end of copper wire just inside a burning candle flame so that it is in the outermost zone (non-luminous zone) of flame (as shown in Figure). Keep the copper wire in this position for about 30 seconds.
We will see that the part of copper wire which is in the outermost zone of the flame becomes red hot. This tells us that the outermost, non-luminous zone of a flame has a high temperature. In other words, the non-luminous zone (or outermost zone) of a flame is the hottest part of a flame.
Activity 5
To Show That the Outermost Zone (Non-Luminous Zone) of a Flame is the Hottest
Take a long copper wire and hold its one end with a pair of tongs. Introduce the other end of copper wire just inside a burning candle flame so that it is in the outermost zone (non-luminous zone) of flame (as shown in Figure).
Keep the copper wire in this position for about 30 seconds. We will see that the part of copper wire which is in the outermost zone of the flame becomes red hot. This tells us that the outermost, non-luminous zone of a flame has a high temperature. In other words, the non-luminous zone (or outermost zone) of a flame is the hottest part of a flame.
A kerosene oil lamp produces a flame exactly similar to the candle flame, consisting of the same three zones. Goldsmiths blow air with a blow-pipe to intensify a kerosene lamp flame for melting and moulding the pieces of gold and silver into desired shapes to make jewellery (see Figure).
When air is blown through blow-pipe into the flame, it helps in the combustion of unburnt fuel and hence makes the flame hotter. We will now discuss the harmful products produced by the burning of fuels.
Burning of Fuels Leads To Harmful Products
The burning of fuels produces harmful products which pollute the air around us. So, the increasing use of fuels has harmful effects on the environment. The important harmful effects produced by the burning of fuels are as follows :
1. The burning of fuels like wood, coal and petroleum products (kerosene, petrol, diesel, etc.) releases unburnt carbon particles in the air. These fine carbon particles are dangerous pollutants which can cause respiratory diseases such as asthma.
2. Incomplete combustion of fuels (due to insufficient air) produces a very poisonous gas called carbon monoxide. Excessive inhaling of carbon monoxide gas can kill a person. We should never sleep in a room with closed door and windows, and having a coal fire burning inside.
his is because when coal burns in an insufficient supply of air in the room (due to closed door and windows), then a lot of carbon monoxide gas is produced. When the persons sleeping in this room breathe in poisonous carbon monoxide gas, they may all die.
3. Burning of fuels releases carbon dioxide into air in the environment. Increased percentage of carbon dioxide in air is causing global warming. This happens as follows: Carbon dioxide gas in the air traps sun’s heat rays by producing greenhouse effect. Global warming is the rise in temperature of earth’s atmosphere caused by the excessive amounts of carbon dioxide in the air.
Due to rise in the temperature of atmosphere, the ice in polar regions will melt very fast, producing a lot of water. This water may cause a rise in the sea-level leading to floods in coastal areas. The low-lying coastal areas may be completely submerged under water leading to the loss of life and property.
4. Burning of coal, petrol and diesel produces sulphur dioxide gas which goes into the air. Sulphur dioxide is an extremely suffocating and corrosive gas. It may damage our lungs. The burning of petrol and diesel in the engines of vehicles also releases nitrogen oxides into the air.
Sulphur dioxide and nitrogen oxides produced by the burning of fuels dissolve in rain water and form acids. The rain water containing acids is called acid rain. Acid rain is very harmful for forests, aquatic animals and buildings.
The Case of CNG
The use of petrol and diesel as fuels in automobiles (vehicles) is being replaced by CNG (Compressed Natural Gas). This is because when CNG burns, it produces very small amount of harmful products. CNG is a clean fuel because it burns without producing smoke. Since the burning of CNG produces much less harmful products and smoke, therefore, the use of CNG as fuel in automobiles has reduced air pollution in our cities.
The Case of Wood
Wood has been used as a domestic and industrial fuel for centuries. In many rural parts of our country, people still use wood as a fuel due to its low cost and easy availability. The burning of wood as a fuel has, however, many disadvantages. Some of the disadvantages of burning wood as fuel are as follows :
- The burning of wood produces a lot of smoke which is very harmful for human beings. Smoke causes respiratory diseases.
- Trees provide us with many useful substances. When trees are cut down to obtain wood for use as fuel, then all the useful substances which can be obtained from trees are lost.
- The cutting down of trees to obtain fuel wood leads to deforestation which is very harmful for the environment.
Wood has now been replaced by coal and other fuels such as LPG. LPG is a better domestic fuel than wood due to the following reasons :
- LPG has a much higher calorific value than wood, so it produces much more heat on burning than an equal mass of wood.
- LPG burns without producing any smoke but burning of wood produces a lot of smoke.
- LPG burns completely without leaving behind any solid residue but wood leaves behind a lot of ash on burning.