Science Chapter 6 Combustion And Flame

Conditions necessary for combustion are :

(i) Presence of combustible substance.

(ii) Attainment of ignition temperature.

(iii) Proper supply of air to provide oxygen.

pollution

kerosene

ignition temperature

water

CNG has replaced petrol and diesel as fuel in automobiles because petrol and diesel produced a lot of unburnt carbon particles and emitted carbon dioxide and nitrogen oxides and sulphur dioxide. These all gases are poisonous gases and cause various environmental hazards. But CNG is safe because it produces these substances in very small amounts thus reducing pollution in cities.

Water is a conductor of electricity, so it can easily conduct electric current and causes danger of electric shocks.

LPG has more calorific value and produces no pollution. Hence, it is better domestic fuel than wood.

 The ignition temperature of paper is less, so it catches fire easily. It does not catch fire when wrapped around aluminium pipe because aluminium absorbs the heat, so paper does not attain its ignition temperature.

 The calorific value of fuel is expressed in Kilojoules per kg (kJ/kg).

(i) CO₂ forms a blanket around fire due to which supply of air (oxygen) is stopped.

(ii) CO₂ also brings down the temperature of the fuel. 

Hence able to control fires.

The green leaves contain some water content with which the ignition temperature of leaves increases and they do not catch fire easily while dry leaves have no water so they catch fire easily.

A goldsmith uses the outer zone (non-luminous zone) of candle flame to melt gold and silver because it is hottest zone and has more temperature.
 

Total mass of fuel = 4.5 kg
Total heat produced = 180,000 kJ

Therefore,

Heat produce by burning 1 kg of fuel = 180,000 kJ/4.5 kg = 40,000 kJ/kg.

So, calorific value of fuel = 40,000 kJ/kg.

Combustion is a chemical process in which a substance reacts with oxygen and gives out energy during the process in the form of either heat or light or both. Rusting is defined as the slow decay of iron metal in presence of air and moisture. This process produce no heat and no light. Also it is very slow process. The process of rusting cannot be called combustion because in this process no heat and light is produced. Due to this reason iron is not considered as combustibIe substance.

The water heated by Ramesh will get heated in a shorter time because he kept his beaker near the hottest zone of the flame. On other hand Abida kept the beaker in the yellow part of the candle flame, which is less hotter than blue part.

The substances which provide heat and light are called fuels.

CNG stands for Compressed Natural Gas.

The substances that undergo combustion are called combustible substances. It may be solid liquid and gas.

The lowest temperature at which a substance catches fire is called its ignition temperature.

Phuljhari requires some heating before it sparks to attain its ignition temperature.

LPG has lower ignition temperature.

The substances which have very low ignition temperature and can catch fire easily with a flame are called inflammable substances.For example methane gas, LPG, etc.

Petrol, alcohol and LPG etc.

Sand or soil and carbon dioxide.

The principle of fire extinguisher is cut off the contact between the fuel and oxygen to control Fire.

Water should not be used for extinguishing oil fires because oil is lighter than water. oil forms a thin layer over the surface of water and flows with the water, thereby spreading fire to larger areas.

In this method, the supply of air is cut off by forming a foam at the surface of fire.

A flame is the shining zone in which a combustible gaseous material undergoes combustion producing heat and light.

Blue colour.

Yellow flame.

The substances which vaporise during burning gives flame.example, kerosene oil and molten wax.

Kerosene oil and molten wax.

The substances which vapourise during burning, give flames. Charcoal does not vaporise, so it does not produce a flame.

Non-luminous zone.

Heating up of the earth's atmosphere due to trapping up of the infrared radiation by carbon dioxide in the atmosphere is called global warming. 

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Carbon tetra chloride fire extinguisher.

Foam type fire extinguisher.

Increasing amount of carbon dioxide is the cause of global warming.

The melting point of gold and silver is very high therfore goldsmiths need maximum temperature. Hence goldsmith use outermost part of the flame because it is the hottest part of flame.

The substances in which combustion takes place are called combustible substances. For example : wood, paper, coal.

The substances in which no combustion takes place are called non-combustible substances. For example : Glass, iron nail. 

A yellow flame comes about when there is insufficient supply of oxygen. It is a luminous flame. In this case the hydrocarbons do not burn completely, the temperature of the flame is lower than that of the blue flame. The yellow flame also leaves behind a black soot or residue.

On rubbing the match stick in the rough surface, the friction converts this work into heat. The heat raises the temperature of the chemical present on the match stick head to its ignition temperature. Due to this the chemical substance catches fire and the match stick starts burning.

When fire brigade arrives, it pours water on the fire. Water cools the combustible material, so that its temperature is brought below its ignition temperature. They cut-off electricity and remove the combustible material from surrounding. This prevents the fire from spreading. Water vapour also surrounds the combustible material, helping in the cutting off the supply of air. So the fire is extinguished.

Wood does not burn with a burning matchstick because the ignition temperature of the wood is high and matchstick burns out soon. It needs to he heated for a longer period so, it does not burn with burning matchstick.

The empty glass catches fire easily, but the glass with water does not because when heated the heat produced is absorbed by the water. Thus, its temperature is lowered and it does not catch fire.

Water is poured on burning wood to extinguish fire because water absorbs large amount of heat and the temperature of wood falls below its ignition temperature and fire is blown out.

Outermost Zone : It is blue coloured because complete combustion takes place in this part due to sufficient amount of oxygen. It is the hottest part of the flame.

Middle Zone : The colour of the middle zone is yellow because incomplete combustion takes place in this part with the lack of oxygen. It is less hot part than outer part of the flame.

Water helps in extinguishing the fire in two ways :

(i) It brings the temperature of the combustible material below its ignition temperature. As a result, the fire gets extinguished.

(ii) The water vapour formed due to the heat of fire cuts to supply of air. Due to the absence of air (supporter of combustion), the fire gets extinguished.

 

The wick in a spirit lamp does not burn while the spirit in it burns because:

(i) The ignition temperature of wick is higher than that of spirit.

(ii) The heat produced by the burning of spirit is quickly transferred to the surroundings and, therefore, the temperature of wick remains below its ignition temperature.

(i) Incomplete combustion of a fuel produces a poisonous gas called carbon monoxide and unburnt carbon particles which cause pollution. 

(ii) Incomplete combustion of a fuel produces less heat and hence leads to the wastage of fuel.

Most of the, vehicles on the road and industries emit out gases like oxides of nitrogen and sulphur. Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to formmore acidic pollutants, known as acid rain.

In case of fire due to oils, water is not useful as it is lighter than oil and settles down below the oil particles. In such cases, sand or soil should be used. They cut off the supply of air to the fire and it is put off.

Carbon dioxide covers the surface of the burning substance and act as blanket and hence does not allow the oxygen to reach the substance. In absence of oxygen the fire gets put off.

No, the petrol will not catch fire. This is because in a closed tank full of petrol, there is no air to support the combustion of petrol. In the absence of supporter of combustion, petrol cannot burn or catch fire.

When paper is heated alone, its ignition temperature is reached quickly due to which it catches fire easily. When the paper wrapped around an aluminium pipe is heated, then the heat supplied to paper is transferred to aluminium pipe by conduction. Due to the continuous transfer of heat from paper to aluminium pipe, the paper does not got heated to its ignition temperature quickly and hence does not catch fire easily.

(a) Sodium hydrogen carbonate.

(b) The heat of fire decomposes sodium bicarbonate to produce carbon dioxide gas. This carbon dioxide covers the fire like a blanket and cuts off supply of air to the burning substance. Due to this the fire gets extinguished.

(c) Potassium hydrogen carbonate.

(a) Spontaneous combustion.

(c) Rapid combustion.

(c) Explosive combustion (or Explosion).

(d) Spontaneous combustion.

Ordinary water conducts electricity to some extent. So, when water is thrown over the burning electrical appliance or burning electric wires, it can give electric shock to the person involved in fire-fighting. Thus, water should not be used to extinguish fires caused by electricity. 

At airports or petrol pumps the probability of breaking fire from oils are more. In case of fires due to oils, soda-acid fire extinguisher is not successful, because it uses water to cool the combustible substance. But water is lighter than oil and settles down the oil particles and fire cannot be controlled. In such case Hydrocarbon fire extinguisher is used which has turkey red oil, which causes the foaming of carbon dioxide under pressure. The foam covers the surface of burning substance and cuts off the supply of air to control fire.

Combustion is of three types :

(i) Rapid combustion : When gases burn rapidly to produce heat and light it is called rapid combustion.

(ii) Spontaneous combustion : When any material like phosphorus burns on its own without any apparent cause, it is called spontaneous combustion.

(iii) Explosion : When combustion takes place with sudden release of heat and light and a large amount of gas in form of bang, it is called explosion as in case of crackers and bombs.

Fuels are very useful for us. But there are certain disadvantages of burning fuels.

(i) Fuels like wood, coal, petroleum, etc. which are carbon fuels generally release unburnt carbon particles which are quite harmful. They cause various respiratory and skin diseases.

(ii) In most of the cases carbon dioxide is released during burning of fuels. This carbon dioxide is released in environment. This carbon increases the  temperature of the earth, which is called global warming.

(iii) Incomplete combustion of fuels release many poisonous gases like carbon monoxide in the environment. These poisonous gases create air pollution, such air if inhaled can be fatal.

(iv) Combustion of coal and diesel generates sulphur dioxide. It is a corrosive gas and it causes suffocation.

(v) Burning of petrol releases nitrogen oxides. These oxides of nitrogen and sulphur dioxide combine with rain water to cause acid rain. it affect aquatic life and natural environment.

Carbon dioxide molecules have the ability to absorb infra red radiations reflected from the earth. This trapping of infra red rays by the carbon dioxide molecules increases the temperature of the atmosphere. This is also known as green house effect.

Two gases that are responsible for causing green house effect are :

(i) Carbon dioxide

(ii) Methane

The burning of fossil fuels like coal and petroleum products causes a lot of air pollution.

(i) The burning of coal produces air pollution like carbon dioxide, carbon monoxide sulphur dioxide and smoke. Though carbon dioxide is not a poisonous gas but excess of carbon dioxide in the atmosphere can lead to undesirable changes in climate due to increased green house effect. Carbon monoxide and sulphur dioxide are poisonous gases which affect the health of human beings. Sulphur dioxide also leads to acid rain.

(ii) The major air pollutants produced by burning of petrol in automobiles are carbon dioxide, sulphur dioxide, nitrogen oxides and some load compounds. All these pollutants released from the exhausts of the engine into the air are harmful.

ignition

spontaneous

combustion

flame

combustible

luminous

ignition

outermost

hotter

fire extinguisher

D.

LPG

A.

catch fire easily

B.

only glow

B.

luminous zone

D.

oxides of sulphur and nitrogen

B.

water

B.

kJ/kg

C.

slow oxidation

C.

inner Zone

(b) The flame flickers and produces smoke because there is no air in case.

(c) In case flame finally goes off because air is not available.

(a) Kerosene lamp



b) Candle




c) Bunsen burner




(ii)
Colour of kerosene lamp flame: yellow,
Colour of candle flame: yellow,
Colour of bunsen burner flame: blue.