Combustion and Flame

The process of burning of substances in air with the release of heat and light is called combustion.

Combustion

A substance that Burns in air to produce heat and light is called Combustible Substance

Eg: Wood, Petrol

A substance that does not burn in air is called a non combustible substance

Eg: Stone, Metal

During combustion substances like coal or charcoal react with oxygen in air to form carbon dioxide.

Carbon + Oxygen= Carbon dioxide + Heat Hydrogen reacts with oxygen to form water. Hydrogen + Oxygen = Water + Heat Methane reacts with oxygen which burns to

produce carbon dioxide and water. Methane + Oxygen = Carbon dioxide +

Water + Heat

Combustion is a chemical change

Combustion takes place only if there is

Combustible Substance Air( Oxygen) Temperature

Conditions Necessary for Combustion

Ignition Temperature

A substance has to heated to a certain minimum temperature before it can catch a fire. This temperature is different in different objects. In petroleum the ignition temperature is very less.

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

http://www.youtube.com/watch?v=9d6tseOh21Y

http://videos.howstuffworks.com/discovery/29392-assignment-discovery-combustion-video.htm

Videos:

TYPES OF COMBUSTION

Rapid combustion – occurs when a substance burns rapidly to produce heat and light. Ex- LPG , paper , wick of an oil lamp.

Spontaneous combustion – occurs when a substance starts burning suddenly even without being heated. Ex- white phosphorus whose ignition temperature is 35°C

Explosion – is a combustion reaction that occurs suddenly when a material is heated liberating a large amount of heat , light and sound.Ex- firecrackers.

EXTINGUISHING A FIRE

An uncontrolled fire can be disastrous – it can destroy life and property in minutes.

For the prevention of an uncontrolled fire each city has fire stations equipped with fire-fighting equipment . Water and fire extinguishers are used to put out fires.

EXTINGUISHING A FIRE

. Remove the combustible material.

. Reduce temperature to the ignition temperature of the combustible substance.

. Cut off air supply.

TYPES OF FIRE EXTINGUISHERS

Dry powder fire extinguisher – it contains baking soda (NaHCO3).When this mixture is thrown over a fire , baking soda decomposes to release carbon di oxide which extinguishes the fire.

Soda-acid fire extinguisher – it consists of a small glass bottle filled with sulphuric acid that is supported in a strong iron vessel containing sodium bicarbonate. On striking the knob, the bottle containing acid breaks and carbon di oxide is liberated by the action of the acid on baking soda.

TYPES OF FIRE EXTINGUISHERS

Foam fire extinguisher – in this a solution of aluminium is taken in the glass bottle instead of sulphuric acid. It releases a foam of carbon di oxide that surround the burning substance and cuts off its air supply. This type of fire extinguisher is especially used to extinguish oil fires.

VIDEO

http://www.youtube.com/watch?v=uUpuR4BoAzQ

FUELS

Combustible materials such as wood,coal,gas or oil which are burnt to produce heat or power are known as fuels. Solid fuels are easy to store but difficult

to use. Gaseous fuels are difficult to store; they

have to be compressed at high pressures and are stored in sealed cylinders or tankers. These can be dangerous if a leak develops because the fuel can spread very fast. However gaseous fuels are easy to use.

Liquid fuels can be stored easily and are easy to use.

WHAT IS CALORIFIC VALUE OF FUEL?

The amount of heat liberated when a unit mass of fuel is burnt gives us a measure of efficiency of a fuel.This is referred to as calorific value of fuel.Calorific value of a fuel can be defined as the amount of heat liberated when one kilogram of the fuel is completely burnt in sufficient supply of oxygen.note: the higher the calorific value the better the fuel.The SI unit of calorific value is joule per kilogram (J/kg). However, the unit kilojoule per kilogram (Kj/g).

CHARACTERISTICS OF A GOOD FUEL It should have a high calorific value. It ignition temperature should be neither too low nor

too high. It should be cheap and readily available. It should be easy and safe to transport, handle and

store. It should not cause air pollution upon burning. It should not leave behind solid residue upon

burning.No fuel really meets all the characteristics listedhere. Natural gas is the closest to an ideal fuel.Considerable research is being done to produce efficient fuel cells that use hydrogen and oxygen to produce electricity.

Harmful byproducts formed by combustion

Carbon monoxide and soot are also produced during the combustion of other fuels. They are highly undesirable byproducts of combustion. The fine particles of soot are called suspended particulate matter (spm).They are dangerous pollutants of air and can cause respiratory problems.

Some fuels like diesel & coal contain sulphur, which changes to sulphur dioxide (SO2) upon combustion. Engines give out oxides of nitrogen.

Both sulphur dioxide and the oxides of Nitrogen are undesirable and harmful byproducts of combustion.Besides being poisonous themselves, they dissolve in rainwater to form acids and cause acid rain.

The carbon dioxide released during combustion can result in an increased percentage of gas in air. An atmosphere with excess carbon dioxide traps more heat- this is called greenhouse effect. Many scientists believe that this can result in the average temperature of the earth. This phenomenon is called global warming. Global warming can lead to the melting of polar ice causing flooding of low lying costal lands and also cause climate changes.

https://www.youtube.com/watch?v=dP-tg4atr5M&feature=player_detailpage

CANDLE FLAME

When a candle is lit the wax around the wick melts. This wax rises up through the wick and vapourises.

A flame has four different zones –1. The zone of non combustion-The dark zone2.The zone of incomplete combustion –The luminous zone3. The zone of complete combustion-The non luminous zone4.Blue zone

Zone of non combustion-The dark zone

This is the innermost zone and contains unburnt wax vapours. Combustion does not take place over here as

there is no air supply. It is also the least hot part of the flame.

Zone of incomplete combustion-The luminous zone

In this zone, the wax vapours do not burn completely as the supply of air is inadequate for complete

combustion. The wax breaks up into carbon and hydrogen. The unburnt carbon particles glow and

impart a pale yellow colour to the flame. This is the middle part of the flame and is moderately hot.

Zone of complete combustion-The non luminous zone

This is the outermost and hottest part of the flame. Since adequate air is available of carbon and

hydrogen to form carbon dioxide and water vapour

The blue zone

This part lies at the bottom of the flame. The blue colour is due to the burning of carbon monoxide produced due to the incomplete combustion of

carbon particles.

Summary The process of burning of substances in air

with the evolution of heat and light is known as combustion.

A substance that burns in air to produce heat and light is called a combustible substance. A substance that does not burn in air is called a non-combustible.

The conditions that must be met for combustion are: presence of a combustible substance, presence of oxygen(air) and attainment of ignition temperature.

The minimum temperature at which a substance catches fire is called it’s ignition temperature.

The three types of combustion are rapid combustion, spontaneous combustion and explosion.

A fire can be extinguished by removing the combustible substance, reducing the temperature to below the ignition temperature or by cutting out the air supply.

Fire extinguishers mainly use carbon dioxide to extinguish fires.

The important criteria for the selection of a fuel are physical state, calorific value, ignition temperature, combustion residue, emissions and cost.

The candle flame has four zones dark zone, luminous zone, non luminous zone and blue zone.

The luminous zone produces light due to the glowing of carbon particles. The non-luminous zone is the hottest.

Calorific value of a fuel an be defined as the amount of heat liberated when one kilo-gram of the fuel is completely burnt in an sufficient supply of oxygen.

Natural gas is the closest to an ideal fuel. Some harmful substances given off

during combustion are suspended particulate matter (SPM), carbon dioxide, carbon monoxide, sulphur dioxide and oxides of nitrogen.

Fill in the blanks1.The chemical reaction in which a fuel is burned to release heat energy is called __________ .Ans. Combustion2.Wood, paper, kerosene and LPG are examples of ________ .Ans. Combustible substances 3.The _________ temperature is the minimum temperature at which a fuel can catch fire.4.Combustion of wood can be classified as __________ combustion.Ans. Rapid5.A soda-acid type fire extinguisher gives out _________ to extinguish a fire.Ans. Carbon dioxide6._________ extinguisher is used to extinguish a fire.Ans. Foam fire

7.The zone that produces light in a candle flame is called the _______ zone.Ans. Luminous8.The amount of heat evolved, in kilojoules, when one kilogram of fuel is burnt completely is called ________ value of the fuel.9._________ is caused by sulphur dioxide and oxides of nitrogen.Ans. Acid rain10. ______ _______ can cause flooding of low lying coastal land.Ans. Global warming11.A substance burnt to obtain heat energy is called a _______ .Ans. Fuel

12.A foam fire extinguisher gives out a foam of _________ .Ans. Carbon dioxide13.The glowing ________ particles in the luminous zone of a candle flame make it look yellow.Ans. Unburnt carbon particles.