Combustion of flame

What is combustion ?• A chemical process in which a substance

reacts with oxygen to give off heat is called combustion. The substance that undergoes is said to be combustible. It is also called a fuel.

Try this activity• Fix a lighted candle on a table. Put a glass chimney

over the candle and rest it on a few wooden blocks in such a way that air can enter the chimney. Observe what happens to the flame. finally, put a glass plate over the chimney. Watch the flame again. What happens in the three cases? Does the flame flicker off? Does it flicker and give smoke? Does it burn unaffected? Can you infer anything at all about

Ignition temperature• You might have heard that when the clothes of a person catch fire, the person is covered with a blanket to extinguish fire. You must have had an experience of burning a piece of paper. Does it burn when a burning matchstick is brought near it? Can you burn a piece of wood by bringing a lighted matchstick near it? Why do you have to use paper or kerosene oil to start fire in wood or coal? Do these experiences tell you that different substances catch fire at different temperatures?

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

Forest fire• During extreme heat of summer, at some

places dry grasses catch fire. From grasses, it spreads to trees, and very soon the whole forest is on fire . It is very difficult to control such fires.

History of matchstick• The history of the matchstick is very old.

More than five thousand years ago small pieces of pinewood dipped in sulphur were used as matches in ancient Egypt. The modern safety match was developed only about two hundred years ago. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

Inflammable substances

• The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

How to control fire• When a fire brigade arrives, what does it do?

It pours water on the fire .Water cools the combustible material so that its temperature is brought below its ignition temperature. This prevents the fire from spreading. Water vapors also surround the combustible material, helping in cutting off the supply of air. So, the fire is extinguished.

Fire extinguisher• The most common fire extinguisher is water.

But water works only when things like wood and paper are on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. Water is also not suitable for fires involving oil and petrol. Do you recall that water is heavier than oil? So, it sinks below the oil, and oil keeps burning on top

How do we get the supply of carbon dioxide? It can be stored at high pressure as a liquid in cylinders. In what form is the LPG stored in cylinders? When released from the cylinder, CO2 expands enormously in volume and cools down. So, it not only forms a blanket around the fire, it also brings down the temperature of the fuel. That is why it is an excellent fire extinguisher. Another way to get CO2 is to release a lot of dry powder of chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate. Near the fire, these chemicals give off CO2.

Types of combustion• We find that the gas burns rapidly and produces

heat and light. Such combustion is known as rapid combustion.

• The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.

Explosion

• A large amount of gas formed in the reaction is liberated. Such a reaction is called explosion. Explosion can also take place if pressure is applied on the cracker.

Flame • Observe an LPG flame. Can you tell the

colour of the flame. What is the colour of a candle flame?

Structure of a flame

goldsmith• Notice that the portion of the copper wire

just outside the flame gets red hot. Does it indicate that the non-luminous zone of the flame has a high temperature? In fact, this part of the flame is the hottest part Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver

What is fuel?

• The sources of heat energy for domestic and industrial purposes are mainly wood, charcoal, petrol, kerosene, etc. These substances are called fuels

Fuel efficiency

• Suppose you were asked to boil a given quantity of water using cow dung, coal and LPG as fuel. Which fuel would you prefer? The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorif ic value.

Burning of fuels leads to harming products

• Moreover cutting of trees leads to deforestation which is quite harmful to the environment.

• Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.

• Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain .

Made by- Suruchi Behera