CBSE NCERTCLASS 8TH COMBUSTION AND FLAME (self study notes)

A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

The substance that undergoes combustion is said to be combustible.

The substances that do not burn on being exposed to flame are called  non-combustible  substances.

like stone, glass, iron nails, etc.

Ignition temperature:  The lowest temperature  at which a fuel catches fi re is called  its  ignition temperature.

Types of combustion : Combustion can be of different types.  It can be spontaneous, rapid, slow or incomplete.

Spontaneous combustion: combustion reaction that occurs without the help of any external heat is called
spontaneous combustion e.g. When white phosphorus is exposed to air at room temperature, it

catches fi re immediately; even without  being lit by a match stick

Rapid combustion : combustion reaction that occurs with the help of any external heat is called Rapid combustion e.g. Bursting of fire crackers, burning of camphor. Magnesium wire in air, gas in a burner and kerosene  in a stove

Slow combustion : Combustion that takes place at a very slow rate is called slow combustion.

During this type of combustion low heat and light are produced. Food oxidized  in our body to release energy is an example of slow combustion.

Glucose + Oxygen   ---------à Carbon dioxide + water + energy

Incomplete combustion : Combustion takes place in the presence of oxygen. If the supply of oxygen

is insufficient, then combustion will be incomplete. This is called incomplete combustion. Carbon forms carbon monoxide when it undergoes incomplete combustion.

                 Carbon +   Oxygen   -------à carbon monoxide

FIRE CONTROL

Fire can be controlled and extinguished by

1.  Removing any combustible materials near the region of fi re.

2.  Cutting off the supply of air by  using sand or blanket.

3.  Bringing down the ignition temperature by using water

Water should not be used for oil fire. Why?

 Oil being lighter, fl oats, spreads and causes severe damage. So, oil fi re should be extinguished by using substances called foamite.

How does fires caused by electrical appliances or installations extinguished?

Fires caused by electrical appliances or installations extinguished by using solid carbon dioxide

or carbon tetrachloride. The risk of electrical shock is too great, if water is used.

FLAME AND ITS STRUCTURE

Different parts of a candle fl ame

Zone of non-combustion: This is the dark zone that lies around the wick. It contains  un burnt gas particles. No combustion takes place here as no oxygen is available.

Zone of partial combustion: In this zone, the hydrocarbons present in the oil gas decompose

into free carbon and hydrogen. The un burnt carbon particles impart a pale yellow colour to the flame. This is the luminous part of the flame.

Zone of complete combustion (blue) : This is the non-luminous thin zone  of the flame. It is the outermost hottest region in the fl ame that is invisible. Here, carbon and hydrogen are completely oxidized to Carbondioxide and water vapour.

Carbon + Oxygen ------à  Carbon monoxide (blue flame)

Carbon monoxide + Oxygen---------à Carbon dioxide + Water (vapour)

Fuel: Any substance that can be burnt or otherwise consumed to produce heat energy is called a fuel. Wood, natural gas, petrol, kerosene, diesel, coal, and LPG are commonly used as fuels

Characteristics of a Good Fuel

1.  It should be cheap and readily available.

2. It should be easy to store, transport and handle.

3.  It should not produce toxic fumes or smoke or other harmful products on combustion.

4.  The amount of soot or ash left behind should be minimum.

5.  It should have a high calorific value.

6.  It should have a low ignition temperature.

Calorific Value: The amount of heat energy liberated when 1 kg of the fuel is burnt completely in oxygen is called the calorific value of the fuel.

Types of Fuels

There are three types of fuels. They are solid, liquid, and gaseous fuels

Solid Fuels

Coal, wood, charcoal, coke, and paraffin are some commonly used solid fuels.

The drawbacks of solid fuels are as follows:

1. They have a high ignition temperature.

2. They produce a large amount of residue (soot, ash) after combustion.

3. Their calorific value is low

Liquid Fuels : Petrol, kerosene, and diesel are some commonly used liquid fuels

which are obtained from petroleum (an oily mixture of hydrocarbons in its crude form). Ethyl alcohol is also a liquid fuel. Locomotives, buses, and lorries use diesel as the fuel.

Gaseous Fuels :Gases such as methane, carbon monoxide and hydrogen are combustible. Natural gas, producer gas, coal gas, water gas, LPG (liquefi ed petroleum gas), and biogas (gobar gas) are other examples of gaseous fuels

Why gaseous fuels do are referred over solid and liquid fuels?

.Gaseous fuels are preferred over solid and liquid fuels because of the following advantages.

•  They have a low ignition temperature.

•  They burn completely (complete combustion) and leave no residue (soot, ash, smoke).

•  They are easy and safe to handle, transport, and store.

• They have a high calorific value.

• They are cheap.

Harmful effects of burning of fuel on the environment.

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases such as asthma.

2. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. Because the carbon monoxide gas produced can kill persons sleeping in that room.

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

4. Burning of coal and diesel releases sulphur dioxide. 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