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Class 8th-Science: Chapter 6-Combustion and flame

Combustion:

In general Definition:

A process of burning something is known as combustion.

In science:

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

Examples of Combustion:

(i) Burning of magnesium ribbon

Mg + O2 —> MgO + Heat + light

(ii) Burning of Coal

C + O2 —> CO2 + Heat + light

Flame:

The hot, glowing and visible part of a fire is known as flame.

Fuel:

Any combustible substance that produces heat (sometime light also) after combustion is known as fuel.

Example of fuel:

Coal, Charcoal, petrol, diesel, wood, CNG(Compressed Natural Gas), LPG(Liquified Petroleum Gas) etc.

“Types of substance on the bases of Combustible:-“

(i) Combustible

A combustible substance is that substance which can burn in air.

Example

Coal, wood, paper etc.

(ii) Non Combustible

A non-combustible substance is that which cannot burn in air.

Example

Iron, glass, gold, etc.

Table 6.1

Necessary condition under which combustion can take place:

(i) presence of oxygen.

(ii) presence of combustible substance.

(iii) Ignition temperature.

Let us discuss above 3 conditions one by one:-

(i) presence of oxygen:-

from the above activity we can see that without air (oxygen) candle flame goes off.

=> oxygen is required for combustion.

(ii) presence of combustible substance:-

for combustion a combustible substance is required because a incombustible substance cannot burn even if there is lot of oxygen.

(iii) Ignition temperature:-

The lowest temperature at which a substance catches fire is known as ignition temperature.

Different materials have different ignition temperature.

For Example:-

A paper catches fire as soon as a burning matchstic is brought near it because it has low ignition temp.

But a piece of wood do not catches fire as soon as we bring a burning matchstick it take a lot time also we use paper of kerosene oil to start fire in wood because wood has higher ignition temperature.

Inflammable substances: the substances which have very low ignition temperature and can easily eatch fire with a flame, are called inflammable substances.

6.2 How Do We Control Fire?

We know following are three essential requirements for producing fire

(i) air (to supply oxygen)

(ii) Fuel (Combustible substance)

(iii) ignition temperature or (heat to raise the temperature above ignition temperature)

So

“Fire can be controlled by removing one or more of these above requirements.”

so fire can be extinguished in 3 ways:-

(i) By cutting off air supply to burning substance.

(ii) By removing fuel(combustible substance)

(iii) By removing heat, so that temp can be lowered down below ignition temperature

Fire Extinguisher:

Anything that can be used to control or stop fire is known as Fire Extinguisher.

For Example: (i) water, (ii) CO2 Carbondioxide atc.

Water

Water is the most common fire extinguisher.

” How water controls fire “

water can control fire because

(i) it has a coding effect

(ii) water makes a layer between burning substance and air (oxygen) this layer acts as a barrier between burning substance and oxygen.

Note.

(i) water is not suitable for Fires involving oil and petrol because water is havier than oil it sinks below the oil keeps burning on top.

(ii) it is also not suitable for Fire on electrical equipment because water can conduct electricity.

6.3 Types of Combustion

Combustion can be classified into three main types

(i) Rapid combustion

(ii) Spontaneous combustion

(iii) Explosion

(i) Rapid combustion:-

The combustion in which a substance burns rapidly and produces heat and light is known as rapid combustion.

for example:

(i) When we bring burning lighter near a gas stone it burns rapidly and gives heat and light.

(ii) The burning of kerosene oil in kerosene stone.

(ii) Spontaneous combustion:

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

For Example

(i) Burning of phosphorus at room temp.

(ii) Spontaneous combustion of coal dust.

(iii) some forest fires are spontaneous due to heat of the sun.

(iii) Explosion:

A very fast combustion in which a large amount of heat, light and sound are produced is known as explosion or explosive combustion.

Example

Fireworks which we explode during Diwali etc.

6.4 Flame

The hot glowing and visible part of a fire is known as flame.

We know that:- colours of candle flame and LPG flame is different but why?

Different flames give diff colours because complete combustion gives blue flame and partial combustion gives yellow flame.

LPG requires less oxygen as compaired to candle that why LPG burns completly in environment but for candle environmentle oxygen is not sufficient and it burns partially.

why some substances give flame while burning and some not.

The substances which vapourise during burning give flame.

Examples: kerosene oil, molten wax etc.

The substance which do not vapourise do not give flame.

different zones of candle flame:

6.6 What is a fuel:

Any combustible substance that produces heat (sometimes light also) after combustion i known as fuel.

For Example:

Coal, wood, charcoal, petrol, diesel, LPG, CNG etc.

Characteriatics of good fuel:-

A good fuel is that which have following properties

(i) It should be easily available.

(ii) It should be cheap.

(iii) It burns easily in air at moderate rate.

(iv) It produces a large amount of heat(high calorific value).

(v) produces less pollution.

(vi) its storage and transportation is easy.

6.7 Fuel Efficiency:

Fuel efficiency will be greater if its calorific value is high.

Calorific value:

The amount of heat energy produced on complete combustion of 1 Kg of fuel is called calorific value.

It is expressed in kilojoule per Kg (KJ/Kg)

Harmful Effects of Burning of fuel:-

(i) unburnt carbon particle can cause respiratory diseases like “Asthma”.

(ii) Incomplete combustion of carbon fuels gives carbon monoxide gas. Which is very poisonous gas.

(iii) Carbondioxide produced during combustion is a major cause of global warming.

(iv) Burning Coal and diexel releases sulphur dioxide (SO2) gas. Which is suffocating as.

Also petrol engines give off oxides of nitrogen oxides of sulphur and nitrogen dissolve in rain water and form acid such rain is called acid rain.

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