Class 8 Chemistry - Combustion And Flame

Estimate the maximum possible temperature of a Bunsen burnur flame.

The hottest part of the Bunsen flame, which is found just above the tip of the primary flame, reaches about $$1,500 ℃$$.

_________ is a substance that reacts with oxygen and burns to release energy.

Fuel is a substance that reacts with oxygen and burns to release energy.

This is the standard definition of fuel.

Explain the principle on which a fire extinguisher works.

There are a few basic principles on which fire extinguisher works:

Smouldering (removal of oxygen)

Cooling (reducing heat or removal)

Starvation (removal of fuel from scene of fire)

Draw a labelled diagram of a candle flame and explain what happens in each zone.

Inner most zone: Black colour

Unburnt part

Least hot

Middle zone: Partial combustion occurs

yellow colour

moderately hot

Outer zone: Complete combustion

Blue colour

Hottest part

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When fewer amounts of oxygen are present, carbon combines with it to give ________ gas.

Carbon and oxygen can combine to form two gases.

When combustion of carbon is complete, i.e. in the presence of plenty of air, the product is mainly carbon dioxide ($$CO_2$$). Sources of carbon include; coal, coke, charcoal.

When combustion of carbon is incomplete, i.e. there is a limited supply of air, only half as much oxygen adds to the carbon, and instead, you form carbon monoxide ($$CO$$). The relative amount of $$CO$$ produced depends on the efficiency of combustion.

Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why?

In glass jar B the candle remains lighted for a longer time as it contains oxygen gas which helps in the combustion of candle but this candle puts off in glass jar A with carbon dioxide gas as it does not help in combustion, so the candle of jar A will flicker out.

A fuel should have high ________ (Calorific /Octane) value.

Write your answer in form of 1 or 2.

Fuel should have a high calorific value. As calorific value is more it produces more energy with which engine can run more efficiently.

The amount of heat released by a unit weight or unit volume of a fuel substance during complete combustion is defined as the calorific value of the fuel.

Light a candle. Keep one hand above the flame and one hand on the side of the flame. Do your hands feel equally hot?

$$\text{No}$$.

The hand above the flame gets more heat due to convection. On the sides, there is no convection and so air does not feel as hot as at the top.

Give reasons why respiration is called slow combustion?

Respiration is called slow combustion because in the respiration process food materials are burned slowly and at a lower temperature to produce the products of ordinary combustion like $$CO_2$$, water and energy.

Is it possible to heat a liquid or gas from above?

No, the liquid or gas is heated by convection in which heat is transferred upwards, neither downwards nor sideways. If we heat a liquid or gas from above then, only the molecules on the surface of the liquid or gas are getting heated up.

Prateek took a piece of burning charcoal and collected the gas evolved in a test tube. Write down word equations of all the reactions taking place in this process?

When charcoal burns in the air it produces an orange glow on the charcoal layer. Charcoal (i.e. Carbon) burns with a gaseous oxygen molecule to produce carbon dioxide as the product.

The chemical reaction can be written as,
$$C_{(s)} + O_{{2}{(g)}} \rightarrow CO_{{2}{(g)}}$$

Carbon dioxide is used in fire extinguishers. Why?

Fire is caused by the three elements i.e. fuel, oxygen, and heat. If one of the three elements is removed fire will stop. Carbon dioxide removes one element, i.e oxygen.

$$CO_2$$, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. Therefore, $$CO_2$$ is used in fire extinguishers.

What is ignition temperature?

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

Make a labelled diagram of a candle flame.

How $$CO_2$$ is able to control fire? Explain.

$$CO_2$$ is a non-combustible gas and it is also non-supporter of combustion. It extinguishes fire in two ways:

1. Since it is heavier than oxygen, it covers the fire like a blanket and cuts off the contact between oxygen and fuel.

2. In cylinders, $$CO_2$$ is kept in the liquid form. When released, it expands enormously and cools down. This brings down the temperature of the fuel, which helps in controlling the fire.

List conditions under which combustion can take place.

The conditions required for combustion to take place are:
1. Presence of a fuel
2. Air (or oxygen)
3. Ignition temperature (minimum temperature at which a substance catches fire)

Fill in the blanks:
(a) Burning of wood and coal causes ___ of air.
(b) A liquid fuel, used in homes is _ .
(c) Fuel must be heated to its _ before it starts burning.
(d) Fire produced by oil cannot be controlled by ____ .

(a) Burning of wood and coal causes $$\text{pollution}$$ of air.

(b) A liquid fuel used in homes is $$kerosene$$.

(d) Fire produced by oil cannot be controlled by $$\text{water}$$.

Explain how the use of CNG in automobiles has reduced pollution in our cities.

Combustion of fuels like petroleum causes the addition of many harmful pollutants like un-burnt carbon, particulate matter and many fume gases enter the air and cause respiratory diseases.

Compressed Natural Gas (CNG) produces these harmful products in very less quantity. It is a comparatively cleaner fuel. Therefore, the use of CNG has reduced pollution in our cities.

Compare LPG and wood as fuels.

Which zone of a flame does a goldsmith use for melting gold and silver and why?

A goldsmith uses the outer part of the candle flame for melting gold and silver. This is because, in this zone of flame, the temperature is the highest helping these metals to melt easily.

Why, water cannot be used to extinguish petrol fires?

As water is heavier than petrol, therefore, slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguish the fire. The latter is true if a small quantity of water is poured. Thus the petrol fire cannot be extinguished by water.

A fuel should have high ________ value.

Calorific value of a fuel may be defined as the amount of heat produced on complete burning of 1 gm of fuel. If the fuel has higher calorific value it will have tendency to produce more heat. The fuels with less calorific value tend to burn inefficiently thus causing lots of exhaust and air-pollution. So the correct answer is $$\text{'calorific'}$$.

Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick of the candle flame. Ramesh kept the beaker on the outermost part of the flame. Whose water will get heated in a shorter time?

The water in Ramesh's beaker will heat up in a shorter time. This is because the outermost zone of a flame is the hottest zone in which the complete combustion of fuel is taking place, while the yellow zone (in which Abida had kept the beaker) is less hot as this is the middle zone which is the zone of partial combustion so it is less hot than the outermost zone.

Name the process of converting crude oil obtained from seeds into commercially useful fuel.

The process is known as transesterification. It is used for manufacturing bio fuel. It is an alternative fuel for diesel. It is used for the road transport vehicles.

When the material P mined from the earth is processed, a black solid Q is produced which consists mainly of carbon. When another material R obtained from trees is heated in an insufficient supply of air, it produces another solid fuel S which also consists mainly of carbon. Name P, Q, R and S.

$$P$$: Coal (mined from the Earth)

$$Q$$: Coke (solid, consists mainly of carbon, produced during coal processing)

$$R$$: Wood (obtained from trees)

$$S$$: Charcoal (consists mainly of carbon)

Why CNG is considered a better fuel?

CNG (Compressed Natural Gas) is considered a better fuel because the harmful products produced when burning it are very small in quantity compared to those produced by other fuels like petrol and diesel.

Explain the different zones of a candle flame.

A candle flame consists of three different zones. Each zone has different temperatures.

The first zone is the outer zone: Due to complete combustion, the outer zone is blue. This zone is the hottest when compared to the other zones. This blue coloured zone is the non-luminous part of the flame.
The second zone is the middle zone: The yellow coloured region which is the bright part of the flame is the middle zone. It is moderately hot and partial combustion of fuel takes place.
The third zone is inner zone: The inner zone is dark and mostly consists of unburnt wax vapours.

Explain how $$CO_2$$ is able to control fire.

$$CO_2$$ being hevier than $$O_2$$ covers the fire like blanket also brings down the temperature, Since contact between the fuel and oxygen cut off, and thus the fire come under control.

Compare the usefulness of solid, liquid and gaseous fuels.

Solid fuels:

1. Easy to transport

2. Convenient to store

3. Cost of production is low

Liquid fuels:

1. Require less storage space

2. Clean burning fuels and do not leave ashes after burning

3. High calorific value

Gaseous fuels:

1. Clean burning fuel

2. Do not leave ash or smoke after burning

3. Free from impurities

What is the difference between combustible and non-combustible substances?

Substances that burn or catch fire easily are called combustible substances. Paper, clothes, wood, LPG are all combustible substances. Without these substances, a fire cannot be started.

A non-combustible substance is a substance that is not capable of ignition or burning. Examples of non - combustible materials include portland cement concrete, gypsum concrete or magnesite.

A wax candle burns with yellow flame. Give reason.

Candle burns with a yellow flame because of burning of low energy fuel in a low oxygen content. The incomplete combustion of wax produces small unburnt carbon particles which rise in flame, get heated and give yellow and luminous flame.

How are fuels classified based on their physical state? Give at least two examples of each.

On the basis of physical state, fuels are classified into solid, liquid, and gaseous fuels.

$$Examples$$

Solid: Wood, cowdung cakes and agricultural waste.

Liquid: Kerosene, diesel and petrol.

Gaseous: Biogas, CNG and LPG.

A wet cotton handkerchief soaked in alcohol will not catch fire even it is set to fire. Why?

A wet cotton handkerchief soaked in alcohol will not catch fire because the heat is transferred from the hot handkerchief to water. This will prevent the burning of handkerchiefs.

Give any two advantages of hydrogen as a fuel.

Advantages of hydrogen as a fuel:

The cleanest fuel available when combusted produces no carbon residues.
Highly efficient when compared to other energy sources.

Why is it more difficult to burn some combustible substances than others?

Different combustible substances have different ignition temperatures. The lowest temperature at which a substance catches fire is called its ignition temperature. Substances having high ignition temperature take a longer time to burn as opposed to those with low ignition temperatures.

For instance, wood has a high ignition temperature, so it does not catch fire on its own and needs to be heated for a longer time in order for it to catch fire. On the other hand, a matchstick burns quickly because of the low ignition temperature of the chemical substances on its tip.

During a fire accident, when you are not supported with any fire extinguisher, it is advisable to evacuate the building and close the doors and windows. Why do you think we should close the doors and windows? (Hint : Recall the conditions necessary for combustion)

For combustion/reaction necessary condition is full availabilty of oxygen. Therefore during fire accidents , it is advisable to evacuate the building and close the doors and windows to cut down supply of oxygen.

How can you make a flame thrower with orange?

Inside the cells of citrus fruits peels, inflammable oils and other carbon compounds are present. When peel of orange is passed, the cells are squeezed and the oil is pushed out. When these flammable oils hit the flame, they burn and create and amazing effect as can be seen in the film. It's like a small initially flamethrower.

During an explosion , a large amount of ____ is given out.

During an explosion, a large amount of Energy/ Heat is given out. When nuclear fusion and fission reaction, takes place high amount of energy is given out. In Sun's core, nuclear fusion reaction takes place.

What are the characteristics of a good fuel?

A good fuel should have the following properties:-

i) High calorific values.

ii) Moderate ignition temperature.

iii) Low moisture content.

iv) Low on the non-combustible matter.

v) Low cost.

vi) Easy to store and transport.

Why are gaseous fuels better than the solid and liquid ones?

The calorific value of gaseous fuels is more than solid and liquid fuels.
When a gaseous fuel burns, it gives off a great deal of energy.
Unlike other solid and liquid fuels it does not leaves any ash.
Gaseous fuels produce very little air pollution.
Gaseous fuel is easier to store and transport when compared to solid and liquid fuels.

Due to the above reasons, gaseous fuel is always better than solid and liquid fuels.

Give example of spontaneous combustion.

The ignition of a substance or body from the rapid oxidation of its own constituents without heat from any external source is known as spontaneous combustion.

Example: Phosphorous and sulfur start burning instantaneously at room temperature.

Which zone of candle flame is uses by gold smith to melt gold and silver and why?

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What is spontaneous combustion?

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

combustion

State two disadvantages of incomplete combustion.

Disadvantages of incomplete combustion:
(i) It leads to the formation of soot which is nothing but unburnt carbon which pollutes the atmosphere, blackens cooking utensils.
(ii) It leads to the formation of an extremely poisonous gas called carbon monoxide.

Write characteristics of an ideal fuel.

The Characteristics of an ideal fuel are discussed as follows:

1. The fuel should have a high calorific value.
2. The fuel should have proper ignition temperature.
3. The rate of combustion should be balanced and moderate.
4. The content of non-volatile substances should be as low as possible.
5. There should be no poisonous or residue byproducts on combustion.
6. The fuel should be easily available in plenty.
7. The fuel should be available at a low cost.
8. There should be convenience in transporting the fuel and should be easily storable.

Write the type of heat of combustion.

Types of the heat of combustion:

(1) Complete combustion: This combustion occurs in an unlimited supply of air, oxygen in particular.

(2) Incomplete combustion: It takes place when the air is in limited supply.

(3) Rapid combustion: Rapid energy needs external heat energy for the reaction to occur.

(4) Spontaneous combustion: This means that it requires no external energy for combustion to start.

(5) Explosive combustion: It happens when the reaction occurs very rapidly.

What is indicated by the blue flame of a bunsen burner?

A blue flame colour means complete combustion. This indicates that the gas is being burned efficiently without any unburned and wasted substances.

What type of fuels:
(a) burn with a flame?
(b) burn without a flame?

(a) Fuels which vaporise on heating, burn with a flame.
(b) Fuels which do not vaporise on heating burn without a flame.

Write short notes on fire extinguisher.

A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations.

Fire extinguishers contain different chemicals, depending on the application. Handheld extinguishers, which are commonly sold at hardware stores for use in the kitchen or garage, are pressurized with nitrogen or carbon dioxide $$(CO_2)$$ to propel a stream of a fire-squelching agent to the fire. The active material may be a powder such as potassium bicarbonate $$(KHCO_3)$$, liquid water, and evaporating fluorocarbon, or the propelling agent itself.

Write short notes on gaseous fuels.

Gaseous fuels are a number of fuels that under ordinary conditions are gases. Such gases are sources of potential heat energy or light energy that can be readily transmitted and distributed through pipes from the point of origin directly to the place of consumption.
Examples include LPG and CNG.
Gaseous fuels are used as domestic fuels and in gas burners like the Bunsen burner used in laboratories.
They may also be used in gas heaters, camping stoves, and even to power vehicles, as they have high calorific value.

Why are holes provided at the bottom of a bunsen burner?

Holes let the supply of air to be adjusted for complete combustion.

Mrs. Anita Pandey observed that her cooking utensils are becoming black in colour and the flame of her gas stove is yellowish in colour. She complains about it in the gas company and got the gas stove repaired.
Answer the following question based on the above situation:
How is this problem harmful for our environment?

The yellow flame leads to incomplete combustion. This problem is harmful to the environment as incomplete combustion results in the formation of oxides which are major pollutants of our environment.

Give reason for the following observation:
(a) Air holes of a gas burner have to be adjusted when the heated vessels get blackened by the flame.

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

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

Slags is a non-combustible material.

Glass is a non-combustible material.

Seldie is a combustible substance.

Diesel is a combustible substance.

Mrs. Anita Pandey observed that her cooking utensils are becoming black in colour and the flame of her gas stove is yellowish in colour. She complains about it in the gas company and got the gas stove repaired.
Answer the following question based on the above situation:
What values are promoted by Mrs. Pandey regarding this situation?

The values promoted by Mrs.Pandey regarding this situation are careful observation, decision making and prudency.

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

The $$\text {lowest}$$ temperature at which a substance catches fire is called its $$\text {ignition}$$ temperature.

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

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

When the clothes of a person catch , the person is covered with a __ to extinguish fire.

Answer: fire, blanket

When the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire. The blanket cuts off the supply of oxygen, which is a supporter of combustion. In absence of oxygen, the fire is extinguished.

Chittsmack does not burn by itself.

Matchstick does not burn by itself.

Some substances on combustion produce thea and htlig.

Some substances on combustion produce heat and light.

Wood, paper, CNG are __________ substances.

Wood, paper, CNG are $$\text{combustible}$$ substances.

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

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

Match the following for the flame of a candle
Column A Column B (zone) Column C (colour)
(a) hottest part
(i) innermost zone
(x) blue of
unburnt wax vapours
(b) moderately hot (ii) middle zone of (y) black partial
combustion
(c) least hot (iii) outer zone of (z) yellow

(a) (iii) (x)
(b) (ii) (z)
(c) (i) (y)

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.

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

Boojho wants to separate the following materials as combustible and non-combustible. Can you help him? Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, candle, wood.

Given materials can be classified into combustible and non-combustible as,

Combustible	Non-Combustible
Straw, Paper, Charcoal, wood, candle, cardboard	Chalk, Copper coin, Stone, iron rod, Glass

Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.

A Flame consist of three zones:
1. The first zone is called the Outer zone:
Due to complete combustion, the outer zone is blue. This zone is the hottest in temperature when compared to the other zones. This blue coloured zone is the non-luminous part of the flame.

2. The second zone is called the Middle zone:

The yellow colour region which is the bright part of the flame is the middle zone. It is moderately hot and partial combustion of fuel takes place.

3. The third zone is called as Inner zone:

The least hot region of the flame is the inner zone. This inner zone is black due to the presence of unburnt wax vapours.

Anu should boil the water in the hottest zone of the flame, which is the non-luminous zone or the outermost zone of the flame.

If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen
burner flame, what will you observe? Will it produce a flame?

The iron wire becomes red and glows. It doesn't produce a flame.

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

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

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

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

What do you understand by fuel efficiency?

Fuel efficiency is the amount of energy produced on burning 1 kg of complete fuel. This is determined by caloric value. This is expressed in kJ/kg

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ____________.

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its Calorific Value

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

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

Cracker on ignition produces sound. Why?

Crackers when ignited produce sound, heat, and light. This process is known as an explosion. The explosion is a chemical reaction that occurs due to the presence of different gases.

You are provided with three watch glasses that contain milk, petrol, and mustard oil, respectively. Suppose you have brought a burning candle near these materials, which material(s) will catch fire instantly and why?

If a burning candle is brought near milk, petrol, and mustard oil, petrol will burn instantly as it has a very low ignition temperature.

The calorific values of petrol and CNG are 45,000 kJ/kg and 50,000 kJ/kg, respectively. If you have a vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

The calorific value of compressed natural gas is higher than petrol. So, CNG will be a good preference. Also, it releases fewer environmental pollutants and also costs less than petrol.

Complete the crossword Figure with the help of the clues:

Across
Non-metal which catches fire if exposed to air (10)
The lowest temperature at which a substance catches fire is called its (8)
The most common fire extinguisher. (5)

Down-
A chemical process in which a substance reacts with oxygen to give off heat. (10)
Petrol is used as in automobiles. (4)
It is as hard as stone and black in color. (4)

Across-

1-Phosphorus

3- Ignition

5-Water

Down-

2-Combustion

4-Fuel

6-Coal

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Fill in the blanks:
The supporter of combustion in air is ______.

The supporter of combustion in air is $$\underline {oxygen}$$.

Oxygen is present in air and it supports combustion. Without it, combustion will not occur even if a fuel is present and it is ignited.

Give two examples each for a solid, liquid, and gaseous fuel along with some important uses.

For Solid
Fuels we can take coal and wood. Coal is used in factories and industries while wood is mostly used in homes.

For Liquid
Petrol and kerosene are examples of liquid fuel. Kerosene is mostly used in stoves to cook food while petrol is used in vehicles.

For Gases

CNG and LPG are considered gaseous fuels. CNG is used to drive vehicles while LPG is used in vehicles and homes also.

Give short answers:
What is a fuel?

Fuel: The substances which on burning produce heat energy are called fuels. For example, charcoal, wood, diesel, petrol, etc.

A good fuel is one that produces a large amount of heat, burns at moderate temperature, is readily available, cheap, and does not leave behind any undesirable substances.

Give two examples of fuel.

A material that is burnt to produce heat is called fuel.

Examples:

L.P.G.
Petrol
Coal

Complete the crossword Figure with the help of the clues:
Across
Non-metal which catches fire if exposed to air. (10)
The lowest temperature at which a substance catches fire is called its .(8)
The most common fire extinguisher. (5)

Down-
A chemical process in which a substance reacts with oxygen to give off heat. (10)
Petrol is used as in automobiles.(4)
This fossil fuel is hard as stone and black in colour. (4)

Across-

1-Phosphorus

3- Ignition

5-Water

Down-

2-Combustion

4-Fuel

6-Coal

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Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Because of the following reasons, wood is not a fuel of choice:

(i) It has a high ignition temperature, so starting a fire in wood is a little tough.

(ii) Burning wood produces many harmful gases such as carbon monoxide and carbon dioxide. These gases are toxic and also cause air pollution.

(iii) To use wood as a fuel, we would have to cut trees. Trees are very essential for living beings and so, cutting them is not encouraged.

What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire?

Below are the essential components to produce fire-

(i) Fuel- To give combustion

(ii)Air-Most essential for letting the fire burn

(iii) Heat- To provide ignition temperature

The fire extinguisher has carbon dioxide in it, which helps to reduce the flow of the air between the fuel and air. It happens because $$\mathrm{CO}_2$$ is heavier than oxygen and settles between air and oxygen and helps in controlling the fire.

Which fuel has the highest calorific value?

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$$ (kJ/kg)$$.

Hydrogen has the highest calorific value. It is approximately $$150000\ kJ/kg$$.

Why are carbon and its compounds are used as fuels for most applications?

Carbon in all its allotropic forms burns in oxygen to give carbon dioxide along with the release of heat and light. Most carbon compounds also release a large amount of heat and light on burning. Hence carbon and its compounds are used as fuels for most applications.

What are the limitations of using hydrogen as a fuel?

Limitations:

Hydrogen is a gas that burns explosively in the air.
The distribution and storing of hydrogen gas are not easy.
Hydrogen energy is expensive.

A candle brought near the mouth of a jar containing hydrogen gas starts burning but is extinguished when pushed inside the jar.

Hydrogen is a combustible gas but it does not support the combustion process. When the candle is brought near the mouth of a jar containing Hydrogen with the help of Oxygen in the atmosphere. When pushing the candle inside the jar oxygen supply gets cut off hence the fire gets extinguished.

What is combustion?

The burning of a substance in oxygen is called combustion. For example, the carbon in charcoal or coal undergoes combustion in the presence of oxygen to give carbon dioxide.

$$Carbon + Oxygen \rightarrow Carbon\ dioxide$$

Why is water not used to put off the fire caused due to electricity?

Water is a good conductor of electricity. The person gets an electric shock when it comes in contact with the water which is used to extinguish the fire. Due to this reason, water is not used to put off the fire caused due to electricity.

Complete the statement by filling in the blank/s with the correct words.
Water is added to the atmosphere by _and .

Water is added to the atmosphere by burning fossil fuels and respiration. During combustion or burning, carbon from fossil fuels combines with oxygen in the air to form carbon dioxide and water vapor. Aerobic respiration. Carried out by the vast majority of organisms, this involves oxygen. The by-products of the reaction are water and carbon dioxide both of which are eliminated as waste products.

................ gas is necessary for combustion.

Oxygen supports the chemical processes that occur during combustion. Therefore, $$\text{Oxygen}$$ gas is necessary for combustion.

Define combustion. List the substance undergoing complete and incomplete combustion.

A chemical reaction that occurs when oxygen combines with other substances to produce heat and usually light is called combustion. For combustion of any substance, the following three things are necessary.

1. Oxygen

2. Particular or optimum temperature.

3. Fuel

When a fuel undergoes complete combustion, it releases the maximum amount of energy from the fuel being reacted and produces carbon dioxide and water as the product. Incomplete combustion is also a reaction between oxygen and, fuel but the products are carbon monoxide, water, and soot.

What will you do if a fire breaks out in the house of your neighbor?

First of all, we will contact a fire emergency number as soon as possible. Until the fire brigade arrives, if we see a person's clothes on fire, we can wrap him in a blanket and try to put out the fire. We can also use water to put out fire in places where there's no electrical equipment or oil.

What did you see? Make note of the observation made.
Keep it for some time as it is. Make note of the observation made.

A bright white flame/light is seen when a magnesium strip is placed over the flame of a lighted candle. This happens because magnesium reacts with oxygen.

Wood will take more time as compared to paper. For combustion of any substance that substance should attain a definite temperature which is higher for the wood so, it takes time to burn.

A substance catches fire at a definite temperature which is known as an ignition point. If the temperature of a paper cup does not reach its ignition point, it won't start burning. It will burn when the paper cup reaches its ignition temperature.

Give two examples of two solid fuels.

Coal and wood are examples of solid fuels. Coal is used in industries and factories and is also used to generate electricity.
Wood is used mostly in rural homes as fuel.

Why do we have holes in the formation of lantern / primus/gas stove?

We have holes in the formation of lantern/gas stove because when air moves upwards it needs to be replaced with the freshh air otherwise the light will go off.

Explain the three zones of a candle flame.

Three zones of a candle flame are as follows:

1. The outermost zone, also known as the non-luminous zone, is the zone of complete combustion and has a blue flame. It is the hottest zone of the candle flame.

2. The middle zone, also known as the luminous zone, is the zone that is moderately hot with limited oxygen supply and has a yellow flame.

3. The innermost zone is the zone is formed just around the wick of the candle flame as the candle burns. It is also known as the dark zone.

Prepare a list of combustible substances.

Straw from a broom: Burns
Match stick: Burns
Paper: Burns
Cardboard: Burns
Nail: Does not burn
Straw: Burns
Glass: Does not burn
Stone: Does not burn

Prepare a list of some combustible and non combustible substances.

Combustible substances:

Methanol, Cotton, Biogas, Wood, Charcoal.

Non-combustible substances:

Carbon dioxide, Stone, Portland cement, Glass, Water.

In which places do we find 'fire extinguishers'?

We can find fire extinguishers at various offices, at corridors and stairways of a building, in kitchens and garages, in laboratories, in warehouses.

Name the types of fossil fuels that are found in nature.

Three types of fossil fuels are found in nature are:

Mineral coal
Petroleum (Mineral oil)
Natural gas

For cooking, why are L.P.G./P.N.G. better than the other fuels?

L.N.G/P.N.G are better due to the following reasons:

1. L.N.G/P.N.G is a clean fuel (comparable with biogas); it releases fewer pollutants than any other fuel.

2. L.N.G/P.N.G gives off lower greenhouse gas emissions than other fuels.

3. L.N.G/P.N.G is easy to light and provides instant heat after lighting.

4. L.N.G/P.N.G cooks fast, is convenient to use, easy to store and transport.

Is the heat produced by the combustion of different fuels the same? Discuss

No, the heat produced by the combustion of different fuels is not the same because their calorific value is not the same.

Observe the fast and complete combustion of flat paper. Understand that the combustion of a crumbled paper makes more smoke.
List the conditions for the complete combustion.

Conditions for complete combustion.

Dryness
Fast evaporation
Materials should reach a certain temperature that is required for combustion.

Observe the fast and complete combustion of flat paper. Understand that the combustion of a crumbled paper makes more smoke.
How does partial combustion cause environmental pollution?

Besides the fuel loss and time loss, carbon monoxide produced during partial combustion.

Name the gas produced during the complete combustion of fuels.

The major gas that is produced during the complete combustion of fuels is carbon dioxide.

Answer the following questions:
(a) Which is the main constituent of LNG?
(b) What are the uses of CNG?

LNG is $$85$$ to $$ 95 \%$$ methane, along with a few percent ethane, even less propane, and butane, and trace amounts of nitrogen.

CNG is primarily used as a fuel for buses and trucks that return to base for refueling regularly. CNG is also used as a fuel for cars.

Name the main constituent of CNG.

The main component of the CNG (Compressed Natural Gas) is methane.

Write the full form of LNG and CNG.

LNG - Liquefied Natural Gas CNG - Compressed Natural Gas

Liquefied natural gas, or LNG, is natural gas in its liquid form.

CNG is stored under high pressure and is used for power generation.

Take three papers of the same size. Keep one stretched. Crumble the next. Make the third paper wet using water. Burn each of them over a candle flame using pincers. Compare the burning of each.

Stretched paper	Crumbled paper	Wet paper
$$\bullet$$ Burns very well	$$\bullet$$ Burns slowly	$$\bullet$$ Burns very slowly.
$$\bullet$$ Completely becomes ash.	$$\bullet$$ Some ash is produced
	$$\bullet$$ Some part is left unburnt.

What are the drawbacks of partial combustion?

Drawbacks of partial combustion is:

• Fuel loss

• Wastage of time
• Economic loss

• Atmospheric pollution
• More smoke is produced
• $$CO_2, CO$$ (carbon monoxide) are produced as byproducts.

Filling in the blanks.
Inflammable tendency is higher in ________ fibres.

The inflammable tendency is higher in $$synthetic$$ fibre.

They easily catch fire and melts. So while working in places where combustion takes place synthetic fibres should not be worn.

Write down any two examples of combustible substances.

Those substances which can burn are called combustible substances.

For Example Cloth, straw, cooking gas, kerosene oil, coal, charcoal, wood, leaves, paper, wax, hydrogen gas, ethanol, methane.

Flammable materials are combustible materials that ignite easily at ambient temperatures.

Write down three essential conditions for the combustion of fuels.

Three essential conditions for the combustion of fuels are:

There must be Fuel to burn.
There must be Air to supply oxygen.
There must be Heat (ignition temperature) to start and continue the combustion process.

It is advisable to keep stirring a heap of waste while burning it. How does the stirring help the combustion? Explain.

Stirring makes sure sufficient oxygen is made available to burn the waste material, which increases the rate of combustion.

List out some of the hazards of incomplete combustion of fuels.

The incomplete combustion of fuels produces carbon monoxide, which is a poisonous and potentially fatal gas for humans. Also, incomplete combustion causes a lot of wastage of fuels.

Write down disadvantages of partial combustion.

Partial combustion pollutes the air by producing excess smoke, soot, carbon monoxide. It also leads to wastage of fuel.

Making a volcano.Heap up some ammonium dichromate powder on a tile. Deposit on it the chemical present on a match stick, and ignite. Write down the changes happening there.
Change in colour:
Change in amount:
Exchange of energy: ____

Change in colour- orange-red Change in quantity - decreases Energy exchange - exothermic

Give reasons
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

(a) Water is a conductor of electricity, so it can easily conduct electric current and cause danger of electric shocks or short-circuits. Therefore, water can not be used to control the fire involving electrical equipment.
(b) LPG is a better domestic fuel as it does not produce smoke and unburnt carbon particles, which cause respiratory problems.
(c) Paper by itself catches fire easily because it has low ignition temperature but when wrapped around an aluminium pipe its temperature is lowered due to aluminium metal absorbing the heat supplied to paper. So it does not catch fire.

Write any four characteristic of an ideal fuel.

Characteristic of an ideal fuel:
(1) The fuel should have a high calorific value.
(2) The fuel should have a proper ignition temperature.
(3) The rate of combustion should be balanced and moderate.
(4) The content of non-volatile substances should be as low as possible.

I: When a hydrocarbon is combusted in air, carbon monoxide is the major product
II: Air contains much more nitrogen than oxygen.

When a hydrocarbon is combusted in air, carbon monoxide is the major product.

$$CH_4+2O_2\rightarrow CO_2+2H_2O$$

Air contains much more nitrogen than oxygen.

Both the statements are correct and the statement II is correct explanation for statement 1

So the answer would be TTCE.

What kinds of fire extinguishers are most often used for extinguishing an electrical fire?

$$CO_2$$ extinguishers are predominantly used for electrical fire risks and are usually the main fire extinguisher type provided in computer server rooms. They also put out Class $$B$$ fires (flammable liquids, such as paint and petroleum).

What is the use of oxyacetylene flame?

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Why do goldsmiths use outermost flame to burn gold and silver?

A flame mainly contains three parts; the outermost part, the middle part and the innermost part. The outer part of the flame provides the maximum heat, and it produces a blue color.

Goldsmiths melt gold and silver with the use of outermost part of the flame because gold and silver have a high melting point and they require a maximum amount of heat.

The outermost part of the flame contains a high amount of heat, and it also allows the particles to undergo complete combustion.

What is meant by rapid combustion ? Give an example.

Rapid Combustion is a type of combustion in which a large amount of heat and energy is released.

Example:- Burning of matchstick, combustion of LPG

Mrs. Anita Pandey observed that her cooking utensils are becoming black in colour and the flame of her gas stove is yellowish in colour. She complains about it in the gas company and got the gas stove repaired.
Answer the following question based on the above situation:
What can be the reason for this sooty flame?

One of the reason for sooty(yellowish) colour can be that the inlets for air in the stove get blocked which leads to incomplete combustion of fuel giving yellowish colour to flame.

Explain combustion reaction with example.

Combustion reaction: Carbon compounds burn in oxygen to produce carbon dioxide along with release of large amount of heat and light.

$$ Carbon + Oxygen \rightarrow Carbon\ dioxide + Heat \ and \ light $$

Why does coal not burn by itself in air, but once initiated by a flame, continues to burn?

Activation energy for the combustion reaction is very high and is not available at room temperature.

On applying flame, to a part of the coal and air in contact with the flame absorb heat which provides the necessary activation energy and combustion starts. The heat liberated further provide activation energy for the combustion to continue.

Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire, he poured water to extinguish the fire. Do you think this action was suitable? If yes, why? If not, why not? In such a condition what should Manu have done?

Oil is lighter than water. When one will pour water on burning oil, it'll settle down at the bottom of the vessel. But, pouring soil or sand may help to extinguish the fire as they reduce the flow of air. The particles will not let the air pass through the oil and fire will stop. This happens because the flow of oxygen stops. And without an oxygen burning fire is not possible.

Certain gases are given below.
Hydrogen, oxygen, nitrogen, chlorine, carbon dioxide

a)Which is a combustible gas?
b)Which gas support combustion?
c)Which gas has the tendency to limit combustion?
d)Which gas resists combustion?

a) Hydrogen
b) Oxygen
c) Nitrogen
d) Carbon dioxide

Light a candle and observe its flame.
- How many parts are visible?
- Which are they?

A candle flame consists of three-zone. The first zone is called the outer zone, which is blue in colour and, it is the hottest. The second zone is called the middle zone. The zone is yellow coloured and is the brightest part of the flame. The third zone is called the inner zone, which is black in colour due to the presence of unburnt wax vapour. This is the least hot region.

Forest fire produces a lot of air pollution. Write in brief about the reasons for forest fires.

Following reasons can be considered-

(i) Dry grass in the forest when gets very dry, high temperature helps it to burn which causes a forest fire.

(ii)Lightning from the sky can also be a reason.

(iii) Sometimes when villagers use fire to ward off animals, it burns the dry grass and results in forest fire.

(iv) Friction between bamboos produces heat and burns the bamboo which also results in a forest fire.

Column A	Column B (zone)	Column C (colour)
(a) hottest part	(i) innermost zone	(x) blue of unburnt wax vapours
(b) moderately hot	(ii) middle zone of	(y) black partial combustion
(c) least hot	(iii) outer zone of	(z) yellow

Combustion And Flame - Class 8 Chemistry - Extra Questions