Living Science 2020 2021 for Class 8 Science Chapter 6 - Combustion And Flame

Answer:

Yes, when a hydrocarbon fuel is burnt, it gives out carbon dioxide and water vapours. Hydrocarbons are made up of carbon and hydrogen. When they are burnt in the presence of oxygen, carbon reacts with oxygen to form carbon dioxide and hydrogen reacts with oxygen to form water vapours. 

Example: When we burn methane gas in the presence of a sufficient amount of oxygen, it gives carbon dioxide gas and water vapours.

${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$

Answer:

The lowest temperature at which a combustible substance starts burning in the presence of air is called ignition temperature.
The substance that has very low ignition temperature and hence, can catch fire easily are called inflammable substances.

Answer:

To extinguish a fire, we need to cut the supply of oxygen from the combustible substance. Other than removing the combustible substance, we can use the following methods to extinguish the fire: 

(i) We can use water to extinguish the small and short-range fire.
(ii) We can also use a CO₂ extinguisher to put out the heavy fire or fire started due to oils. CO₂ extinguisher uses carbon dioxide gas to cut the supply of oxygen from the combustible substance.

Answer:

Carbon dioxide is heavier than air and it settles down and cuts the supply of oxygen, which extinguishes the air.

Answer:

No, the innermost part of the candle is not the hottest.
The outermost zone of a candle's flame is the hottest zone of the flame and is also a zone of complete combustion. The innermost zone is the least hot zone.

Answer:

Hydrogen gives off the maximum heat when 1 kg of it is burnt. Amount of heat produced when 1 kg of fuel is burnt is called its calorific value. Unit of calorific value is kilojoule per kilogram(kJ/kg)

Answer:

Yes, it is good fuel.
Reasons for good fuel:
1. It has a very high calorific value.
2. It is non-polluting in nature.
3. It has moderate ignition temperature.
 

Answer:

During the combustion of hydrocarbons, carbon dioxide gas is released which is one of the main components of global warming. Increased level of carbon dioxide traps the heat of sun inside the earth atmosphere, which results in increasing the temperature of the earth.

Answer:

If a fuel while burning does not get sufficient air, then carbon monoxide is released which is a very harmful pollutant and can cause death if breathed in.

Answer:

(a) low ignition temperature and high calorific value.
Good fuels should have low ignition temperature (the lowest temperature at which a substance catches fire) and high calorific value (produce a large quantity of heat).

Answer:

(b) one kilogram of the fuel is completely burnt.

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 kilojoule per kg (kJ/kg).

Answer:

(b) carbon monoxide

Incomplete combustion of fuels produces carbon monoxide gas. It is dangerous to burn coal in a closed room because it produces carbon monoxide gas, which can kill persons sleeping in that room.

Answer:

(d) coal

Sulphur is found as a trace element in coal. When coal is burnt, sulphur combines with oxygen to form sulphur dioxide (SO₂). SO₂ does not react with most chemicals found in the atmosphere and can travel long distances. Further, it reacts with water vapors in the atmosphere and causes acid rains.

Answer:

(d) any one of these

Job of a fire extinguisher is to cut off the supply of air, or to bring down the temperature of the fuel, or both. In some cases, a flame can also be extinguished by just removing the combustible substance.

Answer:

(a) non-luminous zone.

Non-luminous zone is a zone of complete combustion. It is the outermost and the hottest zone of the flame with plenty of oxygen available from the surrounding atmospheric air.

Answer:

(a) SPM

Suspended Particulate Matter (SPM) consists of mist, dust, fumes and smoke. Common chemicals that constitute SPM are nickel, arsenic and lead. When these particles are inhaled, they tend to accumulate in the lungs and cause respiratory problems and damage the lungs.

Answer:

(c) hydrogen

The calorific value of a fuel is expressed in kilojoule per kg (kJ/kg).
The calorific values of coal, CNG, hydrogen and petrol are as follows:
Coal: 25,000$-$33,000 kJ/kg
CNG: 50,000 kJ/kg,
Hydrogen: 150,000 kJ/kg
Petrol: 45,000 kJ/kg

Answer:

The process of burning of substances with the evolution of heat and light is called combustion.

Answer:

Most fuels are made up of carbon and hydrogen.

Answer:

 (a) Carbon dioxide gas is formed, when a fuel burns in sufficient supply of air.
 (b) Carbon monoxide gas is formed, when a fuel burns in insufficient supply of air.

Answer:

False.
Different combustible substances catch fire at different temperatures.
A combustible substance cannot catch fire or burn if its temperature is lower than its ignition temperature.

Answer:

False.
Substances that have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

Answer:

True.
The combustion in which a material suddenly bursts into flames without the application of any apparent cause is called spontaneous combustion.
Example: Phosphorous burns in air at room temperature.

Answer:

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

Answer:

False.
Water cannot be used if electrical equipment is on fire because it can conduct electricity and cause harm.
It is also not suitable to use water to extinguish fires involving inflammable materials like oil and petrol because water is heavier than oil and sinks below it. On the other hand, oil continues to burn on the top.

Answer:

In a soda-acid type fire extinguisher, sodium hydrogen carbonate (NaHCO₃) reacts with sulphuric acid to give out carbon dioxide.

Answer:

The central zone in a candle flame is the zone of no combustion.

Answer:

The amount of heat liberated when one kilogram of a fuel is completely burnt in sufficient supply of oxygen is called calorific value of the fuel.

Answer:

The heat liberated by a fuel is expressed in kilojoule per kg (kJ/kg).

Answer:

False.

Fuels should have a moderately low ignition temperature. A moderately low ignition temperature makes a fuel useful.

Answer:

The fine particles released on burning carbon-containing fuels are called SPM (Suspended Particulate Matter).

Answer:

Combustion is a chemical process wherein a substance reacts with oxygen to produce energy in the form of heat or light or both. It is an oxidation process because it involves the burning of a fuel, completely or partially in the presence of air. However, the presence of oxygen is not must for oxidation to take place. Elements such as magnesium also burn in an atmosphere of nitrogen or chlorine to form magnesium nitride or magnesium chloride, respectively.

Answer:

Answer:

It is more difficult to burn some combustible substances than others because different substances catch fire at different temperatures. Some substances have a low ignition temperature; therefore, they can catch fire more easily than others. Ignition temperature plays a crucial role in determining the combustibility of different substances.

Answer:

Combustion of a fuel requires:

• Presence of a combustible substance such as coal, natural gas, wood, etc
• Presence of air to supply oxygen
• Availability of heat to raise the temperature of the fuel beyond its ignition temperature

Answer:

Both kerosene and coal are fuels, but we need extra care to store kerosene oil because the ignition temperature of kerosene oil is lower than that of coal. If kerosene oil is heated a little, it will catch fire easily. On the other hand, coal needs more heat to catch fire.

Answer:

If a fuel has an ignition temperature lower than the average room temperature, combustion will occur suddenly on its own. Such a combustion is known as spontaneous combustion.

Answer:

Combustion that occurs very rapidly with the evolution of large amount of heat and light accompanied by loud sound is called explosion. Large amount of gas is also released during explosion. Hydrogen gas is an example of one of the explosive gases.

Answer:

Water is not suitable for extinguishing a fire in an electrical appliance. If water is poured on an electrical fire, it can conduct electricity and cause electric shocks to the person trying to extinguish the fire.

Answer:

(a) limited supply of air
If the supply of air is limited, it can result in incomplete combustion, which, in turn, produces carbon monoxide gas instead of carbon dioxide gas.
 

Answer:

Different methods can be used to extinguish fires. They are as follows:

• Cooling off the burning material.

• Excluding oxygen from fire

• Removing fuel from fire

• Using a flame inhibitor

Answer:

Water cools the combustible material and the temperature is brought below the ignition temperature of the combustible material . This prevents fire from spreading. Water vapours also surround the combustible material and help in cutting off the air supply.
Water can be used when solid materials like paper, wood etc. are on fire. However, it is not suitable for extinguishing fire in electrical appliances. If water is poured on an electrical fire, it can conduct electricity and cause electric shock​s to the person trying to extinguish the fire. Moreover, water should not be used in fires involving hot cooking oil or fat because it can spread such fires.

Answer:

Carbon dioxide is heavier than air and does not support combustion. Carbon dioxide extinguishes fire by displacing oxygen or by taking away oxygen from the fire triangle. Carbon dioxide is also very cold when it comes out of the extinguisher. Thus, it reduces the temperature of the fuel as well.

Answer:

The amount of heat liberated, when one kg of a fuel is completely burnt in sufficient supply of oxygen, is called the calorific value of the fuel.
 
Hydrogen is not used as fuel despite the highest calorific value because:
 

• It is lighter than air; thus, it is not easily available.
• It cannot be used in liquid form because its melting point is $-$253⁰C.
• Hydrogen gas in external air may spontaneously ignite.
• Hydrogen fire, while being extremely hot, is almost invisible; thus, it can lead to accidental burns.
• Hydrogen poses unique challenges due to its ease of leaking, low-energy ignition and wide range of combustible fuel-air mixtures.

Answer:

We know that matter can exist in three states $-$ solid, liquid and gas.
On this basis, fuels can be classified as follows:
Solid fuels: Examples: Wood, coal, cattle-dung cakes, bagasse (sugarcane from which juice has been extracted), etc.
Liquid fuels: Examples: Kerosene, liquefied petroleum gas (LPG), fuel oil, petrol, diesel, etc.
Gaseous fuels: Examples: Natural gas, coal gas, water gas, producer gas, biogas, acetylene, etc.

Answer:

Combustion of most of the fuels releases carbon dioxide and nitrogen dioxide in the environment. Increase in their concentration in the air causes global warming.

Answer:

A soda-acid type fire extinguisher contains concentrated solution of sodium hydrogen carbonate (NaHCO₃) in a small cylinder fitted with a glass bottle filled with sulphuric acid. When fire breaks out, the cylinder is hit on a hard surface to break the bottle. Once the bottle breaks, the acid reacts with sodium hydrogen carbonate to evolve carbon dioxide and water in the form of vapours. Carbon dioxide, being a non-supporter of combustion, forms a blanket over the fire and extinguishes it. Water, because of its cooling effect, brings down the temperature below the ignition temperature to check burning.

Answer:

Different zones of a candle flame



As shown in the above diagram, a candle flame has four zones. They are as follows:
 
Innermost zone: The flame starting from the wick has a small blue colour zone. Here, carbon monoxide produced by the incomplete combustion of carbon particles burns to give blue color.
 
Central zone: It lies above the innermost zone, where no combustion takes place. It is not a hot zone. It consists of unburnt vaporised particles of the wax.
 
Luminous zone:  It is luminous and consists a pale yellow glow above the central zone. It is formed by incomplete combustion of wax vapors in limited supply of oxygenated air.
 
Non-luminous zone:  It is the outermost and the hottest zone of the flame above the luminous flame. It is formed by complete combustion of wax vapours with plenty of oxygen available from the surrounding atmospheric air.

Answer:

Characteristics of a good fuel are as follows:
1. High calorific value: In order to have high fuel efficiency, a good fuel must have high calorific value.
2. Moderate ignition temperature: A good fuel should have moderate ignition temperature.
3. Burns easily at a moderate rate: A good fuel should burn easily in sufficient air at a moderate rate.
4. Low content of non-combustible material: A good fuel should have low content of non-combustible material.
5. Easy to store and handle: A good fuel should be easy to transport and store at a low cost and without causing any harm.
6. Lower moisture content: A good fuel should have less moisture content.
7. Fuels on combustion should not leave behind any harmful product such as CO, SO₂, etc.
8. A good fuel should have low volatile matter content.

Answer:

Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants, which cause respiratory diseases such as asthma.
Incomplete combustion of these fuels gives carbon monoxide gas, which is a very poisonous gas.
Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air causes global warming.
Use of fossil fuels damages the environment with the realease of toxic gases like oxides of nitrogen and sulphur. Such gases are responsible for acid rain, which, in turn, is very harmful for crops, buildings and soil. Moreover, nitric oxide and CFCs are also responsible for depletion of ozone layer.

Answer:

Gases burn with a flame because they vaporise on burning. Wood initially burns with a flame because of the presence volatile gases in it. When all the gases escape, it glows without a flame.

Answer:

Coal meets nearly all the conditions of being a good fuel. It is economically cheap, easily available and produce no harmful odour on burning. It can be transported easily and its combustion can be controlled. Therefore, coal can be considered as a good fuel.

Answer:

The fuel that burns without giving out water vapour is coke, which is a pure form of carbon. It has high carbon content and low amount of hydrocarbon.
Water vapour is released from a fuel if it contains hydrogen. However, coke has very low hydrogen content and hence, water vapour is not given out. It simply burns giving carbon dioxide gas.

    $\mathrm{C}+O_2\underset{}{\overset{}{\to }}{\mathrm{CO}}_2$

Answer:

The head of a safety match contains antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorous. When the match is struck against the rubbing surface, some red phosphorous gets converted into white phosphorous. This immediately reacts with potassium chlorate in the matchstick head to produce heat to ignite antimony trisulphide and start the combustion.

Answer:

Yellow flame indicates that the flame tube is starved of oxygen.
There can be various reasons for less oxygen like air holes of the burner may be clogged causing the fuel to burn insufficiently due to insufficient supply of air, or less amount of fuel is available; i.e., the cylinder is about to get empty.

Answer:

Hot gas is generally less dense than cool gas. Flame is a very hot gas. As the flame burns, it takes the oxygen from the atmospheric air and heats the surrounding atmospheric gases. Therefore, the hot air around the flame is pushed up because it is less dense. As the air around the flame is pushed up, the air around the flame is drawn towards the flame to take the space of the lighter gases, which are pushed up. Meanwhile, the surrounding cold air is pulled down by gravity and the flame is pressurised all around by the atmospheric gases. This elongates the flame upward. 

Answer:

Candle is a solid cylindrical block of wax with a wick embedded in it.
a) A candle starts melting when it comes in contact with the heat of the flame. The change in shape and size of the candle takes place. This indicates a physical change from solid state to liquid state. It is possible to get the wax back after it melts.
b) A wax candle acts as a fuel when we burn the candle. It is basically carbon. The wax oxidises (burns) in the flame to yield water and carbon dioxide, which dissipate in the air around the candle in a reaction, which also yields light and heat.  The unburnt carbon is deposited as a black substance called soot. This signifies a chemical change. It is not possible to get the wax back after it burns.

Answer:

For a fire to burn, all three elements $-$ heat, fuel and oxygen $-$ of the fire triangle must be present. If any of these is cut off, fire will be put out. When a fire blanket completely surrounds the person whose clothes caught fire, it seals the person’s body surface around the fire and cuts off the oxygen supply to the fire; thereby, putting out the fire . Thus, fire blanket acts as a fire extinguisher and stops the fire from spreading.

Answer:

During hot summers, due to hot environment, the speed of hot air is high. This causes the dry leaves and trees to collide each other. This collision results in friction, which, in turn, produces a spark. Dry leaves easily catch those sparks and start burning. The burned leaves further spread due to air and cause the forest fire to expand.

Answer:

The ignition temperature of green leaves is higher than that of the dry leaves because they contain moisture in them. Therefore, its easier to burn dry leaves but not green leaves.

Answer:

1. The ignition temperature of the wood is very high. Matchstick is not able to produce enough heat and raise the temperature to the ignition temperature of the wood.  So, wood does not set on fire by the matchstick.

2. Kerosene has a much lower ignition temperature than wood. When kerosene starts burning, due to the heat produced the temperature increases and equals to the ignition temperature of the wood, and the wood starts burning too.

3. Green leaves contain water molecules, so they can't be burn easily but dry leaves have no water content so they can be burnt easily.

4. Petroleum jelly is a mixture of hydrocarbons. 
Two properties of petroleum jelly used to light the fire are:
(i) Petroleum jelly contains carbon, and carbon can be easily burnt.
(ii) Petroleum jellies have a high calorific value.

 

Answer:

The yellow flame coming out of the LPG cylinder because a sufficient amount of oxygen is not available for the burning of LPG.
Yellow flame shows the release of carbon monoxide which is poisonous gas and blue flame also releases a lot of unburnt carbon particles which make utensils black.
So, after cleaning of stove burner, a sufficient amount of oxygen is available which doesn't lead to the production of carbon monoxide and release less unburnt carbon. Due to which blackening of utensils stops.
Values shown by Namita is that she is well aware of environmental problems and health risks caused due to the production of carbon monoxide.