Solved examples for class 8 science : Chapter 6

    Combustion and Flame

    Q.1. The condition necessary for combustion of substances is/are
    a) combustible substances
    b) supporter of combustion
    c) heat required for ignition temperature
    d) all of these
    Answer.(d) all of these

    Q.2. Which of the following is not an inflammable substance?
    a) Petrol
    b) LPG
    c) Coal
    d) Kerosene

    Answer. (c) coal

    Q.3. Rusting of iron and respiration are examples of
    a) spontaneous combustion
    b) slow combustion
    c) fast combustion
    d) none of these

    Answer: (b) slow combustion
    Q.4. The hottest part of the flame is
    a) outermost non-luminous zone
    b) dark inner zone
    c) the lowest blue zone
    d) none of these

    Answer. (a) outermost non-luminous zone

    Q.5. An ideal fuel should have…………………………..

    a) very strong odour
    b) low calorific value
    c) very high ignition temperature
    d) moderate ignition temperature
    Answer (d) moderate ignition temperature

    II. Fill in the blanks by selecting appropriate words from the given list:
    List : CO2, oxygen, ignition, explosion, water
    Q.1. A substance does not catch fire if its temperature is lower than the …………………temperature.
    Q.2. The oxidation of the combustible substances takes place at high speed producing an …………………. .
    Q.3. Combustion of substances may be complete or incomplete depending on the amount of………………………… available during the process.
    Q.4. The most common substance used to extinguish fire is…………………………
    Q.5. Electrical and oil fires are extinguished by directing a jet of ……………………stored as a liquid in cylinders under high pressure.

    1- ignition,
    2- explosion,
    3- oxygen,
    4- water,
    5- $CO_{2}$

    III. Match the items of Column A with those of Column B.

    Answer: 1-c, 2-d, 3-b, 4-e, 5-a,

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Burning of a substance is actually a chemical change.
    Q.2. The substances which bum readily are known as combustible substances.
    Q.3. Each substance needs the same amount of heat to start burning at different temperatures.
    Q.4. Burning of fuels in sufficient quantity of oxygen produces the poisonous gas called carbon monoxide.
    Q.5. 5. When we light a candle or matchstick, it burns with a yellow flame.

    Answer: 1-T, 2-T, 3-F, 4-F, 5- T

    V. Very Short Answer Questions.

    Q.1. Write an example of spontaneous combustion.
    Answer: Burning of yellow phosphorus

    Q.2. Why does incomplete combustion take place?
    Answer: When the quantity of oxygen available to the burning substance is insufficient, incomplete combustion takes place.

    Q.3. What substance is used to extinguish electrical and oil fires?
    Answer: Carbon dioxide

    Q.4. Which kind of substances are burnt with a flame?

    Answer: Only those solid combustible materials that vaporise on heating bum with a flame.

    Q.5. Write the coldest region of the candle flame.

    Answer: Dark inner zone is the coldest region of the candle flame.

    VI. Short Answer Questions.

    a) Why do substances such as a piece of paper, dry grass, LPG, kerosene oil, etc., catch fire more easily than a log of wood or a piece of coal?

    Answer: Substances such as a piece of paper, dry grass, LPG, kerosene oil, etc., catch fire more easily than a log of wood or a piece of coal. This is because each substance needs different
    amounts of heat to start burning at different temperatures called its ignition temperature. Piece of paper, dry grass, LPG, kerosene oil, etc., have low ignition temperature compare to a log of wood or a piece of coal

    b) Why can water not be used to extinguish electrical and oil fires?
    Answer: The most common substance used to extinguish fire is water. But it is not used to put off fires in electrical wiring as water conducts electricity and may electrocute the person dousing the fire. Water also cannot be used for fires involving oil or petrol because, water being heavier sinks down and oil keeps burning on top.

    c) Why do different substances burn with different coloured flames?

    Answer: When we light a candle or a matchstick, it burns with a yellow flame. In general, all hydrocarbons or hydrocarbon fuels Like camphor, LPG, kerosene, petrol, etc., burn with a blue or a yellow flame. The metal magnesium burns with a dazzling white Light, sodium with a yellow flame and calcium with a brick red flame. The colour of the flame depends on the combustible substance, its temperature and the amount of air supplied during burning. So, different substances burn with different coloured flames.

    d) What do you mean by combustion? Describe the necessary conditions for combustion of substances.
    Answer: The process in which a substance combines chemically with oxygen to produce heat and light energy is called combustion.

    Three conditions are necessary for combustion of substances:
    (i) Presence of a combustible substance.
    (ii) Presence of a supporter of combustion.
    (iii) Heat to raise the temperature of combustible substance to its ignition temperature.

    e) Discuss how to control fire for different conditions. Explain with proper examples.

    Answer: The principle of extinguishing fire involves the removal of the condition which support combustion.
    These are :
    (i) Removal of all combustible substances.
    (ii) Cutting off the supply of air (oxygen).
    (iii) Cooling the combustible substances below their ignition temperatures.

    When a building is on fire, firemen direct a jet of water on the building. Water along with its vapour help to cut off the supply of oxygen and also bring down the temperature of the combustible materials.
    When a person’s clothes have caught fire a blanket is wrapped around his body to cut off the supply of oxygen.

    f) Discuss pollution caused by burning of different fuels.
    Answer: Combustion of fuels like wood, coal, petrol, diesel, LPG, etc., produce gases Like carbon dioxide and carbon monoxide. In addition, the fossil fuels release oxides of sulphur and nitrogen. Oxides of sulphur are released during burning of coal and diesel while oxides of nitrogen are released during burning of petrol. Solid fuels release unburnt carbon particles in the form of smoke on combustion. These gases along with smoke cause air pollution.

    VII. Long Answer Questions.

    Q.1. What are different types of combustion? Explain them with suitable examples.
    Answer: Combustion has been classified into the following types:
    Spontaneous Combustion: Combustion of a substance on its own at room temperature, without being ignited is known as spontaneous combustion. Burning of yellow phosphorus in air, spontaneous forest fires etc are examples of spontaneous combustion.

    (ii) Rapid Combustion: Combustion of a substance in which large amount of heat
    and light are released in a very short time is known as rapid combustion. Burning of coal, wood, candle and hydrocarbon fuels e.g., kerosene, petrol, LPG, etc., are examples of rapid combustion.

    iii) Explosion : It is a type of combustion where huge quantity of heat, light and sound are produced with the evolution of a large amount of gases in a very short span of time. Bursting of fire crackers is an example of explosion.

    iv) Slow combustion: The combustion that takes place very slowly without any ignition at low temperatures is known as slow combustion. The process produces very little amount of heat and no Light. Rusting of iron and respiration are examples of slow combustion.

    Q.2. What do you understand by a flame? Discuss structure of a candle flame briefly.

    Answer: The region where combustion of gaseous materials takes place producing heat and Light is known as a flame.
    A candle flame consists of the following four zones :
    (i) Outermost non-luminous zone: Wax vapours undergo complete combustion to produce carbon dioxide and water since enough air (oxygen) is available from both sides of the flame. The flame is hardly visible and it is the hottest part of the flame.

    (ii) The middle luminous zone: It is the prominent yellow part of the candle flame. Wax vapours undergo incomplete combustion because enough air is not available in this zone. The unburnt carbon particles glow emitting a yellow light. This part of the flame is moderately hot.

    (iii) Dark inner zone: This is the zone of no combustion as oxygen is not available. Thus, it appears black due to the presence of unburnt carbon particles in wax vapours. It is the coldest region of the candle flame.

    (iv) The lowest blue zone: This zone lies at the bottom of the flame. The blue colour is due to burning of wax vapours which gets plenty of air at the base of the flame.

    Q.3. What do you mean by fuel? Discuss different types of fuels and their characteristics and also define calorific value of a fuel.

    Answer: A combustible substance which produces a large amount of heat and light on burning is called Fuel. Coal, wood, petrol, diesel, kerosene, LPG and CNG are some commonly used fuels.

    Fuels can be classified into solid, liquid or gaseous fuels based on their physical states. Solid fuels are those combustible substances which exist in the solid state at room temperature. Wood, coal, charcoal, coke, solidified animal dung, agricultural wastes are some solid fuels. They contain mainly carbon in the free or combined states.

    Petrol, diesel, kerosene, alcohol which are either hydrocarbons or their mixtures are examples of Liquid fuels. They are volatile Liquids which produce combustible vapour.

    Gaseous fuels consists of combustible gases or mixtures of combustible substances. LPG, CNG, natural gas, coal gas, etc., are examples of some gaseous fuels.

    The amount of heat produced when 1 g of fuel undergoes complete combustion is called its calorific value. The calorific value of a fuel is expressed in the unit kilojoule per gram [kJ/g]
    . The S.l. unit of calorific value of a fuel is joule per kilogram [J/kg].

    Reviewed By:
    Krishna Kant Majee
    M.Sc., B.Ed.

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