Olympiad Test: Combustion & Flame - Class 8 MCQ

L.P.G. is a mixture of hydrocarbons. It consists of butane, propane and ethane but butane and propane are the two major constituents of L.P.G.

Calorific value of a substance is the amount of heat released during the combustion of a specified amount of fuel or food. The calorific value is the characteristic of a substance.

Natural gas has the highest calorific value of 50000 kJ/kg Petrol has 45000 kJ/kg Kerosene has 45000 kJ/kg and Coke has 28000 kJ/kg

All the given substances can act as fuels but butane is considered as a good fuel because of the following characteristics:
(i) Exists in gaseous state
(ii) Has high calorific value
(iii) Does not form poisonous products during combustion

The process of a substance burning in the presence of air with evolution of heat is called combustion.

Fuels and hydrocarbons burn in oxygen and form 

CO₂+2O₂→CO₂+2H₂O+heat

and give out heat. 

CH₄+2O₂→CO₂+2H₂O+heat

Combustion is an oxidation reaction accompanied by heat and light.

Diamond is a non-combustible substance.

The temperature at which a substance starts burning on heating is called ignition temperature. Highly    inflammable substances have a low ignition temperature. Hence, they catch fire easily.

A burning substance will be extinguished if the temperature falls below its ignition temperature.

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. Candle shows rapid combustion, whereas cracker and white Phosphorous come under explosion and spontaneous combustion respectively.

The combustion in which heat, light and sound are produced is known as explosive combustion.

The correct combination is a - 3, b - 4, c - 2, d - 1
(i) Dark inner zone -Unburnt vapours of wax
(ii) Blue zone - Complete combustion
(iii) Luminous zone ? Partial combustion
(iv) Non-luminous zone - Hottest part (no carbon)

The blue zone in L.P.G. flame indicates the hottest zone where complete combustion takes place.

Natural gas consists of methane, ethane, propane etc., of which methane forms major composition of about 95%.

The Bergius Process is a very simple process for converting brown coal completely into crude oil. It was invented by Dr. Friedrich Bergius. In this process, brown coal, also known as lignite is powdered and mixed with heavy oil and heated with hydrogen under high pressure of about 200-250 atm. and a temperature of about 748 K in the presence of iron oxide as catalyst. The vapours on condensation give a liquid resembling crude oil. This is called synthetic petrol, which on fractional distillation gives petrol (gasoline).

To arrange the fuels in increasing order of calorific value:

1. Hydrogen gas: Hydrogen has the highest calorific value among the listed fuels, around 141.8 MJ/kg (MegaJoules per kilogram).2. Kerosene oil: Kerosene oil has a high calorific value, ranging from 42.5 to 46.2 MJ/kg.3. Charcoal: Charcoal has a moderate calorific value, typically around 33 MJ/kg.4. Wood: Wood has the lowest calorific value on this list, with an average of around 15 MJ/kg (depending on the wood type and moisture content).
- Correct Order: 4. Wood, 3. Charcoal, 2. Kerosene oil, 1. Hydrogen gas
- Explanation:
- Wood: Lower calorific value due to higher moisture content.
- Charcoal: More calorific value than wood due to lower moisture content.
- Kerosene oil: Higher calorific value compared to charcoal.
- Hydrogen gas: Highest calorific value due to complete combustion producing only water vapor.
- Correct Answer: D: 4, 3, 2, 1

The incomplete combustion of a fuel gives carbon monoxide (CO), soot (carbon particles), and water vapor. Incomplete combustion occurs when there is not enough oxygen to allow the fuel to burn completely, which results in these harmful byproducts instead of producing only carbon dioxide (CO₂) and water, as in complete combustion.

1 kg of fuel when burnt gives 20,000 joules of heat energy. 50,000 joules of heat energy will be produced by 2.5 kg of fuel as given below. 1 kg fuel produces 20kJkg⁻¹ heat ? fuel produces 50kJkg⁻¹ heat 

Combustion is burning of a substance in the presence of oxygen. During this process heat energy is released, it is therefore an oxidation and exothermic reaction.