Solved Questions on Combustion and Flame
1. A certain substance has an ignition temperature of 300oC. If the substance is exposed to a constant flame with a temperature of 250oC, what will happen?
a) The substance will not ignite.
b) The substance will ignite and burn completely.
c) The substance will partially ignite but not sustain the flame.
d) The substance will ignite, but the combustion will stop as soon as the flame is removed.
Answer: a) The substance will not ignite. The ignition temperature of a substance is the minimum temperature required for it to catch fire and sustain combustion. In this case, the constant flame temperature of 250°C is lower than the ignition temperature of the substance (300°C), so it will not ignite.
2. Consider the following statements and choose the correct option:
Statement 1: Using a lid to cover a burning pan can extinguish the fire by cutting off the supply of oxygen.
Statement 2: Using water to extinguish an electrical fire is safe and effective.
a) Statement 1 is correct but statement 2 is incorrect.
b) Statement 1 is incorrect but statement 2 is correct.
c) Both statements are correct.
d) Both statements are incorrect.
Answer: a) Statement 1 is correct but statement 2 is incorrect.
Statement 1: Using a lid to cover a burning pan can indeed extinguish the fire by cutting off the supply of oxygen, which is necessary for combustion.
Statement 2: Using water to extinguish an electrical fire is NOT safe and effective. Water is a good conductor of electricity, and using it on an electrical fire can lead to electrical shock or spread the fire further. It is recommended to use a fire extinguisher specifically designed for electrical fires.
3. In an experiment, a piece of paper is set on fire using a magnifying glass to focus sunlight. Which of the following is the most likely reason for the paper to catch fire?
a) The ignition temperature of paper is very low.
b) The magnifying glass lowers the ignition temperature of the paper.
c) Sunlight contains a large amount of oxygen.
d) The magnifying glass concentrates heat on a small area, raising the temperature above the ignition temperature.
Answer: d) The magnifying glass focuses sunlight on a small spot, increasing the intensity of heat energy at that point. This concentrated heat raises the temperature of the paper at that spot above its ignition temperature, leading to combustion and catching fire. The other options (a, b, and c) are not directly related to the phenomenon of using a magnifying glass to start a fire with sunlight.
4. In the following question, you will find an assertion and a reason. Select the appropriate option that applies.
Assertion: The ignition temperature of paper is higher than that of gasoline.
Reasoning: Paper is a solid fuel, and solid fuels generally have higher ignition temperatures compared to liquid fuels like gasoline.
a) Both assertion and reasoning are correct, and the reasoning justifies the assertion.
b) Both assertion and reasoning are correct, but the reasoning does not justify the assertion.
c) The assertion is correct, but the reasoning is incorrect.
d) Both assertion and reasoning are incorrect.
Answer: a) Both assertion and reasoning are correct, and the reasoning justifies the assertion.
The assertion states that the ignition temperature of paper is higher than that of gasoline, which is true. The reasoning provides a valid explanation for this, stating that solid fuels, like paper, generally have higher ignition temperatures compared to liquid fuels like gasoline. The reasoning justifies the assertion.
5. During a fire safety demonstration, four different fire extinguishers were used to put out fires of different types - water for wood, foam for petrol, carbon dioxide for electrical, and sand for oil fires. Which statement best explains the choice of extinguishers for specific fires?
a) Water extinguisher breaks down the fuel molecules to stop combustion.
b) Foam extinguisher displaces oxygen and smothers the fire.
c) Carbon dioxide extinguisher adds a cooling effect to the fire.
d) Sand extinguisher chemically reacts with the fuel to stop combustion.
Answer: b) The choice of extinguishers for specific fires is based on their ability to tackle the fire effectively. Foam extinguisher is used for petrol fires because it forms a blanket over the fuel and cuts off the supply of oxygen, which is essential for combustion. By smothering the fire, the foam extinguisher helps to extinguish it.
FAQs
1. What are the different types of combustion?
There are three forms of combustion: full combustion, in which the fuel burns in the presence of enough oxygen, incomplete combustion, in which the fuel burns with insufficient oxygen, and spontaneous combustion, in which a material ignites without an external heat source.
2. How are flames classified?
Flames are classed according to their look and colour, which can reveal the temperature and fuel type being burnt. Blue flames indicate full combustion, yellow flames indicate partial combustion and red flames indicate colder temperatures.
3. What factors affect the color and appearance of flames?
Flame colour and appearance are determined by elements such as the kind of fuel consumed, oxygen availability, flame temperature, and the presence of impurities or contaminants in the fuel.
4. What are some examples of fuels that undergo combustion?
Examples of fuels that undergo combustion include natural gas, gasoline, diesel, coal, wood, propane, ethanol, and hydrogen.
5. What are some environmental impacts of combustion?
Combustion processes may worsen environmental problems such as air pollution, greenhouse gas emissions, and climate change. Burning fossil fuels emits carbon dioxide and other pollutants into the atmosphere, contributing to global warming and harming air quality and ecosystems.
