NCERT Class VIII Science Solutions: Chapter 6-Combustion and Flame Part 5

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Question 25:

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?

Answer:

Iron wire will become red hot and glow. It will not produce a flame.

Question 26:

Fill in the blanks using the words given in the box.

Ignition, petrol, combustion, calorific value, combustible, inflammable

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

  2. Wood, Paper, CNG are __________ substances.

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

  4. Ignition temperature of __________ is lower than that of wood.

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

  6. The amount of heat energy produced on complete combustion of of a fuel is called its __________.

Answer:

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

  2. Wood, Paper, CNG are combustible substances.

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

    Image of ignition temperature

    Ignition Temperature

    Image of ignition temperature

  4. Ignition temperature of petrol is lower than that of wood.

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

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

Question 27:

People usually keep Angethi/burning coal in their closed rooms during winter season. Why is it advised to keep the door open?

Answer:

Due to insufficient availability of oxygen in the closed room carbon monoxide gas is produced which can kill persons sleeping in that room.

Question 28:

Write True/False against the following statements and also correct the false statement.

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

  2. Water is the best extinguisher for fires involving electrical equipment.

  3. Alcohol, CNG and LPG are inflammable substances.

  4. Increased concentration of nitrogen in air is believed to cause global warming.

  5. Greater the calorific value better is the fuel.

  6. Middle zone is the hottest zone of a flame.

  7. The substances which vapourises during burning, give flame.

Answer:

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

  2. False – Carbon dioxide is the best extinguisher for fires involving electrical equipment.

  3. True.

  4. False – Increased concentration of carbon dioxide in air is believed to cause global warming.

  5. True.

  6. False – outer zone is the hottest zone of a flame.

  7. True.

Question 29:

Cracker on ignition produces sound. Why?

Answer:

When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound and a large amount of gas is liberated. Such a reaction is called explosion. The gases produced are heated by the heat evolved in the reaction.

Question 30:

What do you understand by fuel efficiency?

Answer:

Fuel efficiency is determined by its calorific value which is the amount of heat energy produced on complete combustion of 1 kg of a fuel. The calorific value of a fuel is expressed in kJ/kg.

A good fuel should have high calorific value. That means it should produce large amount, of heat per unit mass.