Chapter 6 – Combustion And Flame

A substance which reacts with oxygen giving heat is called a combustible substance. Which, one of the following is a combustible substance?

• (a)iron nail
• (b)glass
• (c)stone piece
• (d)wood

Which one of the following has the highest calorific value?

• (a)kerosene
• (b)biogas
• (c)LPG
• (d)petrol

Magnesium ribbon on burning in air produces

• (a)magnesium oxide, water and light
• (b)magnesium oxide and heat
• (c)magnesium oxide, heat and light
• (d)magnesium oxide, water and heat

Which of the following is not a combustible substance?

• (a)camphor
• (b)glass
• (c)straw
• (d)alcohol

The substance that does not burn with flame is

• (a)LPG
• (b)camphor
• (c)dry grass
• (d)charcoal

On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of non- availability of

• (a)oxygen
• (b)water vapours
• (c)carbon dioxide
• (d)wax

If a person’s clothes catches fire, the best way to extinguish the fire is to:

• (a)throw water on the clothes.
• (b)use fire extinguisher.
• (c)cover the person with a woolen blanket.
• (d)cover the person with a polythene sheet. COMBUSTION AND FLAME 31

The substance expected to have the highest ignition temperature out of the following is

• (a)kerosene
• (b)petrol
• (c)coal
• (d)alcohol

Choose the correct statement about inflammable substances from the following. They have:

• (a)low ignition temperature and cannot catch fire easily.
• (b)high ignition temperature and can catch fire easily.
• (c)low ignition temperature and can catch fire easily.
• (d)high ignition temperature and cannot catch fire easily.

Choose the incorrect statement from the following. Forest fires are usually due to:

• (a)carelessness of humans
• (b)heat of sun
• (c)cutting of trees
• (d)lightning strike

The calorific value of a fuel is expressed in a unit called

• (a)kilojoule per litre
• (b)kilogram per mililitre
• (c)kilojoule per gram
• (d)kilojoule per kilogram

In villages, people use wood as fuel because:

• (a)it is considered to be an ideal fuel.
• (b)of its easy availability and low cost.
• (c)it is environment friendly.
• (d)it catches fire easily.

Which among the following is considered as the cleanest fuel?

• (a)cow dung cake
• (b)petrol
• (c)kerosene
• (d)hydrogen gas

Choose the incorrect statement from the following. A good fuel is one which:

• (a)is readily available.
• (b)produces a large amount of heat.
• (c)leaves behind many undesirable substances.
• (d)burns easily in air at a moderate rate.

Shyam was cooking potato curry on a chulha. To his surprise he observed that the copper vessel was getting blackened from outside. It may be due to:

• (a)proper combustion of fuel.
• (b)improper cooking of potato curry.
• (c)improper combustion of the fuel.
• (d)burning of copper vessel. 32 E XEMPLAR PROBLEMS

Fill in the blanks in the following sentences.

• (a)A ____________ process in which a substance reacts with __________ to give off heat is called combustion.
• (b)When the clothes of a person catch __________, the person is covered with a __________ to extinguish fire.
• (c)The __________ temperature at which a substance catches fire is called its __________ temperature.
• (d)The substances which have very ________________ ignition temperature and can easily catch fire with a flame are called __________ substances. (e) The substances which vapourise during __________, give flame.

Some words (underlined) in the following sentences are jumbled up. Write them in their correct form.

• (a)Seldie is a combustible substance.
• (b)Slags is a non-combustible material.
• (c)Chittsmack does not burn by itself.
• (d)Some substances on combustion produce thea and mafel. (e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.

Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why?

Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.

Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities? COMBUSTION AND FLAME 33

Boojho wants to separate the following materials as combustible and non-combustible. Can you help him? Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, candle, wood.

Indicate whether the following statements are True or False. Also write the false statements in their correct form.

• (a)Air is necessary for combustion.
• (b)Magnesium is a non-combustible metal.
• (c)Carbon dioxide is an excellent fire extinguisher.
• (d)Calorific value of wood is higher than that of coal.

Match the items of Column A with the items of Column B. Column A Column B (a) Oxides of sulphur and

• (i)fire extinguisher nitrogen (b) CNG
• (ii)incomplete combustion of coal (c) Oxygen
• (iii)very low ignition temperature (d) inflammable substance
• (iv)acid rain (e) carbon dioxide (v) necessary for combustion (f) carbon monoxide (vi) fuel for automobiles.

Match the following for the flame of a candle. Column A Column B Column C (zone) (colour) (a) hottest part

• (i)innermost zone (x) blue of unburnt wax vapours (b) moderately hot
• (ii)middle zone of (y) black partial combustion (c) least hot
• (iii)outer zone of (z) yellow complete combustion 34 E XEMPLAR PROBLEMS

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?

Fill in the blanks using the words given in the box. ignition, petrol, combustion, calorific value, combustible, inflammable

• (a)A chemical process in which a substance reacts with oxygen to give off heat is called __________.
• (b)Wood, paper, CNG are __________ substances.
• (c)The lowest temperature at which a substance catches fire is called its __________ temperature.
• (d)Ignition temperature of __________ is lower than that of wood. (e) The substances which have very low __________ temperature and can easily catch fire with a flame are called __________ substances. (f) The amount of heat energy produced on complete combustion of 1kg of a fuel is called its __________.

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

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

• (a)A physical process in which a substance reacts with oxygen to give off heat is called combustion.
• (b)Water is the best extinguisher for fires involving electrical equipment.
• (c)Alcohol, CNG and LPG are inflammable substances.
• (d)Increased concentration of nitrogen in air is believed to cause global warming. (e) Greater the calorific value, better is the fuel. (f) Middle zone is the hottest zone of a flame. (g) The substances which vapourise during burning, give flame.

Cracker on ignition produces sound. Why?

What do you understand by fuel efficiency? COMBUSTION AND FLAME 35

You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?

Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable. If yes, why? If not, why not? In such a condition what should Manu have done?

What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.

Give two examples each for a solid, liquid and gaseous fuel alongwith some important uses.

The calorific values of petrol and CNG are 45000 kJ/kg and 50,000 kJ/kg, respectively. If you have vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Forest fire produces a lot of air pollution. Write in brief about the reasons of forest fires.

Complete the crossword Fig. 6.1 with the help of the clues : Fig. 6.1 36 E XEMPLAR PROBLEMS Across Down 1. Non-metal which catches 2. A chemical process in which fire if exposed to air (10) a substance reacts with oxygen to give off heat. (10) 3. The lowest temperature at 4. Petrol is used as a __________ which a substance catches in automobiles. (4) fire is called its __________ temperature. (8) 5. The most common fire 6. It is as hard as stone and extinguisher. (5) black in colour. (4)

This question refers to a figure in the original PDF.