Page 65 - Chemistry ICSE Class IX

Chemical Changes and Reactions 53

Illustrative Questions

Q.1. Which gases are produced by complete and incomplete combustion of coal? Which one of them is poisonous?
Explain how it affects the human system.
Ans. Gas produced by complete combustion: Carbon dioxide (CO )
2
Gases produced by incomplete combustion: Carbon monoxide (CO) and small quantity of carbon dioxide (CO )
2
Carbon monoxide is a poisonous gas. Carbon monoxide when inhaled combines with haemoglobin to form
carboxyhaemoglobin. As a result, the oxygen-carrying capacity of haemoglobin decreases and the concentration
of oxygen in blood decreases. This causes suffocation leading to death.
Q.2. Why does a heap of coal or wood not catch re at room temperature though suf cient quantity of oxygen is
present in the air?
Ans. The ignition temperature of coal or wood is much higher than the room temperature. So, coal or wood does not
catch re at room temperature though enough oxygen is present in the air.
Q.3. Why can a burning matchstick light a splinter of wood but not a log of wood?
Ans. The heat liberated from a burning matchstick is enough to raise the temperature of a splinter of wood to its
ignition temperature. The same quantity of heat is not suf cient to raise the temperature of a log of wood to its
ignition temperature. As a result, a burning matchstick can light a splinter of wood but not a log of wood.
Q.4. A piece of cloth soaked in a mixture of alcohol and water, on exposure to heat quickly catches re which gets
extinguished after a short while without burning the cloth. Explain why it is so.
Ans. The ignition temperature of alcohol is lower than that of the cloth. When exposed to heat, alcohol catches re.
The heat produced due to the burning of alcohol is not suf cient to raise the temperature of the wet cloth to its
ignition temperature. Thus, the re gets extinguished after alcohol is burnt and the cloth remains unburnt.
Q.5. How can water be boiled in a paper cup without burning it?
Ans. The ignition temperature of paper is much higher than the boiling point of water, i.e., the temperature of the
boiling water (100°C) is much lower than the ignition temperature of the paper. When a paper cup containing
water is heated, the heat is transferred to the water. As a result, water in the cup starts boiling, and the temperature
of the paper cup does not rise beyond that of the boiling water. Thus, it is possible to boil water in a paper cup
without burning it.
Q.6. Why does a piece of paper burn with yellow ame?
Ans. Paper contains cellulose. When it burns rapidly, the oxygen from the surrounding atmosphere is suddenly used
up and the supply of the air is cut-off. This leads to an incomplete combustion and the unburnt carbon particles
make the ame yellow.
Q.7. Why does a blue ame due to the burning of LPG become yellow (after a short while), when a drop of water falls
over it?
Ans. LPG burns with a blue ame. When a drop of water falls over the ame, the temperature of the ame gets
lowered considerably. This results into an incomplete combustion of LPG (only for a moment), making the ame
yellow due to the presence of unburnt carbon particles.
Q.8. Why is the re hot?
Ans. Fire is produced due to the combustion of any combustible material, such as wood, coal, paper, and straw.
Combustion is an exothermic process. Therefore, heat is liberated during combustion. This heat makes the re
hot.
Q.9. Fuel A is a compound of carbon, hydrogen and oxygen, while fuel B is a compound of carbon and hydrogen
only. Which one of these will have a lower ignition temperature if both have the same number of carbon atoms
in their molecules?
Ans. Fuel B contains carbon and hydrogen which are highly combustible. Fuel A contains carbon, hydrogen and
oxygen. Oxygen is not a combustible substance. Therefore, fuel B will have lower ignition temperature than fuel
A.
Q.10. The calori c value and ignition temperature of fuel A are 80 kJ/g and 20°C respectively. These values for fuel
B are 60 kJ/g and 100°C respectively. On burning, fuel A gives off CO , SO and SO while fuel B gives off H O
3
2
2
2
and CO . Which one of the two fuels is better? Give two reasons to support your answer.
2