Class 8 Science Chapter 4 Important Extra Questions

When the clothes of a person catch fire, the person is covered with a blanket to extinguish fire. Why?
When the clothes of a person catch fire the person is covered with a blanket to extinguish fire because blankets cut off the supply of oxygen, that is the supporter of fire.

Fix a lighted candle on a table. Put a glass chimney over the candle and rest it on a few wooden blocks in such a way that air can enter the chimney. Observe what happens to the flame. Now remove the blocks and let the chimney rest on the table. Again, observe the flame. Finally, put a glass plate over the chimney. Watch the flame again. We find that for combustion, air is necessary.
The candle burns freely in case (a) when air can enter the chimney from below. In case (b), when air does not enter the chimney from below, the flame flickers and produces smoke. In case (c), the flame finally goes off because the air is not available.

The head of the safety match contains only antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and a little red phosphorus (which is much less dangerous). When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus. This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulphide and start the combustion.

With the help of an activity show that it is very essential for a substance to reach ignition temperature to burn.
Make two paper cups by folding a sheet of paper. Pour about 50 mL of water in one of the cups. Heat both the cups separately with a candle. If we continue heating the cup, we can even boil water in the paper cup. The heat supplied to the paper cup is transferred to water by conduction. So, in the presence of water, the ignition temperature of paper is not reached. Hence, it does not burn.
The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples of inflammable substances are petrol, alcohol, Liquified Petroleum Gas (LPG), etc.

Carbon dioxide (CO2) is the best extinguisher for fire. As, carbon dioxide being heavier than oxygen, covers the fire like a blanket. It can be stored at high pressure as a liquid in cylinders. When released from the cylinder, CO2 expands enormously in volume and cools down. So, it not only forms a blanket around the fire, it also brings down the temperature of the fuel. It can be used as wood or plastic fire, oil fire, gaseous fire, electrical fire, cooking oil fire etc. That is why it is considered as an excellent fire extinguisher.

Write down the types of combustion?
There are various types of combustion the three important types of combustion are rapid combustion, spontaneous combustion and explosive combustion.

1. Rapid combustion: The combustion reaction in which a large amount of heat and light are produced in a short time is called rapid combustion. When the bring lighted matchstick on a lighter near the burner of a gas stove in the kitchen, the cooking gas starts burning at once producing a lot of heat and some light. So, the immediate burning of cooking gas in gas stove to give light and heat is an example of rapid combustion.
2. Spontaneous combustion: The combustion reaction which occurs on its own (without the help of any external heat) is called spontaneous combustion. In spontaneous combustion the substance suddenly burst into flames and starts burning even without being heated. Spontaneous combustion take place at room temperature. White Phosphorus is a substance which undergoes spontaneous combustion.
3. Explosive combustion: Explosive combustion or explosion is a very fast combustion reaction in which a large amount of heat, light and sound produced is called explosion combustion or explosion. A large amount of gas is released quickly in an explosive combustion. The crackers which we explode during festival work on the explosive combustion of substances.

A flame is a region where combustion of gaseous substance or vapour takes place.
The three zones of flame have different colours and different temperatures.

• The innermost zone of a flame is dark or black. It is the coldest part of the flame and in this innermost zone of a flame consists of hot unburnt vapours of the combustible material.
• The middle zone of a flame is yellow. It is bright and luminous. The middle zone of a flame produces a moderate temperature. This is brightness zone due to the glow of unburned carbon particles.
• The outer zone of a flame is blue. It is the outer zone of complete combustion. It is the hottest and non-luminous zone. The outermost zone has the highest temperature in the flame.

“The burning of fuels like wood, coal and petroleum products releases unburned carbon particles in the air”. Comment.
The releasing of unburnt particles are harmful for human beings and environment. This fine carbon particles are dangerous pollutants which can cause respiratory diseases such as, asthma and many other pulmonary diseases. Carbon dioxide and carbon particles in the air trap Sun’s heat rays by producing greenhouse effect. Due to this rise in the temperature of atmosphere. The ice in polar regions will melt very fast producing a lot of water. Increase percentage of carbon dioxide in air causes global warming.

The increasing fuel consumption has harmful effects on the environment.

1. Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.
2. Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room.
3. Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.
4. Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and nitrogen dissolve in rain water and form acids. Such rain is called acid rain. It is very harmful for crops, buildings and soil.

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

• 1) At high temperature sometimes dry grass catches fire which spreads throughout the forest.
• 2) Campfire me also be a reason.
• 3) Due to the spark of lightning from the sky.
• 4) The use of fires by villagers to ward off wild animals.
• 5) Fire lit intentionally by people living around forests for recreation.
• 6) Fires started accidentally by careless visitors to forest when they threw away a lighted cigarette in the forest.
• 7) The friction of bamboos due to high wind velocity and rolling stones.