TS 8th Class Physical Science Important Questions 8th Lesson Combustion, Fuels and Flame

These TS 8th Class Physical Science Important Questions 8th Lesson Combustion, Fuels and Flame will help the students to improve their time and approach.

TS 8th Class Physical Science Important Questions 8th Lesson Combustion, Fuels and Flame

Question 1.
Why does candle give flame when it ¡s burnt but why coal burns without emitting a flame?
Answer:

1. A fuel catches fire immediately if it is in the form of vapour (or gas).
2. Some wax near the (laine melts and rises through the wick. Then it becomes gas or vapour and then burns. It produces flame.
3. Charcoal is a non-volatile substance. So it did not vapourise on burning and produce no gas. So charcoal does not hum with flame. It glows only, without a flame.

Question 2.
Do all fuels produce the same amount of heat, when they are burnt?
Answer:
1. No.
2. Different substances produce different amounts of heat on burning. We measure this heat in kilojoules per kilogram. It is called its Calorific value’ or ‘Fuel efficiency’.
3. Examples: Calorific values in KJ / Kg:
Wood 17,000 to 22,000
Petrol 45,000

Question 3.
What do we need to burn a material? Have you ever tried burning a piece of paper or wood or coal, a small rock or a pebble? Do all of them (a piece of paper wood or coal, a small rock or a pebble) burn?
Answer:

• Air is necessary to bum a material.
• The material also should be combustible.

B. Paper burns with a flame, It is a combustible material.
C. Wood and charcoal do nut burn with a flame. They are non-volatile substances. They glow only.
D. A small rock or a pebble never burn. They are non-combustible materials.

Question 4.
A few more examples of combustion are given below. Can you explain the reasons for the changes taking place?
a. A slow fire bursts into a flame when air is blown on It, but a candle burning with flame will goes off when air is blown on it. Why?
Answer:
A.

1. In a combustion process, if a flame has to occur, the solid as well as liquid fuels must be converted into gaseous form.
2. In case of solid fuels, the solid chemically decomposes at high temperature to produce gases.
3. So when air is blown over a slow fire, the rate of reaction increases, more heat produces and raises the temperature of the solid fuel.
4. Then the solid fuel decomposes at that high temperature and raises temperature of the solid fuel and liberates gases.
5. These gases burn, producing a flame.

B. In case of a candle, vapours of wax burn and produce flame. When air is blown over the flame, the vapours of wax are carried away by the blowing air. So there will be no vapours of wax left out near the wick of the candle. It means the flame is put off.

Question 5.
If a large quantity of dry grass Is set on fire in forests then It is very difficult to put off the fire. Why?
Answer:

1. When a large quantity of dry grass is burnt, a large quantity of heat is produced.
2. Water is sprayed on this burning grass, to cool it and subside the fire.
3. But before the cool water reaches the burning grass, the water changes into vapour, due to the high heat.
4. This water vapour at a high temperature escapes away. It cannot cool the burning grass.

Question 6.
When an object catches fire, the fire Is put off if we cover ft off with sand or a blanket. Why?
Answer:
There are three essential requirements for producing fire. They are: fuel, air (supplies oxygen), and heat (to raise the temperature of the fuel beyond the ignition temperature). Fire can be controlled by removing one or more of these requirements. If we cover the burning objects off with sand or a blanket, then the air supply is cut off. Then the fire extinguishes.

Question 7.
What could be the reason for the variation in burning patterns among combustible materials?
Answer:
We know that some materials catch fire as soon all they are brought near to a flame, but some materials take a long time to start burning though they are kept near to the flame. The reasons are

1. The ignition temperature is different for different substances.
2. The value of ignition temperature of a substance decides how fast will it catch fire.
3. Substances with a low ignition temperature catches fire readily than a substance with a high ignition temperature.

Question 8.
Why is It difficult to light matches dunng the rainy season?
Answer:

1. In rainy season, the rubbing sides of the matchbox become slightly wet, due to moist weather (damp).
2. Then the ignition temperature of match stick rises.
3. So it is difficult to light matches during the rainy season.

Question 9.
Can you decide the best fuel among different fuels?
Answer:

1. Different types of fuels are selected for different purposes, depending upon various factors like easy availability, cost, calorific value, etc., So we cannot say, which is the ‘best fuel.
2. However, there are some requisites for an ‘ideal fuel’.

Question 10.
What is the criteria to decide a best fuel? Discuss with your friends.
Answer:
Criteria to decide a best fuel are as follows.

1. It should have a high calorific value.
2. It should have relatively a low ignition temperature.
3. It should be relatively cheap and easily available.
4. It should leave no ash after burning.
5. The fuel gases (waste gases after burning) should not contain much undesirable substances.
6. It should be easy to transport to any remote areas.

Question 11.
What is calorific value?
Answer:
“Calorific value of a fuel is the amount of heat energy produced on complete combustion of 1 kilogram of that fuel. Calorific value of a fuel is measured in Kilo Joules per Kilogram (KJ / Kg).

Question 12.
How can we put off the fire if It breaks out?
Answer:

1. If a fire breaks, it can be put off, using lire-extinguishers’.
2. The most common fire extinguisher is water. The other one is, carbon dioxide gas.
3. Water is used only with things like wood and paper on fire.
4. If electric equipment is on fire, water cannot be used. Water is also not suitable for fires involving oil and petrol.
5. So in such cases, carbon dioxide gas is used to control fires.

Question 13.
Why the fire brigade starts the work by putting off the electric mains if a fire breaks out in a house or in any business establishment?
Answer:

1. Water conducts electricity. So it harms those trying to douse the lire, using water.
2. So the fire brigade starts work after putting off the electric mains.

Question 14.
How water helps In eliminating the factors which support the combustion?
Answer:
1. There are three factors which support the combustion. They are:

• Presence of a combustible material or the fuel.
• Supply of air or oxygen.
• High temperature.
  So getting rid of any one of the three factors will help in controlling the fire.

2. The water sprayed cools the combustible material so that its temperature decreases.

Question 15.
Madhu decided to avoid automobiles and started going to school by bicycle. Do you appreciate his decision?
Answer:
The decision is really appreciable. It may be that Madhu’s school can easily be reached on a bicycle, from his house. But in cities, it is not always possible. The school or college for a student or the place of work for an employee may lie at a far off place from his residence.

In such cases, the student or the employee can’t depend on the bicycle. He must choose an automobile to reach the destination, in time. Then he should opt for a ‘Public transport system’ like a bus, instead of a motorbike or car.

Question 16.
Give four examples of non-combustible materials.
Answer:
Some non-combustible materials are :

• Iron nails
• Stone pieces
• Glass
• Brick
• cement

Question 17.
Give an example of a good fuel. How do you choose that fuel? Explain.
Answer:
For the kitchen, it is LPG and for a motor car, it is CNG. These are some good fuels.

Reasons:

1. They are relatively cheap.
2. They burn easily in air at a moderate rate (not slow or explosive).
3. They produce a large amount of heat,
4. They do not leave behind undesirable substances.
5. They are easily available.

Question 18.
What precautions are to be taken while pouring water on fire?
Answer:

1. Water is not suitable for fires involving oil and petrol.
2. If electrical equipment is on lire, water may conduct electricity and harm those trying to douse the fire. So water cannot be used.

Question 19.
Why a wick is not used in gas burners?
Answer:

1. The substances which vapourise (change to vapour) during burning, give flames.
2. For example, kerosene oil and molten wax (liquids) rise through the wick and then vapourise during burning. The vapours catch Fire and burn.
3. But when a combustible material is used in the form of a gas, no wick is needed. The gas burns directly when ignited.

Question 20.
Give supporting arguments for both the statements (1) fire Is useful (2) fire is harmful.
Answer:
Fire is useful in many ways. It is used

• in our houses to cook food and prepare other food items.
• to heat water and other liquids like milk.
• to run industries, which manufacture a variety of useful goods. But at the same time, it creates havoc (a lot of damage) if not handled with care.

Spontaneous combustion: It creates forest fires. It is very difficult to control the forest fires. It is a huge national loss. Sometimes fire breaks out in homes, shops, and factories, due to various reasons. It results in a huge loss of property to the concerned owners, But without fire, there cannot be life on earth. The sun is a hot ball of fire It supports the life on our planet, the earth.

Question 21.
What would happen ¡f oxygen stops to support combustion? – Make a guess. And If it Is the situation for what other fuels are useful.
Answer:
The digestion of food in our stomach is a process of combustion. Food is broken down by reaction with oxygen. Then energy is released. If oxygen loses this property, no man can survive. Then there will be no human beings to guess about anything.

Question 22.
Use of more fuels in our daily life causes air pollution and It Is harmful to human being and the other life on earth. Suggest some remedies to avoid this.
Answer:
Fuel is a source of energy. At the same time. use of more fuels leads to air – pollution. To minimize air pollution, some remedies are suggested.

1. Excessive use and wastage of fuels should be checked.
2. The fuel should be made free from lead, sulphur, etc.
3. The fuel (waste) gases from industries contain harmful ingredients. So these gase. should be freed from pollutants and then let out into air.
4. People should depend more on Public transport system. They should try to avoid the use of their own automobiles like bikes, cars, etc. This reduces the consumption of fuels, resulting in pollution control.
5. As far as possible. renewable sources of energy should replace the traditional fossil fuels. They are wind energy, solar energy, tidal energy, gravitational energy, etc. These fuels do not harm the atmosphere.

Question 23.
Let us assume that you are on the moon. If you try to focuss sunlight on a paper using magnifying glass, does the paper catch fire? Why?
Answer:
1. No
2. Reason: The earths only natural satellite is moon. But the moon has no atmosphere. So there is no oxygen on moon. For paper to catch fire, it requires oxygen. Since there is no oxygen on the moon, the paper did not burn only due to sun’s rays.

Question 24.
“Is combustion possible without the supply of oxygen”? Discuss with your teacher.
Answer:
If oxygen is not involved in the process, it cannot be called, ‘combustion’.
Reason: The definition of ‘combustion’ is ‘combustion is a chemical process in which a substance reacts with oxygen to give heat’. So, combustion must involve, oxygen.

Question 25.
Collect information about the experiments of Joseph Priestly. Write a two-page report describing Priestly’s experiments proving that oxygen Is needed for burning.
Answer:
English Chemist ‘Joseph Priestly’ in the year 1774 obtained a gas by focusing sun’s rays through a convex lens on the red oxide of mercury (HgO). The oxide decomposes, releasing a gas Priestly called that gas. “dephlogisticated air”.

Priestly studied the properties of this gas. He showed that, the gas.

• supports life
• supports combustion and
• is almost insoluble in water.

At the time of Priestly, scientists believed in a theory, called ‘Phlogiston Theory’.Priestly supported this theory strongly. According to this theory, ‘Phlogiston’ is matter, present in all combustible materials. During combustion, materials release phlogiston and so their weight decreases, after combustion.

In 1774, a French Chemist Lavoisier repeated Priestly’s experiments. He assigned the name, ‘oxygen’ (means, acid-producer) to the gas (dephlogisticated air). Lavoisier put an end to the ‘Phlogiston Theory’. He gave the reason for the loss of weight on combustion of materials. He proved it with experimental evidence.

Multiple-choice of questions

Question 1.
Which one of the following gases is a supporter of combustion? ( )
A) nitrogen
B) oxygen
C) hydrogen
D) all
Answer:
B) oxygen

Question 2.
Detonation is an example of ( )
A) slow combustion
B) spontaneous combustion
C) explosion
D) rapid combustion
Answer:
C) explosion

Question 3.
The temperature at which a substance burns is called its ( )
A) melting point
B) boiling point :
C) kindling temperature
D) none
Answer:
C) kindling temperature

Question 4.
A non-renewable source of energy ( )
A) wind energy
B) tidal energy
C) solar energy
D) natural gas
Answer:
D) natural gas

Question 5.
Which of the following is a fossil fuel? ( )
A) water gas
B) producer gas
C) semi-water gas
D) LPG
Answer:
D) LPG

Question 6.
Which zone contains unburnt gas in a Bunsen flame or a candle flame? ( )
A) outer zone
B) middle zone
C) inner zone
D) none
Answer:
C) inner zone

Question 7.
The highest part of a candle flame is ( )
A) dark zone
B) blue zone
C) yellow zone
D) all zones
Answer:
B) blue zone

Question 8.
Oxygen gas can be prepared by heating ( )
A) potassium permanganate
B) lead nitrate
C) potassium chlorate + a pinch of manganese dioxide
D) any of them
Answer:
C) potassium chlorate + a pinch of manganese dioxide