The DAV Class 8 Science Book Solutions Pdf and DAV Class 8 Science Chapter 7 Question Answer – Combustion are essential study tools for DAV public school students in Class 8.
DAV Class 8 Science Ch 7 Question Answer – Combustion
DAV Class 8 Science Ch 7 Solutions – Combustion
Something To Know
A. Fill in the blanks:
Question 1.
A good fuel should have its ignition temperature ________ the room temperature.
Answer:
more than
Question 2.
Burning of cowdung cakes is an example of ________ combustion.
Answer:
slow
Question 3.
The ________ zone of a candle flame, is the coldest.
Answer:
innermost
Question 4.
The non-luminous zone of a candle flame is the ________ part of the flame.
Answer:
hottest
Question 5.
During incomplete combustion of a fuel, ________ gas is formed.
Answer:
carbon monoxide
B. State True or False for the following statements:
1. Coal is an inflammable fuel.
Answer:
False
2. The outermost part of a candle flame contains unburnt carbon particles.
Answer:
False
3. The heat produced, by burning completely one kilogram of a given fuel, is known as the efficiency of that fuel.
Answer:
True
4. All solid fuels have a higher calorific value than liquid and gaseous fluids.
Answer:
False
5. Sulphur dioxide gas, produced during combustion of fossil fuels, causes acid rain.
Answer:
True
C. Tick the correct option:
Question 1.
An example of a non-combustible substance is-
(a) Petrol
(b) Coal
(c) Wood
(d) Iron
Answer:
(d) Iron
Question 2.
A fuel starts burning only when it is heated to-
(a) Its boiling point
(b) Its ignition temperature
(c) Its melting point
(d) A temperature double of the room temperature
Answer:
(b) Its ignition temperature
Question 3.
The highest temperature zone of a candle flame, is its-
(a) Yellow zone
(b) Red zone
(c) Blue zone
(d) Orange zone
Answer:
(c) Blue zone
Question 4.
The amount of heat produced, on complete burning of 10 kg of a given fuel, in pure oxygen, equals H kilojoule. The calorific value, of that fuel, would equal-
(a) (0.1 H) kJ/kg
(b) (10 H) kJ/kg
(c) (H + 10) kJ/kg
(d) (H – 10) kJ/kg
Answer:
(a) (0.1 H) kJ/kg
Question 5.
The gas produced, due to incomplete combustion of carbon fuels, is-
(a) Carbon dioxide
(b) Carbon monoxide
(c) Sulphur dioxide
(d) Nitrogen dioxide
Answer:
(b) Carbon monoxide
D. Answer the following questions in brief:
Question 1.
Define the term ‘combustible material’? Give two examples of combustible materials.
Answer:
A material which can undergo combustion is called a combustible material.
Examples: Wood, coal
Question 2.
State the three conditions that are needed for combustion to take place.
Answer:
Following are the three conditions needed for combustion:
- A combustible substance
- Temperature should be more than ignition temperature
- Oxygen
Question 3.
State the condition under which ‘slow combustion’ of a material takes place.
Answer:
Slow combustion takes place in insufficient supply of air.
Question 4.
Define ‘calorific value of a fuel’.
Answer:
The amount of heat energy produced on burning 1 kg of a fuel is called the calorific value of a fuel.
Question 5.
State any two characteristics of an ideal fuel.
Answer:
The two characteristics of an ideal fuel are:
- It should have high calorific value.
- It should be cheap and easily available.
Question 6.
Why is water not used in controlling electrical fires?
Answer:
Normal water is a good conductor of electricity because it contains some dissolved salt. Using water against electrical fire has the risk of getting electric shock. Hence, water is not used in controlling electrical fires.
E. Answer the following questions:
Question 1.
Draw a diagram showing different zones of candle flame. Label the zone which contains:
(a) Unburnt vapours of wax
(b) Unburnt carbon particles
Answer:
Question 2.
Calorific value of wood is 18,000 kJ/kg. How much of wood is required to produce 360,000 kJ of heat energy?
Answer:
Calorific value of wood = 18,000 kJ/kg
∴ Amount of wood needed to produce 360,000 kJ of heat energy = \(\frac{3,60,000}{18,000}\) = 20 kg
Question 3.
Why are gaseous fuels considered better than liquid or solid fuels?
Answer:
Gaseous fuels are better than liquid or solid fuels because:
- Gaseous fuels have higher calorific value.
- These produce less amount of pollutants.
- It is easier to transport and store gaseous fuels.
Question 4.
Why do we say that ‘burning of fuels’ can cause ‘health hazards’?
Answer:
Burning of fuels causes air pollution. Air pollution is detrimental to our health. Hence, it can be said that burning of fuels can cause health hazards.
Value Based Question
Ameeta was upset because she could not find hear black pencil and her coloured pencils set for completing her painting. Her elder sister, Suneeta helped her to locate them and guided her in colouring her painting. She then told her that we need to be careful as we often need to fulfill a number of conditions for completing our tasks. She gave the example of the phenomenon of ‘combustion’ and told her that combustion can be started and sustained, only if three conditions are simultaneously satisfied.
Question 1.
State the values displayed by Suneeta.
Answer:
Suneeta is helpful and intellectual.
Question 2.
State the three conditions that need to be simultaneously satisfied for starting and sustaining combustion.
Answer:
The three conditions which must be satisfied for starting and sustaining combustion are:
(i) A combustible substance
(ii) Temperature should be more than ignition temperature
(iii) Supply of oxygen or air
Question 3.
Ask the students to think of situations/tasks which need two or more conditions for their completion.
Answer:
Do it yourself
Something To Do
Question 1.
Conduct a survey of houses in your locality and find their average monthly consumption of LPG/PNG.
Answer:
One cylinder is generally sufficient for a household for a month. Students should <A» tkss.
Question 2.
Prepare a report on the precautions that should be taken while using CNG fuel in vehicles.
Answer:
Precautions that should be taken while using CNG fuel in vehicles are:
(i) Do not allow anybody to smoke near the CNG storage unit.
(ii) Do not burn firecrackers near CNG pumps.
Question 3.
Wastes’ have been traditionally disposed off by burning them. Waste combustors’ are used to dispose off hospital and industrial waste. Find out how these combustors work and their effects on the environment.
Answer:
Do it yourself
Question 4.
Find out about the different types of fire extinguishers. Write the type and locations of the fire extinguishers kept in your school.
Answer:
The different types of fire extinguishers are:
- Class A – fires involving solid materials such as wood, paper or textiles.
- Class B – fires involving flammable liquids such as petrol, diesel or oils.
- Class C – fires involving gases.
- Class D – fires involving metals.
- Class E – fires involving live electrical apparatus
DAV Class 8 Science Chapter 7 Solutions – Combustion Important Questions and Answers
I. Fill in the blanks:
1. Petrol is a highly ________ material.
Answer:
inflammable
2. Coal is capable of ________ combustion.
Answer:
slow
3. Spirit is capable of ________ combustion.
Answer:
rapid
4. Phosphorus is capable of ________ combustion.
Answer:
spontaneous
5. Calorific value of LPG is ________ than the calorific value of firewood.
Answer:
higher
II. Tick the correct answer:
Question 1.
Which of these is not required for combustion?
(a) Oxygen
(b) Combustible substance
(c) Carbon dioxide
(d) Ignition temperature
Answer:
(c) Carbon dioxide
Question 2.
Which of these shows spontaneous combustion?
(a) CNG
(b) LPG
(c) Diesel
(d) Sodium
Answer:
(d) Sodium
Question 3.
Which of these shows slow combustion?
(a) Firewood
(b) Petrol
(c) Kerosene
(d) LPG
Answer:
(a) Firewood
Question 4.
Which of these gases is evolved on incomplete combustion of a fuel?
(a) Sulphur dioxide
(b) Nitrogen oxide
(c) Carbon monoxide
(d) Hydrogen
Answer:
(c) Carbon monoxide
Question 5.
What is the colour of the innermost zone of a flame?
(a) Dark black
(b) Blue
(c) Yellow
(d) Red
Answer:
(a) Dark black
III. Answer the following questions:
Question 1.
What is ignition temperature?
Answer:
The lowest temperature at which a substance starts burning is called its ignition temperature.
Question 2.
What is slow combustion?
Answer:
When a combustive material burns at slow or moderate rate, it is called slow combustion. Examples: burning of cowdung cakes, wood, etc.
Question 3.
What is rapid combustion?
Answer:
When a combustible material burns at fast rate, it is called rapid combustion. Examples: burning of petrol and LPG.
Question 4.
What is spontaneous combustion?
Answer:
When a combustible material starts burning at room temperature, it is called spontaneous combustion. Example: Sodium and phosphorus catch fire at room temperature instantaneously.
Question 5.
What is explosive combustion?
Answer:
When a combustible substance completely burns in a very short period of time and releases lot of heat, light and gases, it is called explosive combustion. Examples: burning of firecrackers, gunpowder, etc.
IV. Projects:
Question 1.
Make a list of five combustible substances and five non-combustible substances in your household.
Answer:
Do it yourself
Question 2.
Look for items in your home which have warning sign of inflammable material on their packs. Make their list.
Answer:
Do it yourself