Science STD 8 Chapter 4: Combustion and Flame - Exercises
1. List conditions under which combustion can take place.
Combustion can take place under the following three essential conditions:
- Presence of a fuel or a combustible substance.
- Presence of air (or oxygen) as a supporter of combustion.
- Heating the fuel to its ignition temperature, which is the lowest temperature at which a substance catches fire.
2. Fill in the blanks.
- (a) Burning of wood and coal causes pollution.
- (b) A liquid fuel, used in homes is kerosene.
- (c) Fuel must be heated to its ignition temperature before it starts burning.
- (d) Fire produced by oil cannot be controlled by water.
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
CNG (Compressed Natural Gas) is a cleaner fuel compared to petrol and diesel. It is less polluting because it produces harmful products like carbon monoxide, sulphur dioxide, and nitrogen oxides in very small amounts when it burns. The use of CNG in automobiles has significantly reduced air pollution, contributing to a better environment in our cities.
4. Compare LPG and wood as fuels.
| LPG (Liquefied Petroleum Gas) | Wood |
|---|---|
| It is a cleaner fuel that produces very little smoke and harmful gases. | It produces a lot of smoke and harmful gases, which can cause respiratory problems. |
| It has a very high calorific value (55,000 kJ/kg). | It has a lower calorific value (17,000 to 22,000 kJ/kg). |
| It is easy to store and transport in cylinders. | It is difficult to store and transport. |
| It does not leave behind any residue (ash). | It leaves behind ash as a residue after burning. |
5. Give reasons.
- (a) Water is not used to control fires involving electrical equipment.
Water is a conductor of electricity. If water is used on a fire involving electrical equipment, it can conduct electricity and cause electric shock to the person trying to extinguish the fire. It could also damage the electrical equipment.
- (b) LPG is a better domestic fuel than wood.
LPG is a better domestic fuel than wood because it is a cleaner fuel that burns completely, producing no smoke, which is harmful to human health. It has a higher calorific value, meaning it produces more heat per unit mass, and it is also easier to handle, store, and use.
- (c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Paper by itself catches fire easily because it has a low ignition temperature. However, when a piece of paper is wrapped around an aluminium pipe, the aluminium, being a good conductor of heat, transfers the heat from the paper and dissipates it. This prevents the paper from reaching its ignition temperature, and therefore it does not burn.
6. Make a labelled diagram of a candle flame.
A labelled diagram of a candle flame shows its three main zones: the outermost non-luminous zone (the hottest part), the middle luminous zone (moderately hot), and the innermost dark zone (least hot). The innermost zone consists of unburnt wax vapours, the middle zone has partial combustion, and the outermost zone has complete combustion.
[Image of a labelled diagram of a candle flame]
7. Name the unit in which the calorific value of a fuel is expressed.
The calorific value of a fuel is expressed in the unit **kilojoule per kg (kJ/kg)**.
8. Explain how CO2 is able to control fires.
Carbon dioxide CO2 is an excellent fire extinguisher because it is heavier than oxygen. When sprayed on a fire, it forms a blanket around the fuel, cutting off the supply of oxygen, which is essential for combustion. Additionally, when released from a cylinder, it expands and cools down, which helps to lower the temperature of the fuel, further aiding in extinguishing the fire without causing harm to electrical equipment.
9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
It is difficult to burn a heap of green leaves because they contain a large amount of moisture. This moisture lowers the temperature of the leaves and requires a lot of heat to evaporate. Therefore, the green leaves do not reach their ignition temperature easily. On the other hand, dry leaves have very little moisture, so they reach their ignition temperature quickly and catch fire easily.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
A goldsmith uses the **outermost zone** of a flame to melt gold and silver. This is because the outermost zone is the hottest part of the flame due to complete combustion, which provides the high temperature needed to melt metals like gold and silver.
11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Calorific Value = Total heat produced / Mass of fuel burnt
Calorific Value = 180,000 kJ / 4.5 kg = **40,000 kJ/kg**
12. Can the process of rusting be called combustion? Discuss.
No, the process of rusting cannot be called combustion. Rusting is a slow oxidation process where iron reacts with oxygen and water to form a new substance (iron oxide), but it does not produce heat or light in a noticeable way. Combustion, on the other hand, is a rapid chemical process that produces heat and light. While both involve oxidation, their rates and the products' properties are different.
13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?
Ramesh's water will get heated in a shorter time. This is because the outermost zone of the flame, where Ramesh placed the beaker, is the hottest part of the flame due to complete combustion. Abida placed her beaker in the middle, yellow zone, which is only moderately hot due to partial combustion.
Suggested Activities and Projects
1. Survey the availability of various fuels in your locality. Find out their cost per kg and prepare a tabular chart showing how many kJ of various fuels you can get for every rupee.
Outline: This activity involves researching the availability and cost-effectiveness of different fuels. You would collect data on fuels like LPG, kerosene, wood, etc. You would then use their calorific values to calculate how much energy (in kJ) you can get for a single rupee. This helps you compare the efficiency of different fuels and understand why certain fuels are preferred over others despite their higher cost per kg.
2. Find out the number, type and location of fire extinguishers available in your school, nearby shops and factories. Write a brief report about the preparedness of these establishments to fight fire.
Outline: This project focuses on fire safety. You would survey your school and other places to find out how many fire extinguishers are present, what type they are (e.g., water, foam, CO2), and where they are located. The report would summarise your findings and assess the preparedness of the establishments. For example, a CO2 extinguisher would be appropriate for a server room, whereas a water extinguisher is suitable for a fire involving paper or wood.
3. Survey 100 houses in your area. Find the percentage of households using LPG, kerosene, wood and cattle dung as fuel.
Outline: This is a social survey to understand the types of cooking fuels used in a community. You would collect data from 100 households and then calculate the percentage of homes using each type of fuel. This project helps in understanding local energy consumption trends and the socio-economic factors that influence fuel choices.
4. Talk to people who use LPG at home. Find out what precautions they take in using LPG.
Outline: This activity is about learning and sharing safety practices. By talking to people who use LPG, you would learn about precautions such as checking for gas leaks, keeping the cylinder in a well-ventilated area, and ensuring the stove is turned off when not in use. You would also find out about the importance of turning off the regulator at night or when leaving the house, and having a gas leak detector.
5. Make a model of a fire extinguisher. Place a short candle and a slightly taller candle in a small dish filled with baking soda. Place the dish at the bottom of a large bowl. Light both the candles. Then pour vinegar into the dish of baking soda. Take care. Do not pour vinegar on the candles. Observe the foaming reaction. What happens to the candles? Why? In what order?
Outline: This project is a hands-on demonstration of how a fire extinguisher works. When vinegar (an acid) is poured into the baking soda (a base), it produces a foaming reaction that releases carbon dioxide gas. Since carbon dioxide is heavier than air, it will settle at the bottom of the bowl, cutting off the oxygen supply to the candles. The shorter candle, being at a lower height, will extinguish first, followed by the taller one, as the CO2 blanket rises.