Test: Fire: Friend and Foe- 1 - Class 7 MCQ

Explanation:
The correct answer is option D: both (a) and (b). When energy is released in the form of heat and light, it typically indicates a chemical reaction, which can often result in the production of fire. Here's a breakdown of the answer:
1. Energy release in the form of heat:
- Heat energy is a form of energy that is transferred from one object to another due to a difference in temperature.
- When a chemical reaction takes place, it often involves the breaking and forming of chemical bonds, which can release energy in the form of heat.
- This heat energy can be observed as an increase in temperature or the generation of flames and fire.
2. Energy release in the form of light:
- Light energy is a form of electromagnetic radiation that is visible to the human eye.
- When certain chemical reactions occur, they can produce light energy as a byproduct.
- This emission of light can range from a faint glow to a brilliant display, depending on the nature and intensity of the reaction.
3. Connection between heat, light, and chemical reactions:
- In many chemical reactions, the release of heat and light is closely interconnected.
- The energy released during a chemical reaction can manifest as both heat and light simultaneously.
- For example, combustion reactions, such as the burning of wood or the ignition of a candle, involve the release of both heat and light energy.
Therefore, when energy is released in the form of heat and light, it signifies a chemical reaction and can often result in the production of fire.

Early man and the Unknown:

• Introduction: Early man refers to the primitive humans who lived on Earth thousands of years ago. During this time, early man had limited knowledge and understanding of the world around them.


• Fire:




• Early man was initially unaware of fire and its potential uses.


• Fire was discovered accidentally through natural phenomena such as lightning or volcanic eruptions.


• Once early man discovered fire, it became a crucial tool for survival, providing warmth, protection, and the ability to cook food.




• Air and Water:




• Early man was aware of the existence of air and water as they were essential for their survival.


• However, they may not have had a comprehensive understanding of the properties and scientific aspects of air and water.


• They relied on these elements for breathing, drinking, and other basic needs.




• Conclusion:




• Early man had limited knowledge about the world and its elements.


• While they were aware of the existence of air and water, their understanding was likely rudimentary.


• However, the discovery of fire was a significant milestone for early man's development.


• Over time, early man continued to learn and explore, leading to advancements in their understanding of the natural world.

Note: This detailed response is based on the assumption that the answer "B: Fire" is the correct option for the given question.

Lowering the temperature below the flash point:
Lowering the temperature below the flash point of a substance refers to reducing the temperature at which the substance can ignite and sustain a fire. The flash point is the minimum temperature at which a substance can vaporize to form an ignitable mixture in air.
Here is a detailed explanation of the effect of lowering the temperature below the flash point:
1. Extinguishes the fire: When the temperature is lowered below the flash point, it reduces the availability of heat energy required for the combustion process. As a result, the fire cannot be sustained, and it is extinguished.
2. Reduces the volatility: Lowering the temperature decreases the volatility of the substance, which means it becomes less likely to vaporize and form flammable gases. This reduces the risk of fire or explosion.
3. Increases the safety: By lowering the temperature below the flash point, the risk of accidental ignition or fire is significantly reduced. This promotes a safer environment, especially in areas where flammable substances are present.
4. Prevents reignition: Lowering the temperature below the flash point ensures that even if a fire is temporarily extinguished, it is less likely to reignite due to the reduced availability of heat energy.
5. Does not affect the density: Lowering the temperature does not directly affect the density of the substance. Density is a measure of mass per unit volume and is not influenced by changes in temperature alone.
Therefore, the correct answer is B: extinguishes the fire. Lowering the temperature below the flash point reduces the availability of heat energy, making it impossible for the fire to sustain itself.

Fire is the result of a chemical reaction

• Definition: Fire is a rapid oxidation process that releases heat, light, and various byproducts such as smoke and gases.


• Chemical Reaction: Fire is the result of a chemical reaction known as combustion.


• Combustion: Combustion is a chemical process that occurs when a fuel (such as wood, paper, or gasoline) combines with oxygen in the presence of heat or an ignition source.


• Triangle of Fire: The combustion process is often represented by the "triangle of fire," which consists of three components: fuel, oxygen, and heat. These three elements must be present simultaneously for a fire to occur.


• Chemical Equation: The chemical equation for the combustion of a hydrocarbon fuel, such as methane (CH4), can be represented as follows: CH4 + 2O2 → CO2 + 2H2O + heat


• Energy Release: During the combustion process, the fuel molecules break apart and recombine with oxygen molecules, releasing energy in the form of heat and light.


• Heat Transfer: The heat generated during the combustion process can further sustain the fire by transferring to nearby fuel sources, causing them to reach their ignition temperature and start burning.


• Fire Spread: Fire can spread through a process called convection, where hot gases rise and carry flames to new fuel sources, or through radiation, where heat energy is transferred directly to nearby combustible materials.


• Control and Extinguishing: Fire can be controlled or extinguished by removing one or more elements of the fire triangle. For example, using water can cool down the fire and remove heat, or covering the fire with a fire blanket can remove oxygen.

In conclusion, fire is a result of a chemical reaction known as combustion, which occurs when fuel combines with oxygen in the presence of heat or an ignition source. The combustion process releases heat, light, and byproducts, and it can be represented by a chemical equation. Fire requires the simultaneous presence of fuel, oxygen, and heat, and it can spread through convection and radiation. Fire can be controlled or extinguished by removing one or more elements of the fire triangle.

Man's first experience of fire could be:
There are several theories regarding man's first experience of fire. Let's explore each option and determine the most likely scenario:
1. Lightning:
- Lightning is a natural phenomenon that can ignite fires when it strikes vegetation or other flammable materials.
- Early humans may have witnessed lightning strikes and observed the resulting fires.
- This could have been their first experience of fire.
2. Rubbing stones:
- Another theory suggests that early humans discovered fire by rubbing stones together to create sparks.
- By experimenting with different materials and techniques, they may have accidentally generated flames.
3. Both (a) and (b):
- It is also plausible that early humans encountered fire through both lightning and rubbing stones.
- They may have initially witnessed fires caused by lightning and later learned to create fire themselves using stones.
4. Volcanoes:
- Volcanic eruptions can release molten lava and spew out fiery debris.
- It is unlikely that early humans directly interacted with volcanoes to discover fire, as volcanic activity can be dangerous and destructive.
Conclusion:
- Based on the available evidence, the most probable answer is option D: volcanoes does not seem to be the most likely scenario for man's first experience of fire.
- Options A and B are both valid possibilities, and it is reasonable to assume that early humans encountered fire through a combination of lightning strikes and experimenting with rubbing stones.
Therefore, the answer to the question is option C: both (a) and (b).

Fire is the combination of:


A: Oxygen, carbon and hydrogen
- Fire does not consist of carbon and hydrogen. Carbon and hydrogen are elements that can be found in various compounds, but they are not necessary for fire to occur.


B: Hydrogen and oxygen
- Fire does not consist of hydrogen and oxygen alone. While hydrogen and oxygen can react to form water, fire is a separate chemical reaction that involves the rapid oxidation of a fuel source.


C: Oxygen and nitrogen
- Fire does not consist of oxygen and nitrogen. Nitrogen is an inert gas and does not participate in the combustion process.


D: None of these
- The correct answer is not "None of these" as fire is indeed a combination of certain elements.


Therefore, the correct answer is A: Oxygen, carbon, and hydrogen.

When carbon and hydrogen are combined with oxygen, they:
There are several possible outcomes when carbon and hydrogen are combined with oxygen. Let's explore each option in detail:
1. Can cause fire:
- When carbon and hydrogen react with oxygen in the presence of heat or a spark, they can undergo combustion and release energy in the form of heat and light.
- This reaction is known as combustion or burning and is a common way to produce energy.
- The reaction can be exothermic, meaning it releases heat, and can sustain a fire if the necessary conditions, such as a continuous supply of oxygen and a fuel source, are present.
2. Can cease fire:
- If the supply of oxygen is limited or cut off, the combustion reaction will be starved of oxygen and will eventually cease.
- This can be achieved by smothering the fire with a fire extinguisher, depriving it of oxygen and effectively ceasing the fire.
3. Explode:
- Under certain conditions, such as when carbon and hydrogen are in a confined space with a high concentration of oxygen and an ignition source is introduced, an explosion can occur.
- The rapid release of energy and the generation of high pressure can result in an explosion, which can be extremely dangerous.
4. None of the above:
- It is also possible that when carbon and hydrogen combine with oxygen, none of the above outcomes occur.
- This could happen if the reaction is incomplete or if the conditions are not suitable for any of the mentioned reactions to take place.
In conclusion, when carbon and hydrogen are combined with oxygen, they can cause fire, cease fire, explode, or none of the above, depending on the specific conditions and the presence of an ignition source.

Answer:
The three things needed to make fire are heat, fuel, and oxygen. Let's break down each component:
Heat:
- Heat is the initial spark or flame that ignites the fuel.
- It provides the energy required to start the combustion process.
Fuel:
- Fuel is any material that can undergo combustion and sustain a fire.
- It can be in the form of solids (e.g., wood, paper), liquids (e.g., gasoline, oil), or gases (e.g., propane, natural gas).
Oxygen:
- Oxygen is a necessary component for the process of combustion.
- It reacts with the fuel to produce heat, light, and various combustion byproducts.
When these three elements - heat, fuel, and oxygen - come together in the right proportions, a fire can occur. Removing any one of these components will extinguish the fire.
It is important to note that carbon is not one of the essential components needed to make fire. Carbon is produced as a byproduct of the combustion process and is commonly found in fuels, but it is not necessary for fire to start and sustain.
Therefore, the correct answer is A: Heat, fuel, and oxygen.

The temperature at which every fuel burns is called the flash point. Here's a detailed explanation:
Flash Point:
- The flash point is defined as the lowest temperature at which a fuel or flammable substance can vaporize to form an ignitable mixture in air.
- It is the temperature at which the vapor of a fuel can ignite when exposed to an open flame or spark.
- The flash point is an essential safety parameter as it indicates the potential fire hazard of a substance.
Distinguishing Terms:
- The flush point: This term is incorrect and does not exist in relation to the temperature at which fuels burn.
- The fire point: This term refers to the lowest temperature at which a fuel can sustain combustion once ignited.
- All the above: This option is incorrect as the correct term for the temperature at which fuels burn is the flash point, not the flush or fire point.
Importance of Flash Point:
- The flash point helps to determine the proper handling, storage, and transportation of flammable substances.
- It is used to classify and label hazardous materials according to their flammability.
- The flash point is also crucial for designing safety measures and selecting appropriate fire extinguishing agents.
In Conclusion:
- The temperature at which every fuel burns is called the flash point.
- The flash point is a critical parameter for assessing the fire hazard of a substance and ensuring safety in handling and storage.

Examples of Fuel:
- Wood: Wood is a traditional source of fuel that is commonly used for heating and cooking purposes.
- Coal: Coal is a fossil fuel that is primarily used for generating electricity and is also used in industrial processes.
- Petrol: Petrol, also known as gasoline, is a widely used fuel for vehicles with internal combustion engines.

Explanation:
- Wood is a natural fuel source that has been used for centuries. It is obtained from trees and can be used as firewood or converted into charcoal.
- Coal is a fossil fuel that is formed from the remains of plants that lived and died millions of years ago. It is a valuable source of energy and is used in power plants to generate electricity.
- Petrol is a liquid fuel that is derived from crude oil through a refining process. It is the most commonly used fuel for vehicles with internal combustion engines.
- Other examples of fuel include natural gas, diesel, propane, and kerosene, each of which has its own specific uses and applications.