৪৯তম বিসিএস ⎯ ফলিত রসায়ন [৫৪১]

Global Warming Potential (GWP) measures how much heat a greenhouse gas traps in the atmosphere relative to CO₂ over a specific period (commonly 100 years).

​Approximate 100-year GWP values:
                 CO₂: 1 (reference)
                CH₄: ~28–36
                N₂O: ~265–298
                SF₆: ~23,500
​
So SF₆has the highest GWP by a huge margin, making it one of the most potent greenhouse gases, even though it exists in very small concentrations.

The greenhouse effect occurs when gases in an atmosphere (like CO₂, CH₄, H₂O vapor) absorb infrared radiation emitted by the planet's surface and re-emit it, trapping heat and warming the surface.
Earth has an atmosphere rich in greenhouse gases, so it retains heat.
The Moon, however, has no significant atmosphere—it’s essentially a vacuum. Therefore, infrared radiation from its surface escapes directly into space, preventing a greenhouse effect.

​ Why the other options are incorrect:

ক) The Moon is only ~384,000 km from Earth, so the difference in solar radiation is negligible.

গ) The Moon does reflect some light, but not all (its albedo is only about 0.12).

ঘ) The Moon actually has an extremely weak magnetic field, not stronger than Earth’s.

Greenhouse gases (like CO₂, CH₄, H₂O) are mostly transparent to incoming solar radiation (which is primarily in the visible and some UV region).
Earth absorbs this energy and re-emits it as longwave infrared radiation.
Greenhouse gases absorb and re-emit infrared (IR) radiation, which traps heat in the atmosphere and causes the greenhouse effect.

 Why others are incorrect:

ক) UV: Mostly absorbed by the ozone layer, not greenhouse gases.

খ) Visible: Passes through the atmosphere with little absorption.

ঘ) Microwaves: Absorbed mostly by water in clouds (used in microwave ovens), not the main cause of greenhouse warming.

Water vapor is the most abundant greenhouse gas, but its concentration in the atmosphere depends strongly on temperature.
When CO₂ and other gases cause warming, the air holds more water vapor, which in turn absorbs more infrared radiation, leading to positive feedback and further warming.
However, water vapor is not the initial driver, because it responds to temperature changes rather than initiating them.

​ Why the others are wrong:

ক) Incorrect: Water vapor does not primarily emit heat to space; it traps heat within the atmosphere.

গ) Incorrect: It condenses locally but continuously evaporates elsewhere, maintaining a global presence.

ঘ) Incorrect: Water vapor mainly absorbs infrared radiation, not UV.

Increasing albedo (reflectivity) means more incoming solar radiation is reflected back to space, reducing the amount of energy absorbed by Earth.
Reflective aerosols in the stratosphere mimic volcanic eruptions (like Mount Pinatubo in 1991), which temporarily cooled the Earth by scattering sunlight.
This approach is even considered as a geoengineering method to counteract global warming.

​ Why the others are incorrect:

খ) Increasing ocean temperatures reduces CO₂ absorption because warm water holds less CO₂, worsening greenhouse effect.

গ) More urban concrete surfaces lower albedo (darker than snow/vegetation), causing urban heat island effect, which increases warming.

ঘ) More transpiration = more water vapor, which is a strong greenhouse gas → amplifies warming, not reduce it.

Radiative forcing refers to the difference between incoming solar energy and outgoing infrared energy caused by factors like greenhouse gases, aerosols, or land use changes.
When greenhouse gases increase, they trap more heat, causing positive radiative forcing (warming).
Conversely, aerosols or increased albedo can cause negative radiative forcing (cooling).
This metric is crucial in climate science because it quantifies how different factors drive climate change.

Why not the others?

ক) Total solar radiation at Earth’s surface → That’s insolation, not radiative forcing.
খ) Energy emitted by the Sun → That’s solar irradiance, unrelated to the Earth's energy imbalance.
গ) Reflection by clouds/aerosols → That’s albedo effect, which can influence radiative forcing but is not the definition itself.

The Antarctic region experiences the most severe seasonal ozone depletion, commonly called the “ozone hole.”
This happens during the Southern Hemisphere spring (September–November) due to:
    1. Extreme cold temperatures forming polar stratospheric clouds (PSCs) in winter.
    2. PSCs provide surfaces for chemical reactions that activate chlorine and bromine compounds from CFCs.
    3. When sunlight returns in spring, these compounds rapidly destroy ozone.
Ozone reduction can exceed 50% in this region.

Why not the others?

ক) Arctic region → Also experiences ozone depletion, but less severe due to warmer stratospheric temperatures and less stable polar vortex.
গ) Mid-latitudes → Ozone levels decline here, but the depletion is gradual, not extreme.
ঘ) Equatorial region → Strong UV radiation, but ozone layer is relatively thick and stable, so no major seasonal depletion.

CFC-12 (chemical formula: CCl₂F₂) is a chlorofluorocarbon widely used as a refrigerant and propellant.

In the stratosphere, UV radiation breaks down CFCs, releasing chlorine atoms, which act as catalysts in ozone destruction:
                             Cl+O₃→ClO+O₂
                             ClO+O→Cl+O₂
A single chlorine atom can destroy thousands of ozone molecules before being deactivated.
This process caused the formation of the Antarctic ozone hole.

Why not the others?

ক) CO₂ → Major greenhouse gas, but does not destroy ozone.
খ) Methane → Can affect ozone indirectly via stratospheric chemistry, but it is not the major contributor.
ঘ) Sulfur dioxide (SO₂) → Mainly contributes to acid rain and aerosol formation, not ozone depletion.

Ozone depletion is more severe in the polar regions (especially Antarctic) due to:
     1. Polar vortex: A strong, stable wind pattern that isolates the air over the poles during winter, preventing mixing with ozone-rich air from lower latitudes.
     2. Polar Stratospheric Clouds (PSCs): Extremely cold temperatures allow PSCs to form, providing surfaces for chemical reactions that activate chlorine and bromine from CFCs into ozone-destroying forms.
     3. When sunlight returns in spring, these active chlorine compounds rapidly destroy ozone, causing the ozone hole.

Why not the others?

ক) Stronger UV radiation at the poles → Actually, UV is weaker at the poles than at the equator.
গ) Higher CFC concentration at poles → CFCs mix globally; their concentration is not higher at the poles.
ঘ) Lack of greenhouse gases at poles → Greenhouse gases affect climate, not directly the ozone depletion mechanism.

The Montreal Protocol (1987) is an international treaty designed to phase out the production and consumption of ozone-depleting substances (ODS) such as:
                   CFCs (chlorofluorocarbons)
                  Halons
                  Carbon tetrachloride
                   HCFCs (hydrochlorofluorocarbons)
The protocol has been highly successful, leading to a gradual recovery of the stratospheric ozone layer, particularly over the Antarctic.

Why not the others?

ক) Greenhouse gases → Regulated under the Kyoto Protocol and Paris Agreement, not Montreal.
খ) Acid rain precursors → Such as SO₂ and NOₓ, addressed by separate environmental policies.
ঘ) Volatile organic compounds → Primarily regulated for air pollution and smog formation, not ozone depletion.

Chlorine radicals (Cl) are the primary catalysts in the destruction of stratospheric ozone, especially in the Antarctic ozone hole.
​Mechanism:
1. ​CFCs are broken down by UV radiation, releasing chlorine atoms:
                     CCl₃F→Cl+CCl₂F    (In presence of UV)
2. Chlorine reacts with ozone:
                   Cl+O₃→ClO+O₂
3. ClO reacts with atomic oxygen, regenerating Cl:
                   ClO+O→Cl+O₂
One Cl atom can destroy thousands of ozone molecules before being deactivated, making it extremely effective in polar ozone depletion.
​
​Why not the others?

ক) Hydroxyl radicals (OH) → Important in tropospheric ozone chemistry, but minor in polar stratosphere.
খ) Nitrogen oxides (NOx) → Can influence ozone indirectly but are less important than Cl in Antarctic depletion.
গ) Oxygen atoms (O) → React with ozone naturally, but the catalytic effect is dominated by Cl in the ozone hole.

The Clean Development Mechanism (CDM) is one of the flexible mechanisms under the Kyoto Protocol.

​It allows a developed country (Annex I country) to: 
​    1. Invest in emission reduction projects in a developing country (non-Annex I country).
    2. Earn Certified Emission Reductions (CERs), which can be counted toward the developed country’s emission reduction targets.

​Examples of CDM projects:
       1. Renewable energy (solar, wind, hydro)
       2. Methane capture from landfills
       3. Energy efficiency improvements

Why not the others?

ক) Joint Implementation (JI) → Projects occur between developed countries, generating Emission Reduction Units (ERUs).
খ) Emissions Trading → Allows countries to buy or sell emission allowances but not tied to specific projects in developing countries.
ঘ) Carbon Border Adjustment → A trade measure to tax imports based on carbon content, unrelated to CERs.

The Kyoto Protocol (1997) specifically targets six greenhouse gases (GHGs) that contribute to global warming:

              1. Carbon dioxide (CO₂)
              2. Methane (CH₄)
              3. Nitrous oxide (N₂O) – not NO₂
              4. Hydrofluorocarbons (HFCs)
              5. Perfluorocarbons (PFCs)
              6. Sulfur hexafluoride (SF₆)

Nitrogen dioxide (NO₂) is a pollutant contributing to smog and acid rain, but it is not classified as a major greenhouse gas under Kyoto.

Why not the others?

ক) CO₂ → Primary GHG, directly covered.
খ) CH₄ → Potent GHG, covered.
গ) SF₆ → Extremely potent GHG, covered.

The Kyoto Protocol (1997) set legally binding emission reduction targets for Annex I (developed) countries.
The first commitment period for these countries was from 2008 to 2012.
During this period, Annex I countries were expected to reduce their collective greenhouse gas emissions by an average of 5% below 1990 levels.

Why not the others?

ক) 1999-2000 → Too short; the protocol allowed time for ratification and preparation.
গ) 2013-2020 → This corresponds to the second commitment period (Doha Amendment), not the first.
ঘ) 1997-2005 → The Kyoto Protocol was adopted in 1997, but the commitments only became legally binding in 2008.

Under the Kyoto Protocol, countries were divided into Annex I (developed) and non-Annex I (developing) countries.
Annex I countries had legally binding targets to reduce greenhouse gas emissions during the first commitment period (2008–2012).
Non-Annex I countries, like India, China, Brazil, South Africa, Indonesia did not have binding emission reduction obligations, although they were encouraged to adopt sustainable development practices and could participate in Clean Development Mechanism (CDM) projects.

Assigned Amount Units (AAUs) are a key component of the Kyoto Protocol’s emissions trading system.
Each AAU represents one metric ton of CO₂-equivalent emissions that an Annex I (developed) country is allowed to emit during a commitment period.
If a country emits less than its assigned amount, it can sell surplus AAUs to other countries under the Emissions Trading Mechanism.
This system encourages cost-effective emission reductions by allowing countries to trade emission allowances.
​
​ক) Carbon credits earned through afforestation → That refers to Certified Emission Reductions (CERs) from CDM projects.
গ) Equivalent to CERs from CDM projects → AAUs are assigned allowances, while CERs are project-based reductions.
ঘ) Only used under the Paris Agreement → AAUs are Kyoto Protocol instruments, not part of the Paris Agreement.

The Kyoto Protocol Compliance Mechanism ensures that countries meet their emission reduction targets.
If an Annex I country fails to meet its target during a commitment period, the consequences include:
Making up the shortfall in the next commitment period plus an additional 30% (effectively reducing 1.3 times the deficit). Some sources round it to "twice the shortfall" for simplification.
Suspension from participating in the emissions trading market until compliance is restored.
This mechanism encourages countries to take their targets seriously while providing a structured way to catch up.

Why not the others?

খ) Banned from future UNFCCC negotiations → There is no such permanent ban.
গ) Pay a financial penalty to the Green Climate Fund → Kyoto does not impose monetary fines.
ঘ) Lose all their AAUs immediately → Countries retain AAUs but face restrictions until compliance.

The Resource Conservation and Recovery Act (RCRA, 1976, USA) defines hazardous waste based on specific characteristics:
1. Ignitability – Waste can catch fire under certain conditions (e.g., solvents, fuels).
2. Corrosivity – Waste can corrode metals or destroy living tissue (e.g., strong acids/bases).
3. Reactivity – Waste is unstable or explosive under normal conditions (not listed in the options).
4. Toxicity – Waste is harmful or fatal when ingested, inhaled, or absorbed (e.g., heavy metals, pesticides).
5. Biodegradability, on the other hand, refers to the ability of a substance to decompose naturally by microorganisms.
      **Biodegradable wastes are generally less of an environmental hazard, so this is not a characteristic of hazardous waste.

Cyanide-containing industrial wastewater is highly toxic and requires chemical oxidation for safe treatment.
Alkaline chlorination is the most commonly used method:
Wastewater is made alkaline (pH > 10.5) to prevent formation of toxic hydrogen cyanide (HCN).
Chlorine or sodium hypochlorite is added to oxidize cyanide (CN⁻) to cyanate (OCN⁻), which is far less toxic.
Cyanate can further hydrolyze to ammonia and carbon dioxide.
                                CN⁻+Cl₂→OCN⁻+Cl⁻
This process is rapid, effective, and widely applied in industry.

Why not the others?

ক) Activated sludge process → Biological treatment, not suitable for highly toxic cyanide; microorganisms can be inhibited.
গ) Sand filtration → Physical removal only; does not degrade cyanide.
ঘ) Anaerobic digestion → Used for organic waste; cyanide toxicity prevents microbial activity.

Leachate is the liquid that drains or “leaches” from a landfill, often containing organic matter, heavy metals, and toxic chemicals.
A leachate collection system in a secure (engineered) landfill is designed to:
             1. Collect leachate at the bottom of the landfill.
             2. Prevent it from percolating into the soil and contaminating groundwater.
             3. Allow safe treatment or recirculation of the leachate.
This is a critical environmental protection measure in modern landfill design.

Why not the others?

ক) Prevent dust formation → Addressed by daily cover or sprinkling, not leachate systems.
খ) Reduce methane gas formation → Methane is controlled by gas collection systems, not leachate collection.
গ) Improve thermal stability of waste → Thermal stability is not a primary goal of leachate collection.

The waste minimization hierarchy is a framework used to prioritize waste management practices based on environmental impact:
          1. Source reduction (prevention) – Minimize waste generation at its origin. This is the most preferred option.
          2. Recycling and reuse – Recover materials for use in new products, reducing the need for virgin resources.
          3. Treatment – Physically, chemically, or biologically treat waste to reduce hazards or volume.
          4. Disposal – Safely discard residual waste, usually in landfills or incineration, as the least preferred option.
This hierarchy helps in reducing environmental pollution, conserving resources, and lowering costs.

Why not the others?

ক) Incorrect order; landfilling should be last, not first.
গ) Incorrect order; treatment and disposal come after prevention and recycling.
ঘ) Incorrect order; source reduction should precede recycling.

The chemical and petrochemical industry is the largest global contributor to hazardous waste because it generates:
                Toxic organic solvents
                Acids and bases
                 Heavy metals
                 By-products from chemical reactions
Hazardous wastes from this sector require specialized treatment, storage, and disposal due to their toxicity, flammability, corrosivity, and reactivity.

Why not the others?

ক) Textile industry → Produces dyes and chemicals, but overall hazardous waste quantity is lower.
খ) Pharmaceutical industry → Generates hazardous chemicals in smaller volumes.
ঘ) Food processing industry → Mostly organic waste, generally non-hazardous.

Zero Liquid Discharge (ZLD) is an advanced wastewater management strategy where:
       1.  All wastewater generated in an industrial process is treated.
       2. Clean water is recovered and reused within the plant.
       3. No untreated or partially treated liquid is discharged into rivers, lakes, or groundwater.

Key techniques in ZLD:
        1. Reverse osmosis (RO)
        2. Evaporation and crystallization
        3. Membrane filtration 
​
​Why not the others?

খ) Complete recycling of solid waste → ZLD deals with liquid waste, not solid waste.
গ) Elimination of gaseous emissions → That relates to air pollution control, not ZLD.
ঘ) Disposal of wastewater directly to surface water → Opposite of ZLD; this creates pollution.

High-concentration organic industrial wastewater typically has high BOD (Biochemical Oxygen Demand), meaning it contains a large amount of biodegradable organic matter.
Anaerobic digestion is most suitable because:
       ** It treats high-strength wastewater efficiently.
       ** Microorganisms break down organic matter in the absence of oxygen, producing biogas (methane + CO₂) as a useful by-product.
        **It reduces BOD and COD significantly and is energy-efficient for high-strength waste.

Why not the others?

ক) Trickling filter → Suitable for low to moderate BOD wastewater; less effective for very high-strength industrial waste.
খ) Aerated lagoon → Works for moderate-strength wastewater; requires large land area and long retention time.
ঘ) Chemical precipitation → Used for inorganic pollutants like heavy metals, not organic BOD reduction.

Pig iron is the intermediate product of iron smelting in a blast furnace.
Its composition is:
             Carbon (C): 2.0% – 4.5% – makes it hard and brittle.
             Silicon (Si): 0.2% – 3.0%
            Manganese (Mn): 0.3% – 1.0%
           Phosphorus (P) and Sulfur (S): trace amounts
The high carbon content distinguishes pig iron from wrought iron (0.02–0.08% C) and steel (0.2–2.0% C).

Wrought iron is almost pure iron (very low carbon content, 0.02%–0.08%), giving it:
         High ductility – can be stretched or bent without breaking.
         High malleability – can be hammered or shaped easily.

These properties make it ideal for decorative applications like gates, railings, and grills, where intricate shapes and curves are required.

Why not the others?

ক) High compressive strength → Important for structural support, not decorative shaping.
গ) High hardness → Wrought iron is relatively soft, making it easier to work with.
ঘ) Low corrosion resistance → Wrought iron has better corrosion resistance than steel, but low corrosion resistance would be undesirable.

Red shortness refers to the tendency of steel to become brittle when hot (during forging or hot working).
The primary cause is sulphur impurity in pig iron:
         Sulphur combines with iron to form iron sulfide (FeS).
         FeS melts at a relatively low temperature (~988°C), which causes grain boundary weakness in steel at high temperatures.
As a result, steel becomes brittle and cracks during hot working.

Why not the others?

খ) Phosphorus → Causes cold shortness (brittleness at room temperature), not red shortness.
গ) Manganese → Actually improves hot workability and counteracts red shortness.
ঘ) Silicon → Strengthens steel but does not cause red shortness.

Basic Oxygen Steelmaking (BOS) is a primary method to produce steel from pig iron.
Major role of oxygen:
Blown oxygen reacts with carbon in pig iron to form CO and CO₂, reducing carbon content to the desired level:
                        C+O→CO / CO₂
Oxidizes other impurities such as silicon, manganese, phosphorus, and sulfur, forming oxides that are removed as slag.
This process rapidly converts high-carbon pig iron into low-carbon steel.
​
W​hy not the others?

ক) Increase carbon content in steel → Opposite effect; oxygen reduces carbon.
খ) Improve ductility by adding oxygen into the lattice → Oxygen is not added to the steel lattice; it oxidizes impurities.
গ) Add hardness by forming carbides → Oxygen removes carbon, reducing carbides, not forming them.

The eutectoid composition of steel is 0.8% carbon.
         
​         Steel with < 0.8% C → Hypoeutectoid steel (ferrite + pearlite).
         Steel with = 0.8% C → Eutectoid steel (100% pearlite).
         Steel with > 0.8% C → Hypereutectoid steel (pearlite + cementite).

Therefore, steel around 0.8% carbon is at or slightly below eutectoid, so classified as hypoeutectoid if slightly below 0.8%.

Pig iron is the product of smelting iron ore in a blast furnace and contains:
              Carbon: 2–4.5%
              Silicon, phosphorus, manganese: small amounts
High carbon and impurity content makes pig iron:
              Hard and brittle
             Difficult to forge or shape
Unlike steel, pig iron cannot be used directly for most structural applications without further refining.

Approximate lifetimes of the given gases are:

Methane (CH₄) → ~12 years

Carbon dioxide (CO₂) → complex, but effective lifetime ~30–100+ years (some fractions persist for thousands of years in oceans and soils)

Nitrous oxide (N₂O) → ~114 years

Sulfur hexafluoride (SF₆) → ~3,200 years

In the atmosphere, methane (CH₄) is primarily removed through oxidation by hydroxyl radicals (OH).
The main reaction starts as:
                                        CH₄+OH→CH₃+H₂O
Through subsequent steps, this ultimately produces CO₂ and water (H₂O).
​
 ​Other Options:
 (ক) Photolysis is not significant for methane because methane does not strongly absorb UV radiation.
 (খ) Combustionrefers to burning, which does not occur naturally in the atmosphere under normal conditions.
 (ঘ) Condensation means phase change from gas to liquid, which does not apply to methane.

Fertilizer use in agriculture is the dominant anthropogenic source of nitrous oxide (N₂O).
​ Nitrogen-based fertilizers increase nitrification and denitrification processes in soil, which release N₂O as a byproduct.
​
 ​Other Options:
 (ক) Deforestation contributes mainly to CO₂ emissions, not significant N₂O.
 (খ) Landfills are significant sources of methane (CH₄), not N₂O.
 (গ) Fossil fuel combustion emits CO₂ and NOₓ gases, but contributes less to N₂O compared to agricultural fertilizers.

The stratospheric ozone layer acts as a shield by absorbing most of the harmful ultraviolet radiation, especially UV-B (280–315 nm) and UV-C (100–280 nm).

 ​Other Options:
​ (ক) Visible light mostly passes through the atmosphere and reaches Earth’s surface.
 (খ)Infrared  is absorbed mainly by greenhouse gases like CO₂, H₂O vapor—not ozone.
 (ঘ)Microwave  is not absorbed by ozone; it's used in communication and heating (like in microwave ovens).

Ozone layer depletion allows more harmful UV-B radiation to reach Earth’s surface. This exposure can cause:
     1.Skin cancer (melanoma and non-melanoma)
     2.Cataracts and other eye damage
     3.Suppression of the immune system

​Other Options:
Increased heat waves (ক) and global warming​(ঘ) are linked to greenhouse gas emissions, not directly to ozone depletion.
Respiratory diseases​(গ) are primarily caused by air pollutants like particulate matter, ozone at ground level (tropospheric ozone), and smog—not stratospheric ozone loss.

HFC-134a (1,1,1,2-Tetrafluoroethane) is a hydrofluorocarbon widely used as a refrigerant and an alternative to CFCs because it does not deplete the ozone layer (contains no chlorine).

Other Options:​
SF₆ (ক) is sulfur hexafluoride, used as an electrical insulator, not an HFC.
CF₄ (গ) is carbon tetrafluoride, a perfluorocarbon (PFC), not an HFC.
CH₄ (ঘ) is methane, a simple hydrocarbon and a greenhouse gas, but not an HFC.

Aerosols (tiny particles or droplets in the atmosphere) primarily influence climate by scattering and reflecting incoming solar radiation back into space, which leads to a cooling effect on Earth’s surface.
They can also serve as cloud condensation nuclei, affecting cloud properties and enhancing the cooling effect.

 ​Other Options:
 (ক)Increase methane concentration  – aerosols do not increase methane.
 (খ) Destroy ozone molecules – certain aerosols (like those containing chlorine from CFC breakdown) can participate in ozone depletion, but that is not their main climate role.
 (ঘ) Absorb CO₂– aerosols do not absorb CO₂; greenhouse gases do that.

Emissions Trading (also called Carbon Trading) under the Kyoto Protocol allows countries with excess emission units (because they emit less than their assigned amount) to sell this surplus to other countries that exceed their targets. This creates a carbon market.

 Other Options:
 (ক)Joint Implementation: A mechanism where an Annex I country can earn emission reduction units by investing in another Annex I country’s emission-       reduction project
​ (খ) Carbon Tax: This is a domestic policy instrument, not a Kyoto Protocol mechanism.
 (ঘ)Clean Development Mechanism : Allows Annex I countries to invest in emission-reduction projects in developing countries and earn Certified Emission     
​ Reductions (CERs).

Secondary greenhouse gases are not emitted directly, but form in the atmosphere as a result of chemical reactions or feedbacks from primary gases:
H₂O vapor (খ) increases due to evaporation and as a feedback effect from warming caused by CO₂, CH₄, and other primary greenhouse gases.
​
Other ​Options:
 (ক) ​CO₂– fossil fuel burning, deforestation
​ (গ)CH₄  – agriculture, landfills, fossil fuel extraction
 (ঘ) N₂O – fertilizers and industrial processesdback effect from warming caused by CO₂, CH₄, and other primary greenhouse gases.

Under the Resource Conservation and Recovery Act (RCRA) in the United States, reactive wastes are those that are unstable under normal conditions and can:
1.Explode
2.React violently with water
3.Produce toxic gases spontaneously
Examples include cyanides, sulfides, or certain explosives.

Other options:
(ক)High moisture content : Not a criterion for reactivity.
(খ)Recyclable waste ): This relates to reuse or resource recovery, not reactivity.
(গ)Waste emitting greenhouse gases : This is an environmental concern, but does not define reactive waste under RCRA.

Chemical precipitation is the most effective method for treating heavy metal-contaminated wastewater.
1.It works by adding chemicals (like lime, sulfides, or hydroxides) that react with dissolved metals to form insoluble metal compounds.
2.These insoluble compounds can then be removed by sedimentation or filtration.

 Other options:
 (খ)Activated sludge process: Targets organic pollutants, not heavy metals.
 (গ)Sand filtration : Removes suspended solids, ineffective for dissolved metals.
 (ঘ)Anaerobic digestion: Treats organic matter biologically, not suitable for metals.

Basic Oxygen Steelmaking (BOS) uses a basic flux, typically lime (CaO), to remove impurities like phosphorus (P) and sulfur (S) from molten iron.
The flux reacts with these impurities to form slag, which floats on top of the molten steel and can be removed.

Other options:
(ক)Bessemer Process : The original Bessemer process used an acidic lining, so it could not remove phosphorus effectively.
(খ)Electric Arc Furnace : Primarily melts scrap steel; impurity removal depends on additives, not a standardized basic flux. 
​(ঘ)Open Hearth Process: Can use basic fluxes in some variants, but BOS is more commonly associated with efficient phosphorus and sulfur removal.

​Pig iron is the intermediate product of ironmaking in a blast furnace. Its composition typically includes:​
​1. Carbon: 3–4.5% 
​2.Silicon: 0.8–2.0% 
3.​Manganese: 0.2–1.0% 
4.​Sulfur and phosphorus: trace amounts
​
 ​Other Options:
 ​(ক)0.02% – 0.08% : Too low; typical for refined steel, not pig iron.
​​ (খ)4.5% – 6.0%  : Very high; not found in pig iron.
 ​(গ)0.2% – 3.0%: Broad range, but the standard is 0.8–2.0% for silicon.

High Carbon Steel contains 0.6%–1.5% carbon, giving it:
   1.High hardness
   2.High strength
   3.Good wear resistance
These properties make it ideal for cutting tools, blades, drills, and springs.


​Other options:
(​ক)Mild Steel: Low carbon (≤0.3%), soft, ductile; used for construction, not cutting tools.
(খ)Medium Carbon Steel : 0.3%–0.6% carbon; balanced strength and ductility, used in gears, shafts.
(ঘ)Hypoeutectoid Steel : Carbon <0.8%; can be low or medium carbon steel, not as hard as high carbon steel.

Thermal cracking occurs when hydrocarbons are exposed to high temperatures, causing them to break down into smaller molecules.
In vacuum distillation, the operating pressure is lowered, which reduces the boiling points of heavy fractions.
Lower temperatures mean the hydrocarbons are less likely to thermally crack, allowing the distillation of heavy oils safely.

Other options:
 (ক)Increasing feed rate: Does not prevent thermal cracking; may even reduce separation efficiency.
 (গ)Reducing column temperature: Can help locally but is not practical for distilling heavy fractions.
 (ঘ)Increasing operating pressure: Increases boiling points, which increases thermal cracking risk.

Sulfur dioxide (SO₂) is a major acid rain precursor:
In the atmosphere, SO₂ reacts with water and oxygen to form sulfuric acid (H₂SO₄), which falls as acid rain.

Other options:
(ক)CO₂  – major greenhouse gas, does not cause acid rain.
(খ) CH₄– greenhouse gas, not an acid rain precursor.
(ঘ)SF₆  – potent greenhouse gas, chemically inert, does not contribute to acid rain.

The cement industry is a major contributor to global CO₂ emissions because:
Calcination reaction: Limestone (CaCO₃) decomposes to CaO + CO₂ during clinker production.
Energy consumption: Kilns use large amounts of fossil fuels, adding more CO₂.

Other sectors:
 (ক) Steel industry: Also a significant emitter, mainly from coke and ironmaking, but slightly less than cement in global totals.
 (গ) Food processing and (ঘ)Textile industry : Contribute far less CO₂ compared to cement and steel.

A biogas digester (anaerobic digester) is used to break down organic matter in wastewater or organic waste under anaerobic conditions (absence of oxygen).
During this process, microorganisms decompose the organic material and produce:
 1.Methane (CH₄) – usable as a renewable energy source
 2.Carbon dioxide (CO₂)
 3.A stabilized sludge that can be used as fertilizer

Other options:
(ক)Remove heavy metals : Not the purpose of biogas digesters.
(গ)Neutralize pH : Only minor effect, not the main purpose.
(ঘ)Remove suspended solids : Some settling occurs, but the primary goal is biogas production.

SF₆ is a synthetic gas used primarily in electrical equipment as an insulating and arc-quenching medium.
It has an extremely high Global Warming Potential (GWP) — about 23,500 times that of CO₂ over 100 years.

Other options:
(ক)CO₂: Major greenhouse gas but low GWP per unit.
(গ)CH₄ : High GWP, but not used for electrical insulation.
(ঘ)N₂O : Moderate GWP, used mainly in agriculture, not electrical insulation.

Nitrification and denitrification are biological processes used to remove nitrogen compounds from wastewater:
1.Nitrification: Ammonia (NH₃/NH₄⁺) is oxidized to nitrite (NO₂⁻) and then to nitrate (NO₃⁻) by aerobic bacteria.
2.Denitrification: Nitrate is converted to nitrogen gas (N₂) by anaerobic bacteria, which is released to the atmosphere, effectively removing nitrogen from the water.

Other options:
(খ)Anaerobic digestion : Mainly removes organic matter, produces biogas.
(গ)Chemical precipitation: Removes metals and some phosphates, not nitrogen.
(ঘ)Filtration : Removes suspended solids, not dissolved nitrogen compounds.