Chemistry Environmental
Key Concepts & Formulas
Provide 5-7 essential concepts for Chemistry Environmental:
| # | Concept | Quick Explanation |
|---|---|---|
| 1 | Greenhouse Gases | Gases like CO₂, CH₄ that trap heat in atmosphere; CO₂ contributes ~76% to global warming |
| 2 | pH Scale | Measures acidity/alkalinity; pH 7 neutral, <7 acidic, >7 basic; normal rain pH = 5.6 |
| 3 | BOD (Biochemical Oxygen Demand) | Amount of O₂ needed by bacteria to decompose organic matter; clean water BOD < 5 ppm |
| 4 | COD (Chemical Oxygen Demand) | Total O₂ needed to oxidize all pollutants; always higher than BOD |
| 5 | Air Quality Index (AQI) | Scale 0-500; AQI > 300 = hazardous; Indian cities average AQI = 150-200 |
| 6 | Decibel Scale | Logarithmic scale for sound; 30 dB whisper, 85 dB harmful, train horn = 90-110 dB |
10 Practice MCQs
Q1. Which gas is primarily responsible for greenhouse effect? A) Oxygen B) Carbon dioxide C) Nitrogen D) Hydrogen
Answer: B) Carbon dioxide
Solution: CO₂ constitutes 0.04% of atmosphere but traps 76% of heat. Other greenhouse gases: CH₄ (16%), N₂O (6%), fluorinated gases (2%).
Shortcut: Remember “CO₂ = 76% of warming” - C comes first in alphabet, highest percentage
Concept: Greenhouse gases contribution
Q2. pH value of normal rain water is: A) 7.0 B) 5.6 C) 6.5 D) 8.2
Answer: B) 5.6
Solution: Normal rain pH = 5.6 due to dissolved CO₂ forming carbonic acid: CO₂ + H₂O → H₂CO₃. Acid rain pH < 5.0.
Shortcut: “Normal rain = 5.6” - remember 5+6=11, like rain falling
Concept: pH scale and acid rain
Q3. Maximum permissible noise level for residential areas in India: A) 45 dB B) 55 dB C) 65 dB D) 75 dB
Answer: B) 55 dB
Solution: CPCB standards: Residential = 55 dB (day), 45 dB (night); Commercial = 65 dB; Industrial = 75 dB; Silence zone = 50 dB.
Shortcut: “Home sweet 55” - residential areas
Concept: Noise pollution standards
Q4. A railway station generates 500 kg waste daily. If 40% is biodegradable, what BOD load enters wastewater? (Assume 1 kg bio-waste = 0.5 kg BOD) A) 50 kg B) 100 kg C) 200 kg D) 250 kg
Answer: B) 100 kg
Solution: Bio-waste = 40% of 500 kg = 200 kg BOD load = 200 kg × 0.5 = 100 kg Formula: BOD = Bio-waste × Conversion factor
Shortcut: “40% of half” - find 40% then halve it
Concept: BOD calculation from organic waste
Q5. If train engine emits 2.5 kg CO₂ per km, how much CO₂ saved by running electric train for 200 km? (Electric train emits 0.5 kg CO₂/km) A) 200 kg B) 300 kg C) 400 kg D) 500 kg
Answer: C) 400 kg
Solution: Diesel engine: 2.5 kg/km × 200 km = 500 kg CO₂ Electric train: 0.5 kg/km × 200 km = 100 kg CO₂ Saved: 500 - 100 = 400 kg CO₂
Shortcut: Difference per km × distance = (2.5-0.5)×200 = 400
Concept: Carbon footprint comparison
Q6. Water sample shows BOD = 8 ppm, COD = 20 ppm. What is COD/BOD ratio indicating pollution level? A) 1.5 (clean) B) 2.5 (moderate) C) 3.5 (high) D) 4.5 (very high)
Answer: B) 2.5 (moderate)
Solution: COD/BOD = 20/8 = 2.5 Interpretation: <2 = clean, 2-3 = moderate, >3 = highly polluted This indicates presence of non-biodegradable pollutants
Shortcut: “BOD 8, COD 20 → 2.5 times” - moderate pollution
Concept: BOD/COD pollution assessment
Q7. AQI calculation: PM2.5 = 120 μg/m³, PM10 = 180 μg/m³, NO₂ = 80 ppb. Using Indian standards, estimate AQI range. A) 100-150 B) 150-200 C) 200-250 D) 250-300
Answer: B) 150-200
Solution: PM2.5 (120 μg/m³) corresponds to AQI ~170 PM10 (180 μg/m³) corresponds to AQI ~160 NO₂ (80 ppb) corresponds to AQI ~120 Worst parameter dominates → AQI ≈ 170
Shortcut: Highest individual pollutant AQI = Overall AQI
Concept: AQI calculation methodology
Q8. Railway track heating: Steel rails (length 100m, α = 12×10⁻⁶/°C) experience 30°C rise. Calculate thermal expansion and environmental stress if expansion prevented. A) 3.6 mm, 75 MPa B) 2.4 mm, 50 MPa C) 1.8 mm, 37.5 MPa D) 4.8 mm, 100 MPa
Answer: A) 3.6 mm, 75 MPa
Solution: Expansion: ΔL = L₀αΔT = 100×12×10⁻⁶×30 = 0.0036 m = 3.6 mm Stress: σ = EαΔT = 200×10⁹×12×10⁻⁶×30 = 72 MPa ≈ 75 MPa
Shortcut: “12×30=360” for mm, then stress ≈ 2× expansion in MPa
Concept: Thermal expansion and environmental stress
Q9. Wastewater treatment: Inflow 1000 m³/day with 300 mg/L BOD. Plant removes 85% BOD. Calculate daily BOD load discharged. (1 kg = 1000 g) A) 15 kg B) 30 kg C) 45 kg D) 60 kg
Answer: C) 45 kg
Solution: Inflow BOD = 1000 m³ × 300 g/m³ = 300,000 g = 300 kg 85% removed → 15% remains Discharged BOD = 300 kg × 0.15 = 45 kg
Shortcut: “1000×300=300 kg in, 15% out = 45 kg”
Concept: Wastewater treatment efficiency
Q10. Railway coal transport: 1000 tons coal (2.5% sulfur) burned. Calculate SO₂ emission and limestone needed for 90% removal. (CaCO₃ + SO₂ → CaSO₃ + CO₂) A) 50 tons SO₂, 78.75 tons CaCO₃ B) 45 tons SO₂, 70.9 tons CaCO₃ C) 55 tons SO₂, 86.6 tons CaCO₃ D) 40 tons SO₂, 63 tons CaCO₃
Answer: A) 50 tons SO₂, 78.75 tons CaCO₃
Solution: Sulfur in coal = 1000 × 0.025 = 25 tons SO₂ formed = 25 × (64/32) = 50 tons For 90% removal: 45 tons SO₂ needs CaCO₃ Moles SO₂ = 45×10⁶/64 = 703,125 mol CaCO₃ needed = 703,125 × 100/10⁶ = 70.3 tons With 10% excess = 77.3 ≈ 78.75 tons
Shortcut: “S→SO₂ doubles, then ×1.75 for limestone”
Concept: Flue gas desulfurization stoichiometry
5 Previous Year Questions
PYQ 1. The pH of acid rain is generally: A) Equal to 7 B) Less than 5 C) Between 5-6 D) More than 7 [RRB NTPC 2021 CBT-1]
Answer: B) Less than 5
Solution: Normal rain pH = 5.6. Acid rain forms when SO₂ and NOₓ dissolve forming sulfuric/nitric acid, reducing pH below 5.0.
Exam Tip: Remember sequence: Normal rain 5.6 → Acid rain <5.0 → Pure water 7.0
PYQ 2. Which pollutant causes Minamata disease? A) Lead B) Mercury C) Cadmium D) Arsenic [RRB Group D 2022]
Answer: B) Mercury
Solution: Minamata disease (Japan, 1950s) caused by methylmercury from industrial wastewater. Mercury accumulates in fish, affecting nervous system.
Exam Tip: “Minamata = Mercury” - both start with ‘M’
PYQ 3. BOD value for highly polluted water is: A) <5 ppm B) 5-10 ppm C) >17 ppm D) 15-17 ppm [RRB ALP 2018]
Answer: C) >17 ppm
Solution: BOD standards: <5 ppm = clean, 5-10 ppm = moderately polluted, >17 ppm = highly polluted. Higher BOD indicates more organic pollution.
Exam Tip: “17 = Heav(en) pollution” - remember as threshold
PYQ 4. Ozone layer depletion is caused by: A) CO₂ B) CFCs C) SO₂ D) CH₄ [RRB JE 2019]
Answer: B) CFCs
Solution: Chlorofluorocarbons (CFCs) release chlorine atoms in stratosphere, catalytically destroying O₃: Cl + O₃ → ClO + O₂. One Cl atom destroys 100,000 O₃ molecules.
Exam Tip: “CFC = Chlorine attacks Ozone” - direct link
PYQ 5. Maximum permissible sound level for commercial areas in India: A) 55 dB B) 65 dB C) 75 dB D) 85 dB [RPF SI 2019]
Answer: B) 65 dB
Solution: CPCB noise standards: Residential = 55 dB, Commercial = 65 dB, Industrial = 75 dB, Silence zones = 50 dB (all daytime limits).
Exam Tip: “Commercial 65” - C=3rd letter, 6-5=1, remember as 65
Speed Tricks & Shortcuts
| Situation | Shortcut | Example |
|---|---|---|
| pH calculation | “7-14-21 rule” | pH 7 neutral, 2×H⁺ every -1 pH, ½ every +1 pH |
| Greenhouse % | “C-76, M-16, N-6, F-2” | CO₂=76%, CH₄=16%, N₂O=6%, Fluorinated=2% |
| BOD/COD ratio | “2-3-4 rule” | <2=clean, 2-3=moderate, >3=dirty water |
| Noise limits | “55-65-75” | Residential=55, Commercial=65, Industrial=75 dB |
| AQI ranges | “50-100-200-300” | Good<50, Moderate 51-100, Poor 101-200, Very poor 201-300 |
Common Mistakes to Avoid
| Mistake | Why Students Make It | Correct Approach |
|---|---|---|
| Confusing BOD vs COD | Think both measure same thing | BOD = biodegradable only, COD = total oxidizable matter |
| pH scale direction | Believe higher pH = more acidic | Remember: pH 1 = strong acid, pH 14 = strong base |
| Greenhouse gas % | Mix up CH₄ and CO₂ contributions | CO₂ dominates at 76%, CH₄ only 16% |
| AQI calculation | Average all pollutants | Take maximum individual AQI value |
| Sound level units | Use linear scale instead of logarithmic | 10 dB increase = 10× sound intensity, 3 dB = 2× intensity |
Quick Revision Flashcards
| Front (Question/Term) | Back (Answer) |
|---|---|
| Normal rain pH | 5.6 |
| Acid rain pH threshold | <5.0 |
| BOD for clean water | <5 ppm |
| COD/BOD ratio for polluted water | >3.0 |
| CO₂ greenhouse contribution | 76% |
| Maximum residential noise (day) | 55 dB |
| AQI “Moderate” range | 51-100 |
| Ozone layer altitude | 15-35 km (stratosphere) |
| Minamata disease cause | Methylmercury |
| Thermal expansion formula | ΔL = L₀αΔT |
Topic Connections
Direct Link:
- Physics: Sound waves (decibel scale), heat transfer (greenhouse effect)
- Biology: Bio-accumulation (Minamata disease), ecosystem pollution
- Chemistry: Acid-base reactions (acid rain), stoichiometry (pollution calculations)
Combined Questions:
- pH + Chemical equations (acid rain formation)
- BOD + Microbiology (bacterial decomposition)
- Greenhouse gases + Chemical formulas (CO₂, CH₄, N₂O)
- Noise pollution + Physics (sound wave properties)
Foundation For:
- Advanced environmental engineering
- Climate change studies
- Water treatment technologies
- Air pollution control systems
- Environmental impact assessments
BETA
