Environmental Chemistry

Statement I is correct, but statement II is not correct. The correct form of statement II is,

The number of greenhouse gases are three.

They are $\mathrm{CH}_4, \mathrm{CO}_2$ and $\mathrm{H}_2 \mathrm{O}(\mathrm{g})$.

Photochemical smog is formed by the reaction of sunlight with pollutants like nitrogen oxides ( $\mathrm{NO}_x$ ) and hydrocarbons (VOCs). Among the given compounds, only sulphur dioxide ( $\mathrm{SO}_2$ ) is not a typical component of photochemical smog. Instead, it is mainly associated with classical smog.

Nitrogen dioxide $\left(\mathrm{NO}_2\right)$ is a known air pollutant that acts as a strong lung irritant. When inhaled, it reacts with the moisture in the respiratory tract to form acids ( $\mathrm{HNO}_2$ ) and $\mathrm{HNO}_3$ ) that can damage the delicate lung tissue. Exposure to $\mathrm{NO}_2$ is particularly harmful to children, as it can increase their susceptibility to respiratory infections and lead to acute respiratory illnesses.

Let's examine the other options.

• Sulphure dioxide $\left(\mathrm{SO}_2\right)$ is also a potent lung irritant and a major cause of respiratory problems, especially in asthamtics. However, between the given options, $\mathrm{NO}_2$ is often specifically cited for its role in causing acute respiratory disease.

• Carbon dioxide $\left(\mathrm{CO}_2\right)$ is a simple asphyxiant at high concentrations,

meaning it displaces oxygen. It is not a long irritant.

• Carbon monoxide (CO) is a toxic gas that impairs the blood's ability to carry oxygen by binding to hemoglobin. It is not a lung irritant, Therefore, nitrogen dioxide $\left(\mathrm{NO}_2\right)$ is the correct answer.

The maximum prescribed concentration of zinc in drinking water is $15 \mathrm{mg} \mathrm{dm}^{-3}$. The maximum prescribed concentrations of copper is $1 \mathrm{mg} \mathrm{dm}^{-3}$ Thus, the correct option is $\frac{15}{x}$.

The correct statements are given in I and III. While II and IV are incorrect. Their correct forms are,

II. $\mathrm{NO}_2$ is not present in classical smog. It consist of $\mathrm{SO}_2$, fog, smoke.

IV. pH of rain water is around 5.6 .

🌍 Background:

The depletion of the ozone layer is mainly caused by halogen (Cl, Br) and nitrogen oxides (NOₓ) that catalytically destroy ozone (O₃) molecules in the stratosphere.

The main culprits are:

• CFCs (chlorofluorocarbons) such as CF₂Cl₂ (Freon-12).

• NO and NO₂ (from supersonic aircrafts, etc.) to a limited extent.

🔹 Option A: NO

• Nitric oxide (NO) can indeed react with ozone and participate in catalytic cycles that destroy ozone.

• So NO does contribute to ozone layer depletion.

✅ Primarily responsible.

🔹 Option B: CF₂Cl₂ (CFC-12)

• A chlorofluorocarbon that releases chlorine radicals (Cl·) under UV light in the stratosphere.

• These radicals catalytically destroy ozone.

• CFCs are the principal cause of ozone layer depletion.

✅ Primarily responsible.

🔹 Option C: CH₄ (Methane)

• Methane does not destroy ozone directly.

• In fact, methane can react with Cl· radicals, forming HCl and thereby reducing the concentration of ozone-depleting radicals to some extent.

• It is not a major ozone-depleting substance.

🚫 Not primarily responsible.

🔹 Option D: Cl₂ (Chlorine gas)

• Though molecular chlorine can destroy ozone, it doesn’t reach the stratosphere in large amounts due to high reactivity and solubility.

• However, under certain conditions, Cl₂ produced from CFC photolysis can release Cl atoms – so it is indirectly involved.

✅ Can contribute, though not directly emitted into stratosphere.

✅ Final Answer:

$ \boxed{\text{CH}_4} $

Methane (CH₄) is not primarily responsible for depletion of the ozone layer in the stratosphere.

Eutrophication can lead to decrease in dissolved oxygen. Eutrophication is a phenomenon in which a water body become overly enriched with nutrients, leading to the plentiful growth of simple plant life. Excessive growth of algae and plankton is indicator of this process.

Thermal power plants generates non-biodegradable waste such as fly ash.

Non-biodegradable waste is defined as a substance that cannot be decomposed naturally.

The complete reaction is as follows.

Hence, compound $ X $ is $ \mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CHO} $.

Methemoglobinemia is a rare blood disorder that effects how red blood cells deliver oxygen throughout the body.

One of the main reason behind is excess of nitrate concentration in drinking water.

Photochemical smog primarily consists of secondary pollutants that form when sunlight reacts with pollutants such as nitrogen oxides and volatile organic compounds (VOCs). The common components of photochemical smog include ozone ($\mathrm{O}_3$), nitrogen oxides such as nitric oxide ($\mathrm{NO}$), and peroxyacetyl nitrate (PAN).

Therefore, the correct combination is:

Option D

$\mathrm{O}_3$ (ozone)

$\mathrm{NO}$ (nitric oxide)

$\mathrm{PAN}$ (peroxyacetyl nitrate)

Atmospheric residence time is the average time spent by a molecule in the atmosphere, after it leaves a source and before it encounters a sink.

The tracking models estimate the global average residence time to be around 8.5 days based on Eka-interium data, thus according to recent study the average residence time of water in the atmosphere is $4-5$ day's is not correct. It is around 8.5.

Methane has lowest average atmospheric residence time among the greenhouse gases.

Hence, option (b) is the correct answer.

Out of the gases, I. $\mathrm{SO}_2$, II. $\mathrm{CO}_2$, III. $\mathrm{NO}_2$ and IV. $\mathrm{CH}_4$ Only $\mathrm{SO}_2$ and $\mathrm{NO}_2$ are mainly responsible for acid rain.

Oxides of sulphur can act as air pollutants. e.g. $\mathrm{H}_2 \mathrm{~S}$ is readily oxidised to $\mathrm{SO}_2$ in the atmosphere.

$ \begin{aligned} & \mathrm{H}_2 \mathrm{~S} \xrightarrow{\mathrm{O}_2} \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \\ & \mathrm{H}_2 \mathrm{~S} \xrightarrow{\mathrm{O}_3} \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \end{aligned} $

Hence, option (d) is the correct answer.

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of a sample of water, is called biochemical oxygen demand (BOD). The amount of BOD in the water is a measure of the amount of organic material in the water, in terms of much oxygen will be required to break it down biologically. Clean water would have BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17 ppm or more.

It means BOD value less than 5 ppm indicates a water sample to be rich in dissolved oxygen.