Sciences - Co-ordinated (Double) (0654) | Chemistry of the environment

🌟 Chemistry of the Environment (C10) Study Notes 🌟

Hello future scientist! Welcome to Chapter C10, where we explore the fascinating chemistry behind the world around us—the water we drink and the air we breathe. This chapter connects basic chemical concepts (like combustion and purification) directly to big global issues like climate change and pollution. Don't worry if these topics seem large; we’ll break them down into easy-to-understand chunks!

Understanding environmental chemistry isn't just important for your exam; it helps you become an informed citizen capable of discussing real-world challenges! Let's dive in!

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C10.1 Water

Water Purity: Distilled vs. Tap Water

In chemistry, we often need pure substances. Water is defined as pure if it contains *only* $\text{H}_2\text{O}$ molecules.

1. Testing for the Presence of Water

How do we know if a substance is water, or just looks like water? We use chemical tests involving anhydrous compounds (compounds with no water chemically attached).

• Anhydrous Cobalt(II) Chloride: This is originally blue. If water is added, it turns pink (forming hydrated cobalt(II) chloride).
  Memory Aid: Co-BALT $\rightarrow$ Blue. Water makes it change color.
• Anhydrous Copper(II) Sulfate ($\text{CuSO}_4$): This is originally white. If water is added, it turns blue (forming hydrated copper(II) sulfate).

2. Testing for the Purity of Water

A substance is pure if it has a specific, fixed melting point (m.p.) and boiling point (b.p.) at standard pressure.

• Pure water melts at \(0^\circ\text{C}\) and boils at \(100^\circ\text{C}\).
• If water contains impurities (like salt or mineral ions), its boiling point will be higher than \(100^\circ\text{C}\), and its melting point will be lower than \(0^\circ\text{C}\).

💡 Quick Note: We use distilled water (very pure water) in practical chemistry experiments instead of tap water because tap water contains dissolved chemical impurities that could interfere with or contaminate the results of a reaction.

3. Treatment of Domestic Water Supply

Water taken from rivers or reservoirs needs to be treated before it is safe to drink. This process removes solids, bad smells, and harmful microorganisms (pathogens).

Here is the step-by-step process of domestic water treatment:

1. Sedimentation and Filtration:

   This removes solids (like dirt, sand, and leaves). Large particles settle out in sedimentation tanks (like letting the mud sink to the bottom of a bucket), and then the water passes through sand and gravel filters to remove smaller suspended particles.

2. Use of Carbon:

   The water is treated with activated carbon. This step uses the porous nature of carbon to adsorb (stick to the surface of) chemicals that cause unpleasant tastes and odours.

3. Chlorination:

   A small amount of chlorine gas or chlorine compounds is added to the water. Chlorine is a powerful oxidizing agent used to kill microbes (pathogens) and bacteria, making the water safe to drink.

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C10.2 Air Quality and Climate

1. Composition of Clean, Dry Air (Core)

When we talk about "air," we are talking about a mixture of gases. The composition of clean, dry air is:

• Approximately 78% Nitrogen ($\text{N}_2$)
• Approximately 21% Oxygen ($\text{O}_2$)
• The remaining 1% is a mixture of noble gases (mostly Argon) and Carbon Dioxide ($\text{CO}_2$).

Did you know? Even though $\text{CO}_2$ is only a tiny fraction of the air, its presence is crucial for life (photosynthesis) and climate (greenhouse effect)!

2. Air Pollutants and Their Sources (Core)

Pollutants are substances added to the air, usually due to human activity, that have harmful effects. It is vital to know where they come from.

Pollutant Group 1: Carbon-Based Fuels

• Carbon Dioxide ($\text{CO}_2$):
  Source: Complete combustion (burning with plenty of oxygen) of carbon-containing fuels (like coal, oil, gas, wood).
  Reaction example: \(\text{C} + \text{O}_2 \rightarrow \text{CO}_2\)
• Carbon Monoxide ($\text{CO}$) & Particulates (Soot):
  Source: Incomplete combustion (burning with limited oxygen supply) of carbon-containing fuels.
  Reaction example: \(\text{C} + 1/2 \text{O}_2 \rightarrow \text{CO}\)

Pollutant Group 2: Sulfur and Nitrogen Compounds

• Sulfur Dioxide ($\text{SO}_2$):
  Source: The combustion of fossil fuels (like coal and oil) that contain sulfur compounds as impurities. The sulfur burns: \(\text{S} + \text{O}_2 \rightarrow \text{SO}_2\).
• Oxides of Nitrogen ($\text{NO}_x$, e.g., $\text{NO}$ and $\text{NO}_2$):
  Source: High temperatures inside car engines or industrial furnaces cause atmospheric nitrogen and oxygen to react.

Pollutant Group 3: Methane

• Methane ($\text{CH}_4$):
  Source: Naturally occurring gas from the decomposition of vegetation (anaerobic decay in swamps/landfills) and waste gases from digestion in animals (especially livestock like cows).

3. Adverse Effects of Air Pollutants (Core & Supplement)

These pollutants cause significant damage to human health and the environment:

A. Global Warming and Climate Change (Greenhouse Gases)

The gases $\text{CO}_2$ and $\text{CH}_4$ are known as greenhouse gases. Higher levels of these gases lead to increased global warming, which causes climate change.

The Greenhouse Effect Mechanism (Supplement C10.2.6)

The greenhouse effect keeps the Earth warm enough for life. However, too much of it causes global warming.

1. The Earth absorbs energy from the Sun (mostly visible light).
2. The Earth surface then heats up and emits this energy back as thermal energy (infrared radiation).
3. Greenhouse gases ($\text{CO}_2$, $\text{CH}_4$) in the atmosphere are excellent at absorbing and reflecting this thermal energy.
4. By trapping this outgoing thermal energy, they reduce the thermal energy loss to space, acting like a blanket and causing the Earth's average temperature to rise (global warming).

B. Acid Rain ($\text{SO}_2$ and $\text{NO}_x$)

When sulfur dioxide ($\text{SO}_2$) and oxides of nitrogen ($\text{NO}_x$) dissolve in atmospheric water droplets, they react to form sulfuric acid and nitric acid, falling as acid rain.

• Effects: Damage to trees and crops, corrosion of buildings (especially limestone), and acidification of lakes (harming aquatic life).

C. Health and Toxicity

• Carbon Monoxide ($\text{CO}$): A toxic gas. It binds irreversibly to haemoglobin in red blood cells, preventing them from carrying oxygen, leading to suffocation. It is often called the "silent killer" because it is colorless and odorless.
• Particulates (Soot): Small solid particles. They increase the risk of respiratory problems (like asthma) and cancer when inhaled.
• Oxides of Nitrogen ($\text{NO}_x$) & Sulfur Dioxide ($\text{SO}_2$): In addition to causing acid rain, they cause respiratory problems (e.g., bronchitis).

4. Strategies to Reduce Pollution and Climate Change

A. Reducing Climate Change Effects (Core C10.2.4)

We need to reduce the amount of greenhouse gases ($\text{CO}_2$, $\text{CH}_4$) released into the atmosphere:

• Planting Trees (Afforestation): Trees absorb $\text{CO}_2$ during photosynthesis, locking up carbon in biomass.
• Decreasing Use of Fossil Fuels: Switching from coal/oil to less carbon-intensive sources.
• Increasing Use of Hydrogen and Renewable Energy: Using sources like wind, solar, and hydroelectric power, which do not produce $\text{CO}_2$.
• Reduction in Livestock Farming: Since livestock (cows) produce a large amount of methane ($\text{CH}_4$), reducing farming can lower methane emissions.

B. Reducing Acid Rain Effects (Supplement C10.2.5)

Acid rain is caused mainly by $\text{SO}_2$. To reduce its effects:

• Use Low-Sulfur Fuels: Choose oil or coal with very little sulfur impurity.
• Flue Gas Desulfurisation: This is a chemical method to remove $\text{SO}_2$ *before* it leaves the power station chimney.

  The acidic sulfur dioxide gas is passed through a slurry of calcium oxide ($\text{CaO}$ - a base) or calcium carbonate. The base neutralises the acid gas:

  $$\text{CaO} (s) + \text{SO}_2 (g) \rightarrow \text{CaSO}_3 (s)$$

C. Controlling Car Pollutants (Supplement C10.2.7)

Oxides of nitrogen ($\text{NO}_x$) form in car engines because the high operating temperatures cause nitrogen and oxygen from the air to react:

$$\text{N}_2 (g) + \text{O}_2 (g) \xrightarrow{\text{high temp}} 2\text{NO} (g)$$

To remove $\text{NO}$ and $\text{CO}$, modern cars are fitted with a catalytic converter. This device uses catalysts (like platinum) to turn the harmful pollutants into less harmful products.

The key reaction in the catalytic converter is:

$$2\text{CO} (g) + 2\text{NO} (g) \rightarrow 2\text{CO}_2 (g) + \text{N}_2 (g)$$

(Notice that $\text{CO}$ is converted to $\text{CO}_2$, and the toxic $\text{NO}$ is converted back into harmless $\text{N}_2$ gas.)