Science - Combined (0653) | Chemistry of the environment

The Chemistry of Our Environment (C10)

Hello future scientists! This chapter, "Chemistry of the Environment," is one of the most important in your Chemistry course because it connects the reactions we study in the lab to the world outside—the air we breathe and the water we drink. Understanding these processes helps us tackle global challenges like pollution and climate change. Let's dive in!

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C10.1 Water: Our Essential Resource (Core)

Water is essential, but it rarely exists in a perfectly pure state. We need to know how to identify it and how we make it safe for use.

Testing for the Presence of Water

You can test an unknown substance to see if it contains water using two special chemical compounds that change colour dramatically when they absorb water (become hydrated).

• Anhydrous Cobalt(II) Chloride:
  Test Result: Changes from blue (when dry/anhydrous) to pink (when wet/hydrated).


• Anhydrous Copper(II) Sulfate:
  Test Result: Changes from white (when dry/anhydrous) to blue (when wet/hydrated).

Memory Aid: Think of the Copper test: White to Blue (like the sky and water!).

Tap Water vs. Distilled Water

In practical chemistry, we almost always use distilled water (or deionised water).

• Tap Water contains dissolved mineral ions and other impurities (which is why it tastes good!).
• Distilled Water is pure \(H_2O\). It is essential in experiments because it contains fewer chemical impurities, ensuring that the results we get are due only to the chemicals we intended to react.

The Treatment of Domestic Water Supply

Water taken from rivers or reservoirs needs careful cleaning before it is safe to drink (domestic supply). This process removes solids, bad smells, and harmful microorganisms (microbes).

Here are the three main stages of water purification:

1. Sedimentation and Filtration:
   • Large solids are allowed to settle (sedimentation).
   • The water is then passed through beds of sand and gravel (filtration) to remove the remaining smaller solid particles.
2. Use of Carbon (Activated Charcoal):
   • Water is passed through layers of carbon to remove unwanted tastes and odours. Carbon is very porous and absorbs these small organic molecules.
3. Chlorination:
   • A small amount of chlorine gas is added to the water.
   • Chlorine is a powerful disinfectant used to kill harmful microbes (bacteria and viruses), making the water safe for human consumption.

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C10.2 Air Quality and Climate

The atmosphere is a mixture of gases. While clean air is mostly harmless, human activity has released pollutants that significantly affect our environment and health.

Composition of Clean, Dry Air (Core)

Clean, dry air has a relatively constant composition:

• Nitrogen (\(N_2\)): Approximately 78%
• Oxygen (\(O_2\)): Approximately 21%
• Remainder (1%): This includes a mixture of noble gases (like Argon) and Carbon Dioxide (\(CO_2\)).

Major Air Pollutants: Sources and Effects (Core)

Pollutants are substances in the air that cause harm. You need to know where they come from and what damage they do.

Pollutant	Source	Adverse Effect
Carbon Dioxide (\(CO_2\))	Complete combustion of carbon-containing fuels (e.g., burning petrol).	Increased global warming, leading to climate change.
Carbon Monoxide (CO)	Incomplete combustion of carbon-containing fuels (when oxygen supply is limited).	Toxic gas (poisonous).
Particulates (Soot)	Incomplete combustion of carbon-containing fuels.	Increased risk of respiratory problems (e.g., asthma) and cancer.
Methane (\(CH_4\))	Decomposition of vegetation (rotting plants) and waste gases from digestion in animals (especially livestock like cows).	Increased global warming, leading to climate change.
Oxides of Nitrogen (\(NO_x\))	Produced in car engines (when high temperatures cause nitrogen and oxygen from the air to react).	Contributes to acid rain and causes respiratory problems.
Sulfur Dioxide (\(SO_2\))	Combustion of fossil fuels (like coal and oil) which contain small amounts of sulfur compounds.	The main cause of acid rain.

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C10.2 Supplement: Understanding and Solving Environmental Problems

For students aiming for higher grades (A* to C), understanding the mechanism of global warming and the strategies for reduction is key.

The Greenhouse Effect and Global Warming (Supplement)

The greenhouse effect is a natural process, but when the concentration of greenhouse gases increases due to human activity, it leads to global warming.

How Greenhouse Gases Cause Global Warming

Greenhouse gases, primarily carbon dioxide (\(CO_2\)) and methane (\(CH_4\)), trap heat in the atmosphere. The process involves:

1. Absorption and Reflection of Thermal Energy: Solar energy passes easily through the atmosphere and warms the Earth's surface. The warm Earth then emits thermal energy (infrared radiation).
2. Trapping the Heat: Greenhouse gases in the atmosphere absorb and reflect this thermal energy that is trying to escape back into space.
3. Reducing Thermal Energy Loss to Space: Because this heat is absorbed and re-emitted downwards, the atmosphere acts like a blanket, reducing the thermal energy loss to space and causing the Earth's temperature to rise (global warming).

Analogy: Think of a greenhouse gas as a thick blanket wrapped around the Earth. The blanket keeps the heat from escaping easily.

Strategies to Reduce Climate Change (Supplement)

To combat the effects of rising carbon dioxide and methane levels, several strategies can be employed:

1. Planting Trees (Afforestation):
   Explanation: Trees absorb large amounts of carbon dioxide from the atmosphere through photosynthesis, storing the carbon in their wood and roots. This reduces the overall concentration of \(CO_2\), slowing global warming.
2. Decreasing Use of Fossil Fuels:
   Explanation: Fossil fuels (coal, oil, gas) are the primary source of anthropogenic (human-caused) \(CO_2\) emissions. Using less of them means less \(CO_2\) is released.
3. Increasing Use of Renewable Energy:
   Examples: Wind, solar, hydroelectric power.
   Explanation: These energy sources do not involve combustion, so they produce no \(CO_2\) (or very little) during operation, replacing fossil fuels.
4. Reduction in Livestock Farming:
   Explanation: Livestock, especially cattle, produce significant quantities of methane (\(CH_4\)) through digestion. Reducing farming intensity can decrease methane emissions.

Did you know? Methane is a much more potent greenhouse gas than carbon dioxide, though it stays in the atmosphere for a shorter time.

Strategies to Reduce Acid Rain (Supplement)

Acid rain is primarily caused by sulfur dioxide (\(SO_2\)) and oxides of nitrogen (\(NO_x\)) dissolving in atmospheric water.

• Reducing Emissions of Sulfur Dioxide:
  The main strategy is using low-sulfur fuels, especially in power stations. This prevents the sulfur compounds from being burned and converting into \(SO_2\). (Technologies like 'flue gas desulfurisation' also clean emissions before they leave chimneys.)