Page-by-Page Notes: Water & Air Environment (Environmental Chemistry)
Page 1
- Water basics
- Four-fifths (4/5) of the world’s surface is covered by water.
- Water is formed when hydrogen burns in air; thus water is a compound of hydrogen and oxygen.
- Physical properties: colourless liquid at room temperature and atmospheric pressure; universal solvent (almost all substances dissolve in water to some extent).
- Chemical tests for water
- Test with anhydrous copper(II) sulfate: water turns white anhydrous CuSO₄ to blue crystals.
- Reaction: ext{CuSO}4( ext{s}) + 5 ext{H}2 ext{O}( ext{l})
ightarrow ext{CuSO}4ullet 5 ext{H}2 ext{O}( ext{s}) - Test with anhydrous cobalt(II) chloride: water turns blue anhydrous CoCl₂ to pink (hydration). Hydrated cobalt chloride is CoCl₂·6H₂O, which appears pink; the anhydrous form is blue.
- Reaction (hydration): ext{CoCl}2( ext{s}) + 6 ext{H}2 ext{O}( ext{l})
ightarrow ext{CoCl}2ullet 6 ext{H}2 ext{O}( ext{s})
- How to test for the purity of water
- Boiling point test: Pure water boils at 100^ ext{°C}. If boiling occurs below or above 100°C, impurities are present.
- Freezing point test: Pure water freezes at 0^ ext{°C}. If freezing occurs below or above 0°C, impurities are present.
- Why distilled water is used in practical chemistry
- Distilled water is pure and does not interfere with reactions.
- Distilled water is produced by boiling water into vapour and then condensing the vapour back into liquid in a separate container, removing impurities and minerals.
- Distillation removes dissolved substances that could affect experimental outcomes.
Page 2
- Substances in water from natural sources
- Everyday uses of water:
- Domestic uses: drinking, cooking, gardening, sanitation.
- Agricultural uses: as a drink for animals and for watering crops.
- Industrial uses: solvent, coolant; heated to make steam used to generate electricity.
- Water bodies and sources: lakes, rivers, and underground water sources (groundwater).
- Water from natural sources may contain a variety of substances:
- Dissolved oxygen (DO)
- Metal ions (e.g. Ca²⁺, K⁺, Na⁺, Mg²⁺)
- Plastics
- Sewage
- Harmful microbes
- Nitrates from fertilisers
- Phosphates from fertilisers and detergents
- Pathways into water: many substances enter water sources when rain washes them into lakes, rivers, or groundwater. Some substances are naturally occurring, but many are a direct result of human activities.
Page 3
- Beneficial vs harmful substances in water
- Beneficial substances:
- Dissolved oxygen (DO): essential for aquatic life.
- Metal compounds: some provide essential minerals (e.g., calcium, magnesium).
- Potentially harmful substances:
- Metal compounds: some are toxic (e.g., aluminium, lead).
- Plastics: harmful to aquatic life (risk of entanglement and starvation if stomachs fill with plastic).
- Sewage: contains harmful microbes causing disease.
- Nitrates and phosphates from fertilisers: promote growth of aquatic plant life, which can lead to deoxygenation of water. This can cause damage to aquatic life in a process called eutrophication (you do not need to know the details of this process).
Page 4
- Water treatment basics
- Untreated water contains soluble and insoluble impurities:
- Insoluble impurities: soil, pieces of plants, and other organic matter.
- Soluble impurities: dissolved calcium, metallic compounds, and inorganic pollutants.
- Step 1: sedimentation / filtration
- Water is pumped into sedimentation tanks and allowed to stand for a few hours.
- Heavier materials (mud, sand, and other particles) settle to the bottom forming a sediment layer.
- The water is then filtered through sand and gravel to remove smaller particles.
- Step 2: filtration / treatment with carbon (charcoal)
- This step removes unpleasant tastes and odours from the water.
- Step 3: chlorination
- Bacteria and other microorganisms are too small to be trapped by the filters, so chlorine is added to kill bacteria and other microorganisms.
- Diseases such as cholera and typhoid can arise from consumption of untreated water.
Page 5
- Air composition
- Air is a mixture of several gases; the composition varies by place.
- Approximate composition of air:
- Nitrogen: 78\%
- Oxygen: 21\%
- Carbon dioxide: 0.03\%
- The other gases (including Ar, Kr, Xe, Rn, He, Ne, and water vapour): 0.97\%
- Note: The normal composition of air does not contain hydrogen.
Page 6
- Air pollution: pollutants, sources, and effects
- Carbon dioxide (CO₂)
- Source: Complete combustion of carbon-containing fuels (fossil fuels).
- Example reaction: ext{CH}4 + 2\text{O}2 \rightarrow \text{CO}2 + 2\text{H}2\text{O}
- Adverse effects: Increases global warming, leading to climate change.
- Carbon monoxide (CO)
- Source: Incomplete combustion of carbon-containing fuels (fossil fuels).
- Example reaction (incomplete): ext{C}8\text{H}{18} + 9\text{O}2 \rightarrow 5\text{CO} + 2\text{CO}2 + 9\text{H}_2\text{O}
- Adverse effects: Toxic/poisonous; binds to haemoglobin, reducing oxygen transport in the blood.
- Particulates (soot)
- Source: Incomplete combustion of carbon-containing fuels (fossil fuels); may produce CO and C.
- Example reactions: 2\text{CH}4 + 3\text{O}2 \rightarrow 2\text{CO} + 4\text{H}2\text{O}
and/or \text{CH}4 + \text{O}2 \rightarrow \text{C} + 2\text{H}2\text{O}
- Adverse effects: Respiratory problems; cancer risk.
- Methane (CH₄)
- Sources: Waste gases from digestive processes of animals; decomposition of vegetation; bacterial action in swamps, rice paddy fields, and landfill sites.
- Adverse effects: Increases global warming, contributing to climate change.
- Oxides of nitrogen (NOx)
- Sources: Reaction of nitrogen with oxygen at high temperatures (e.g., in car engines, high-temperature furnaces) and also produced by bacterial action in soil.
- Adverse effects: Produces photochemical smog; dissolves in rain to form acid rain; damages crops and water supplies; irritates lungs, throats, and eyes; causes respiratory problems.
Page 7
- Sulfur dioxide (SO₂)
- Source: Combustion of fossil fuels containing sulfur compounds: S + O₂ → SO₂.
- Major source: Power stations.
- Environmental impact: Dissolves in rain to form acid rain, with similar effects to oxides of nitrogen on metals and ecosystems.