Using Resources (Water Treatment, Life Cycle, Recycling)

Quick-Fire Definitions

Sustainable development: Meeting the needs of the current generation without compromising the ability of future generations to meet their own needs.
Potable water: Water that is safe to drink - not pure in the chemistry sense, but contains dissolved substances at safe levels.
Desalination: The process of removing dissolved salts from sea water to produce potable water (by distillation or reverse osmosis).
Life cycle assessment (LCA): An assessment of the environmental impact of a product at every stage of its life, from raw material extraction to disposal.
Corrosion: The destruction of materials by chemical reactions with substances in the environment (e.g. Rusting of iron requires water and oxygen).
Composite: A material made from two or more different materials bonded together, combining properties from each (e.g. Concrete, carbon fibre).

Finite & Renewable Resources

Natural resources from the Earth provide everything humans need. These resources fall into two broad categories:

Finite resources: Non-renewable. Once used, they cannot be replaced over a human timescale. Examples: fossil fuels (coal, oil, gas), metal ores.
Renewable resources: Can be replenished naturally at a rate fast enough for human use. Examples: timber, crops, fresh water (via the water cycle).

Sustainable development means meeting the needs of the current generation without compromising the ability of future generations to meet their own needs.

Potable Water

Potable water is water that is safe to drink. It is not pure water in the chemistry sense - it contains dissolved substances, but at levels low enough to be safe.

How Potable Water is Produced in the UK

Choosing a source: Rain water is collected in reservoirs, or water is taken from rivers and aquifers.
Filtration: Water passes through filter beds to remove large debris, sediment, and some microorganisms.
Sterilisation: Chlorine, ozone, or UV light is used to kill harmful bacteria and other pathogens.

Remember: potable ≠ pure. Potable water contains minerals but at safe concentrations. Pure water (in chemistry) contains only water molecules.

Comparing Water Treatment Methods

Feature	UK freshwater treatment	Desalination
Source	Rivers, reservoirs, aquifers (fresh water)	Sea water or brackish water
Key steps	Filtration → sedimentation → sterilisation (chlorine/ozone/UV)	Distillation or reverse osmosis
Energy cost	Relatively low	Very high (making it expensive)
Where used	Countries with sufficient rainfall	Arid regions (e.g. Middle East, parts of Africa)

Required Practical: Water Purification

In this practical you test the pH, dissolved solids, and clarity of water samples, then purify a sample by distillation.

Test the pH of the water using universal indicator or a pH meter.
Measure dissolved solids by evaporating a known volume and weighing the residue.
Distil the water sample - boil in a flask, cool the vapour in a condenser, and collect the distillate.
Compare the distillate with the original sample - it should have a neutral pH and fewer dissolved solids.

Distillation produces pure water but requires large amounts of energy, making it impractical for large-scale use in the UK.

Waste Water Treatment

Sewage and agricultural/industrial waste water must be treated to remove harmful substances before it is released back into the environment.

Treatment Stages

Screening: Removal of large objects (rags, sticks).
Sedimentation: Solids settle out as sludge. The liquid part is called effluent.
Aerobic biological treatment: Effluent is passed through filter beds or aeration tanks where aerobic bacteria break down organic matter.
Sludge treatment: Sludge is digested in large tanks by anaerobic bacteria, producing biogas (methane) as a useful by-product.

Waste water must undergo several stages of treatment, separating liquid effluent from solid sludge, before it is safe to release or reuse.

Desalination

In regions with limited fresh water, sea water can be made potable by desalination - removing dissolved salts.

Methods

Distillation: Heating sea water to boil, then condensing the steam. Very energy-intensive.
Reverse osmosis: Forcing sea water through a membrane at high pressure. The membrane allows water molecules through but traps salt ions. More energy-efficient than distillation.

If asked about desalination problems, mention the high energy cost (makes it expensive and leaves a carbon footprint) and the concentrated brine waste that must be disposed of.

During reverse osmosis, high pressure forces water molecules out of the salty sea water and through a semi-permeable membrane, leaving the dissolved salts behind.

Phytomining & Bioleaching

Traditional mining can be uneconomical for low-grade ores. Biological methods offer an alternative:

Phytomining

Plants are grown on land containing low-grade ore. They absorb metal compounds through their roots. The plants are then harvested, burned, and the ash (which contains the metal) is processed to extract the metal.

Bioleaching

Bacteria are used to produce a leachate (solution) containing dissolved metal compounds from low-grade ores. The metal is then extracted from the solution, often by displacement using scrap iron or electrolysis.

These methods are slower but cause less environmental damage than traditional mining. They use less energy and can process low-grade ores that would be uneconomical to mine conventionally.

Life Cycle Assessments (LCA)

An LCA assesses the environmental impact of a product at every stage of its life - from "cradle to grave".

The Four Stages

Raw materials: Extracting and processing resources.
Manufacturing: Energy and pollution from making the product.
Use: Energy consumption, emissions, and maintenance during the product's lifetime.
Disposal: Landfill, incineration, recycling - each has different impacts.

A Life Cycle Assessment (LCA) evaluates the environmental impact of a product from the extraction of raw materials to its final disposal. Recycling creates a loop that minimises new resource extraction and reduces waste.

LCA Stage	What is assessed	Example (plastic bag)
Raw materials	Extracting and processing resources	Crude oil extraction, cracking to make ethene
Manufacturing	Energy use and pollution from production	Polymerisation, moulding, printing
Use	Energy, emissions, maintenance during lifetime	Single use (very short lifetime)
Disposal	Landfill, incineration, or recycling impact	Non-biodegradable → landfill for centuries

Limitations

Some aspects of an LCA are difficult to quantify objectively (e.g., "impact on environment" can be measured in different ways). This makes it possible for LCAs to be biased or misleading.

When evaluating LCAs, mention that while they provide useful data, they involve subjective value judgements about the relative importance of different environmental impacts.

Reduce, Reuse & Recycle

The most effective strategy for conserving resources is the waste hierarchy:

Reduce: Use fewer resources in the first place (best option).
Reuse: Use products again for the same or different purpose.
Recycle: Process used materials into new products.

Benefits of Recycling

Reduces the amount of waste sent to landfill.
Conserves finite resources (metals, fossil fuels).
Reduces energy consumption (recycling aluminium uses 95% less energy than extracting from ore).
Reduces greenhouse gas emissions.

Corrosion & Prevention Chemistry Only

Corrosion is the destruction of materials by chemical reactions with substances in the environment. The most common example is the rusting of iron.

Conditions for Rusting

Iron requires both water and oxygen to rust:

iron + water + oxygen → hydrated iron(III) oxide (rust)

Preventing Corrosion

Method	How it works	Example
Painting / oiling / greasing	Creates a physical BARRIER preventing air and water from reaching the iron surface	Cars, bridges, railings
Galvanising	Coating with ZINC. Acts as barrier AND sacrificial protection - zinc corrodes preferentially even if scratched	Buckets, corrugated iron roofs
Sacrificial protection	Attaching blocks of a MORE REACTIVE metal (zinc or magnesium). The reactive metal oxidises instead of the iron	Ship hulls, underground pipelines
Electroplating	Coating with a thin layer of another metal using electrolysis	Chromium plating on taps and car parts

Aluminium doesn’t corrode visibly because it forms a tough, protective layer of aluminium oxide (Al₂O₃) on its surface that prevents further reaction.

Alloys, Ceramics & Composites Chemistry Only

Different materials have different properties, making them suitable for different uses.

Pure metals have regular layers of identically sized atoms that easily slide over one another when force is applied. In an alloy, different-sized atoms disrupt these layers, locking them in place and making the material much harder.

Material type	Key properties	Examples
Alloys	Harder and stronger than pure metals (different-sized atoms disrupt regular layers, preventing sliding)	Steel (iron + carbon), Brass (copper + zinc), Bronze (copper + tin)
Ceramics	Hard, strong, heat-resistant, very high melting points, but brittle	Glass, clay pottery, cement, bricks
Polymers	Cheap, lightweight, easily moulded; many are non-biodegradable	Poly(ethene), PVC, nylon, polyester
Composites	Combine properties of two+ materials bonded together	Concrete (aggregate + cement), Fibreglass (glass fibres + resin), Carbon fibre (carbon fibres + polymer)

The choice of material depends on the required properties: strength, flexibility, density, cost, resistance to corrosion, and environmental impact.

The Haber Process Chemistry Only

The Haber process is used to manufacture ammonia (NH₃) on an industrial scale.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Conditions

Temperature: ~450°C (a compromise - low enough for a reasonable yield, high enough for a reasonable rate).
Pressure: ~200 atmospheres (high pressure favours the forward reaction as there are fewer moles of gas on the right).
Catalyst: Iron (speeds up the reaction without being consumed).

The Haber process creates ammonia efficiently by recycling unreacted nitrogen and hydrogen gases, establishing a continuous industrial loop.

NPK Fertilisers

Ammonia is used to make ammonium salts (e.g., ammonium nitrate, NH₄NO₃) which are used as fertilisers. Fertilisers provide nitrogen (N), phosphorus (P), and potassium (K) to help crops grow.

The Haber process conditions are a compromise. A lower temperature would give a higher yield but too slow a rate. A higher pressure would give a better yield but equipment costs are prohibitive.

Le Chatelier’s Principle applied to the Haber Process

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol (exothermic forward)

Temperature (450°C): The forward reaction is exothermic. A lower temperature would increase the yield (Le Chatelier’s shifts to oppose cooling → forward). BUT rate would be too slow. 450°C is a compromise - reasonable yield and acceptable rate.

Pressure (200 atm): 4 moles of gas on the left → 2 moles on the right. High pressure shifts equilibrium to the side with fewer moles → forward → more NH₃. BUT very high pressures are expensive and dangerous. 200 atm is a compromise.

Iron catalyst: Speeds up both forward and reverse reactions equally - does NOT change yield but reaches equilibrium FASTER.

Recycling: Unreacted N₂ and H₂ are recycled back over the catalyst to improve overall conversion.

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