Topic 10 of 10

Using Resources

Learn how we use the Earth's finite resources sustainably — from treating water and extracting metals to life cycle assessments and the future of materials.

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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

  1. Choosing a source: Rain water is collected in reservoirs, or water is taken from rivers and aquifers.
  2. Filtration: Water passes through filter beds to remove large debris, sediment, and some microorganisms.
  3. 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.

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

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

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.

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

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

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:

  1. Reduce: Use fewer resources in the first place (best option).
  2. Reuse: Use products again for the same or different purpose.
  3. 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

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

  • Painting/oiling/greasing: Creates a barrier to air and water.
  • Galvanising: Coating with zinc. Even if scratched, zinc corrodes preferentially (sacrificial protection).
  • Sacrificial protection: Attaching blocks of a more reactive metal (e.g., zinc/magnesium) — the reactive metal corrodes instead of the iron.
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

Alloys

Alloys are mixtures of metals (sometimes with small amounts of carbon or other elements). The different-sized atoms disrupt the regular layers, making the material harder and stronger.

  • Steel: Iron + carbon (and sometimes other metals).
  • Brass: Copper + zinc.
  • Bronze: Copper + tin.

Ceramics

Ceramics (glass, clay, cement) are non-metallic solids. They are typically hard, strong, and heat-resistant, with very high melting points. They are brittle (break rather than bend).

Polymers

Synthetic polymers are cheap, lightweight, and can be moulded into shape. However, many are not biodegradable, causing long-term disposal problems.

Composites

Composites are made from two or more different materials bonded together, combining properties from each. Examples:

  • Concrete: Aggregate (stones) + cement
  • Fibreglass: Glass fibres + resin
  • Carbon fibre composites: Carbon fibres + polymer resin — very strong and lightweight

The Haber Process

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).

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.