Earth’s Resources & Sustainability
Everything we make starts with a natural resource — something from the Earth, oceans or atmosphere that provides warmth, shelter, food or transport. Chemistry’s job is to turn these into useful products while keeping an eye on whether they will run out.
- Natural resources — materials from the Earth, oceans and atmosphere (eg metal ores, crude oil, water, air, timber). Agriculture supplements them with food, timber, clothing and fuels.
- Finite (non-renewable) resources — ones that are being used up faster than they form, or don’t reform at all: fossil fuels and metal ores.
- Renewable resources — ones that can be replaced as fast as they’re used, on a human timescale: eg timber (trees can be replanted).
- Sustainable development — development that meets the needs of today without compromising the ability of future generations to meet their own needs.
Many natural products can be supplemented or replaced by agricultural or synthetic alternatives:
- Rubber (natural latex from trees) is replaced by synthetic polymers designed for the job.
- Fertilisers boost the yield of natural food crops (the Haber process, section 10).
You should be able to distinguish finite from renewable resources and interpret data about resources from charts, graphs and tables — including using orders of magnitude to judge how significant a figure really is.
- “Finite” and “non-renewable” mean the same thing — the terms are interchangeable.
- Renewable doesn’t mean instant. Timber is renewable but trees still take years to regrow — sustainability is about the rate of use versus replacement.
🧪 Exam-style questions
Which of these is a renewable resource? Tick (✓) one box.
What is meant by sustainable development? Tick (✓) one box.
Which natural product has been largely replaced by synthetic alternatives? Tick (✓) one box.
A metal’s known reserves are about 1012 tonnes, and it is used at about 1010 tonnes per year. Using orders of magnitude, roughly how long will the reserves last? Tick (✓) one box.
Potable Water
Water of the right quality is essential for life. Potable water is water that is safe to drink — it has low levels of dissolved salts and microbes. Crucially, potable water is not the same as pure water.
- Pure water (chemistry) contains only H2O molecules — nothing dissolved.
- Potable water is safe to drink but contains dissolved substances (small, safe amounts of minerals and salts). So it is not chemically pure.
Making potable water from fresh water
In the UK, rain gives fresh water (low in dissolved substances) that collects in lakes, rivers and reservoirs (surface water) or underground in aquifers (ground water). To make it potable it is filtered then sterilised — filtration is the same separating technique you met in C1:
Fresh water is made potable in two key steps: filtration removes insoluble solids, then sterilisation (chlorine, ozone or UV) kills microbes.
Desalination — when fresh water is scarce
Where fresh water is limited (hot, dry regions), potable water can be made from sea water or salty water by desalination: removing the salt by distillation or by reverse osmosis (forcing water through a membrane that holds back the ions). Both need large amounts of energy, so desalination is expensive and used only when there is no better source.
Part A — analysis. Measure the pH with universal indicator, and find the dissolved solids: weigh an evaporating basin, add a known volume (eg 25 cm3) of the sample, evaporate the water, then re-weigh — the mass gained is the dissolved solid. (Don’t overheat, or you decompose the solids.)
Part B — purification by distillation. Heat the sample so water boils off, then condense the vapour and collect it — the distillate is pure water. Test it: pure water boils at exactly 100 °C and leaves no residue on evaporation.
- Independent variable: the water sample tested (the source, eg fresh, sea or distilled water) — or, in Part B, the purification step (before vs after distillation).
- Dependent variables: the pH, the mass of dissolved solids left per known volume after evaporation, and whether the distillate is pure water (a sharp boiling point at exactly 100 °C with no residue).
- Control variables: the same volume of each sample, and the same method and apparatus each time (same balance, same evaporating basin, evaporate to dryness without overheating).
Simple distillation: boil the salt water, condense the steam in a cooled tube, and collect pure water. The salt is left behind in the flask.
- “Potable means pure.” No — potable water still contains dissolved substances; it’s just safe to drink.
- Confusing the two steps. Filtration removes insoluble solids; sterilisation (chlorine/ozone/UV) kills microbes. Filtering does not kill bacteria.
- Forgetting why desalination is a last resort. It uses a lot of energy, so it’s expensive — only used where fresh water is unavailable.
🧪 Exam-style questions
How does potable water differ from pure water? Tick (✓) one box.
Fresh water is passed through filter beds. Why? Tick (✓) one box.
Name one method used to sterilise water in the UK.
Show answer
Any one of: chlorine, ozone or ultraviolet (UV) light. 1 mark (These kill microbes — sterilisation is a separate step from filtration.)
Sea water is made potable by desalination. Name the substance removed, and give one reason desalination is only used when necessary. Tick (✓) one box.
After distilling a water sample, how could you show the distillate is pure water? Tick (✓) one box.
Waste Water Treatment
Homes, farms and factories all produce waste water that must be cleaned before it returns to rivers and the sea. Sewage and agricultural waste water need organic matter and harmful microbes removed; industrial waste water may also need harmful chemicals removed.
Sedimentation splits the waste into sludge (digested without air, anaerobically) and effluent (treated with air, aerobically). The methane from the sludge is even used as an energy source.
- Screening and grit removal — take out large solids (twigs, plastic) and grit.
- Sedimentation — in a settlement tank, heavy solids sink to form sludge; the liquid on top is effluent.
- Anaerobic digestion of the sludge — bacteria break it down without air; this releases methane (used for energy) and leaves a digested solid used as fertiliser.
- Aerobic biological treatment of the effluent — air is pumped in so aerobic bacteria break down the remaining organic matter.
Be ready to compare the ease of producing potable water from different sources:
- Ground / fresh water — easiest: low in dissolved substances, so just filter and sterilise.
- Waste water — harder: lots of organic matter, microbes and toxins to remove, so many treatment stages — but it still uses less energy than desalination.
- Salt / sea water — hardest: desalination removes dissolved salt and needs the most energy.
🧪 Exam-style questions
What is the first stage of sewage treatment? Tick (✓) one box.
Sedimentation separates the waste water into two parts. What are they? Tick (✓) one box.
How are the sludge and the effluent each treated? Tick (✓) one box.
Explain why it is more difficult to produce potable water from waste water than from ground water.
Show answer
- Ground water is already low in dissolved substances, so it only needs filtering and sterilising. 1 mark
- Waste water contains a lot of organic matter, microbes (and toxins). 1 mark
- So waste water needs many more treatment stages to remove them, making it harder (and more expensive). 1 mark
Extracting Metals from Low-grade Ores H
This section (biological methods of metal extraction) is Higher Tier only, on both Combined and Triple papers.
The Earth’s metal ores are finite, and the rich, high-grade ores are running out — copper especially. Mining low-grade ores (which contain only small amounts of metal) the traditional way means digging up, moving and disposing of huge amounts of rock. Two biological methods avoid that damage.
First, a quick recap from C4: a metal is locked inside its ore as a compound, and freeing it normally takes a chemical reduction step (heating with carbon, or electrolysis). The biological methods below are alternatives aimed at low-grade ores.
Most metals are found as compounds inside ores. Crushing and concentrating the ore is physical — the metal is only freed by a chemical reduction step, using carbon or electrolysis.
- Phytomining — plants are grown on land containing the metal. They absorb metal compounds through their roots, which become concentrated in the plant. The plants are harvested and burned, and the ash contains the metal compounds.
- Bioleaching — bacteria are used to break down low-grade ores, producing a leachate solution that contains metal ions (eg copper).
Either way you end up with a metal compound or solution, not the metal itself.
Phyto- means plant, and leaching means washing soluble compounds out into solution — the names tell you how each method works.
The metal compounds are then processed to obtain the metal. For copper, this is done by:
- Displacement — adding scrap iron to a solution of a copper compound (iron is more reactive, so it displaces the copper), or
- Electrolysis of the solution.
- Advantages — they let us use low-grade ores and mining waste, and avoid the digging, transport and rock disposal of traditional mining, so there is less environmental damage.
- Disadvantages — they are slow, and bioleaching can produce toxic substances that must be treated. The metal still has to be extracted by displacement or electrolysis.
🧪 Exam-style questions
Name what phytomining uses to extract metals.
Show answer
Plants. 1 mark (They absorb metal compounds and are then burned; the ash contains the metal compounds. Bioleaching uses bacteria.)
In bioleaching, name what produces the solution (leachate) containing metal ions.
Show answer
Bacteria. 1 mark (They break down low-grade ore to produce a leachate solution of metal ions.)
How can copper be obtained from a solution of a copper compound? Tick (✓) one box.
Give one advantage of phytomining and bioleaching over traditional mining. Tick (✓) one box.
Copper-rich ores are running out. Evaluate the use of phytomining and bioleaching to extract copper, compared with traditional mining. This is a levels-of-response question — weigh the benefits against the drawbacks and reach a justified conclusion. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): benefits and drawbacks discussed, ending in a justified conclusion.
- Level 2 (3–4): describes benefits and at least one drawback.
- Level 1 (1–2): one or two simple relevant points.
Indicative content — any of the following are credited.
Benefits:
- They can extract copper from low-grade ores (and mining waste) that would otherwise be uneconomic, conserving the dwindling high-grade ores.
- They avoid the digging, moving and disposal of huge amounts of rock, so there is far less landscape damage, noise and dust than traditional mining.
- Allow also: less energy than digging/transporting huge amounts of rock; less habitat destruction; uses waste tips/spoil heaps.
Drawbacks:
- Both methods are slow.
- Bioleaching can produce toxic, acidic leachate that harms the environment if it escapes.
- You still only get a metal compound or solution — the copper must then be extracted by displacement with scrap iron or by electrolysis, which costs energy.
- Allow also: phytomining needs land/growing time; lower copper recovery per tonne than smelting rich ore.
Do not accept: “biological methods make no waste / cause no harm at all”; “they give pure copper directly”.
Conclusion (needed for Level 3) — any justified verdict scores: as high-grade copper ores run out, these biological methods are increasingly worthwhile — they let us use resources we otherwise couldn’t, with much less environmental damage, even though they are slow and still need a final extraction step. 6 marks