Chemistry of the Environment

• If you can say what the evidence is, where the substance came from, what it does, what you would do about it, and what your answer still does not fix — you have Topic 10.
• Solids → smells → microbes. Big things first, then the things you can taste, then the things you cannot see at all. The order is the order of what you could detect with your own senses.
• Five steps, five marks. If you can write this chain for all six pollutants, subsection 10.3 is finished.
• One strategy, two problems solved — that is worth noticing, and it is worth saying in an evaluation answer.
• If you can rebuild matrix 6 from memory — six rows, four columns — you can answer almost any structured question in subsection 10.3.
• Eighty-four prompts, and every one of them is a single sentence in the answer. That is what Topic 10 is: a large number of small, exact facts held in the right pairs.
• Sixty-two ticks is the whole of Topic 10. Nothing else is assessed, and nothing on this list can be left out.

• State the two chemical tests for the presence of water and the exact colour change each one shows.
• State that pure water has a fixed melting point and a fixed boiling point, and give the values at standard atmospheric pressure.
• State the substances that may be present in natural water: dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates and phosphates.
• State that ammonium salts and nitrates are used as fertilisers, and that NPK fertilisers supply nitrogen, phosphorus and potassium.
• State the approximate composition of clean dry air.
• State one source and one adverse effect for each of carbon dioxide, carbon monoxide, particulates, methane, oxides of nitrogen and sulfur dioxide.
• State the word equation for photosynthesis.
• Describe how you would carry out each test for water and what you would observe.
• Describe the three stages of the treatment of a domestic water supply and the purpose of each stage.
• Describe how oxides of nitrogen are formed inside a car engine.
• Describe photosynthesis as the reaction between carbon dioxide and water that produces glucose and oxygen, in the presence of chlorophyll and using energy from light.
• Describe strategies used to reduce the effects of climate change and of acid rain.
• Explain why distilled water is used in practical chemistry rather than tap water.
• Explain why some substances in natural water are beneficial and others are harmful, naming the substance in each case.
• Explain, in terms of absorption and re-emission of thermal energy, how carbon dioxide and methane reduce the loss of thermal energy from the Earth to space.
• Explain why a chosen strategy addresses a particular pollutant, and state a limitation of that strategy.
• Explain how a catalytic converter changes harmful exhaust gases into less harmful products, and write a balanced equation for one such reaction.
• Write and check the balanced symbol equation for photosynthesis.

Why it matters. This is the reaction that takes carbon dioxide back out of the air, so it is the chemical basis of the strategy “plant trees”. It is also the one equation in Topic 10 that candidates most often write from memory and get slightly wrong — a missing 6, a glucose formula with the wrong hydrogen count, or light energy written on the left as though it were a substance.

Domestic Water Treatment

The treatment of water to make it safe for a domestic supply. Sedimentation and filtration remove solids, carbon filtration removes tastes and odours, and chlorination kills microbes. The purpose is safe drinking water, not chemically pure water: dissolved substances remain after treatment, which is why laboratory work uses distilled water instead.

Clean Dry Air

Air with no pollutants and with water vapour excluded from the stated composition. It is approximately 78 per cent nitrogen and approximately 21 per cent oxygen by volume, with the small remainder made up mainly of the noble gases, chiefly argon, together with carbon dioxide. Water vapour is left out because the amount present varies from place to place and from day to day.

Greenhouse Gas

A gas in the atmosphere that absorbs thermal energy radiated by the warmed surface of the Earth and re-emits it in all directions, so that some returns towards the Earth and less is lost to space. Carbon dioxide and methane are the two greenhouse gases named in this topic. They are molecules spread thinly through the atmosphere, not a solid layer or a reflective surface.

Photosynthesis

The reaction in which green plants make glucose and oxygen from carbon dioxide and water, in the presence of chlorophyll and using energy from light. Its balanced symbol equation is six carbon dioxide plus six water giving one glucose plus six oxygen. Light energy and chlorophyll are conditions written above the arrow, not reactants; the reaction removes carbon dioxide from the atmosphere.

Water Purity

Water is pure when it contains only water. The evidence is physical rather than chemical: pure water has a fixed melting point of about 0 degrees Celsius and a fixed boiling point of about 100 degrees Celsius at standard atmospheric pressure. Dissolved substances lower the melting point, raise the boiling point, and cause both changes to happen over a range of temperature instead of at one value.

Test for Water

A chemical test that shows whether water is present in a sample. Anhydrous cobalt(II) chloride changes from blue to pink and anhydrous copper(II) sulfate changes from white to blue when water is added. Both tests detect the presence of water only; neither shows whether the water is pure.

Distilled Water

Water that has been purified by distillation, so that it contains fewer dissolved chemical impurities than tap water. It is used in practical chemistry because dissolved substances in tap water could react with the reagents being used and give misleading results. Distillation removes dissolved solids; it is not the same process as the treatment of a domestic water supply.

Acid Rain

Rain made acidic by pollutant gases dissolved in atmospheric water. Sulfur dioxide, released when fossil fuels containing sulfur compounds are burned, and oxides of nitrogen, formed in car engines, are the two pollutants responsible. Acid rain damages buildings and structures made of limestone and of metal, and harms trees and life in lakes and rivers.

Natural Water

Water in rivers, lakes, reservoirs and groundwater, which is never only water. It may contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates from fertilisers and phosphates from fertilisers and detergents. Dissolved oxygen is essential for aquatic life and some metal compounds supply essential minerals; the remaining substances, and metal compounds that are toxic, are harmful.

Fertiliser

A substance added to soil to supply the elements plants need for growth. Ammonium salts and nitrates are used as fertilisers. An NPK fertiliser supplies nitrogen, phosphorus and potassium, which are used for improved plant growth. Nitrates and phosphates washed off farmland into rivers and lakes become pollutants, causing deoxygenation of the water and damage to aquatic life.

Catalytic Converter

A device fitted to the exhaust system of a vehicle that converts harmful gases in the exhaust into less harmful products. Carbon monoxide and nitrogen monoxide react over the catalyst to give carbon dioxide and nitrogen. The catalyst increases the rate of the reaction and is not used up, so it goes on working for the life of the vehicle.

Air Pollutant

A substance released into the atmosphere that has an adverse effect on health or the environment. Six are studied at this level: carbon dioxide from complete combustion of carbon-containing fuels; carbon monoxide and particulates from incomplete combustion; methane from decomposition of vegetation and waste gases from animal digestion; oxides of nitrogen from car engines; and sulfur dioxide from combustion of fossil fuels containing sulfur compounds.

Global Warming

The rise in the average temperature of the Earth's surface caused by higher levels of greenhouse gases in the atmosphere, which reduce the amount of thermal energy lost to space. Global warming in turn leads to climate change, meaning long-term changes in weather patterns. Carbon dioxide and methane are the greenhouse gases responsible in this topic.

• “The cobalt(II) chloride paper turned pink, so the water is pure.” Defect Over-claim: the conclusion goes beyond the evidence. Chemistry The colour change shows only that water is present. Sea water, muddy water and sugar solution all give it. Repair Replace “is pure” with “contains water”. Transfer What evidence would show purity? A sharp melting point at about \(0\,{}^\circ\mathrm{C}\) and a sharp boiling point at about \(100\,{}^\circ\mathrm{C}\).
• “Add water to copper(II) sulfate and it turns from blue to white.” Defect The colour change is written backwards. Chemistry The anhydrous solid is white; adding water hydrates it and it becomes blue. Blue to white is what happens on heating. Repair Write “white to blue”, and always say “anhydrous” when naming the reagent. Transfer Which direction does cobalt(II) chloride go? Blue to pink on adding water.
• “The liquid boiled at 100 \({}^\circ\mathrm{C}\), so it is water.” Defect Identity claimed from a single measurement. Chemistry A sharp boiling point shows the sample is pure; it does not prove which pure substance it is. Repair “The data are consistent with pure water.” Transfer A sample melts sharply at \(-38.8\,{}^\circ\mathrm{C}\). Pure? Yes. Water? No.
• “Pure water always boils at exactly 100 \({}^\circ\mathrm{C}\).” Defect A condition has been dropped. Chemistry The boiling point depends on the external pressure. At altitude, pure water boils below \(100\,{}^\circ\mathrm{C}\) and is still pure. Repair Add “at standard atmospheric pressure”. Transfer A laboratory 2000 m up records \(93\,{}^\circ\mathrm{C}\) for a sharp boil. Impure? Not necessarily — check the pressure first.
• “Distilled water is used in the laboratory because tap water is dirty.” Defect Wrong reason; “dirty” is not a chemical statement. Chemistry Tap water has been treated and is clear. The problem is the dissolved chemical impurities that survive treatment. Repair “Distilled water contains fewer chemical impurities, which could otherwise react with the reagents.” Transfer Why does silver nitrate give a faint precipitate with tap water? Dissolved chloride is present.
• “Distilled water is sterile.” Defect Confuses a separation with a disinfection. Chemistry Distillation separates by boiling point. Killing microbes is what chlorination does, in a different process for a different purpose. Repair Claim only “fewer chemical impurities”. Transfer Which stage of domestic treatment deals with microbes? Chlorination.
• “Metal compounds in water are toxic and must all be removed.” Defect One-sided: the syllabus requires both sides. Chemistry Some metal compounds supply essential minerals; others are toxic. Repair Name the compound, then give the verdict for that compound. Transfer Give one beneficial and one harmful example of a substance from the same category. Essential minerals; toxic metal compounds.
• “Chlorination removes the solids from the water.” Defect The purposes of two stages have been swapped. Chemistry Chlorine kills microbes. Solids are removed by sedimentation and filtration. Repair One purpose per stage: solids, then tastes and odours, then microbes. Transfer Which stage would you use on water full of silt? Sedimentation and filtration.
• “Carbon kills the bacteria.” Defect Wrong purpose for the carbon stage. Chemistry Carbon removes tastes and odours. It has no disinfecting action. Repair “Carbon removes tastes and odours; chlorination kills microbes.” Transfer Water tastes marshy but is free of microbes. Which stage? Carbon.
• “Filtration removes the dissolved salts.” Defect Confuses dissolved with suspended. Chemistry Dissolved substances are in solution and pass through a filter bed with the water. Filtration removes solids. Repair “Filtration removes solids; dissolved substances pass through.” Transfer Which process does separate dissolved solids from water? Distillation.
• “After the three stages the water is pure \(\mathrm{H_2O}\).” Defect Over-claim about the product of treatment. Chemistry No stage removes dissolved substances, so the treated water is safe to drink but not chemically pure. Repair “Safe to drink”, not “pure”. Transfer Why is tap water not used in a titration? It still contains dissolved chemical impurities.
• “Water treatment is just distillation on a large scale.” Defect Two different processes conflated. Chemistry Treatment is settling, filtering, carbon and chlorine, aimed at safety. Distillation evaporates and condenses the water, aimed at removing dissolved solids. Repair Name the three treatment stages when a domestic supply is asked about. Transfer Which produces water with fewer dissolved solids? Distillation.
• “Ammonium salts are not fertilisers, only nitrates are.” Defect Half the syllabus statement is missing. Chemistry Both ammonium salts and nitrates are used as fertilisers, and ammonium nitrate belongs to both families. Repair Learn the pair together: “ammonium salts and nitrates”. Transfer Name a fertiliser that is both. Ammonium nitrate.
• “NPK fertilisers supply nitrogen, phosphorus and potassium so the plant can respire.” Defect Right elements, wrong reason. Chemistry The syllabus reason is improved plant growth, not respiration. Repair “…which are used for improved plant growth.” Transfer How many marks does the NPK statement usually carry? Three: the elements, and the reason.
• “Nitrates are pollutants, so fertilisers should be banned.” Defect Ignores that the verdict depends on location and amount. Chemistry On a field, nitrate is a nutrient giving improved plant growth. Washed into a river, the same nitrate causes deoxygenation and damages aquatic life. Repair Say where the substance is before you judge it. Transfer What property lets it travel between the two? It is soluble in water.
• “Air is about 70% nitrogen and 30% oxygen.” Defect Wrong figures, and no remainder. Chemistry Approximately 78% nitrogen and approximately 21% oxygen, with a small remainder of mainly noble gases and carbon dioxide. Repair Learn all three parts, including the word “approximately”. Transfer What roughly is 78 + 21? 99 — which is why the remainder is about 1%.
• “Clean air contains no carbon dioxide.” Defect Treats a normal component as a contaminant. Chemistry Carbon dioxide is part of the remainder of clean dry air. The adverse effect belongs to higher levels of it. Repair “The remainder is mainly noble gases and carbon dioxide.” Transfer What is the required wording for its effect? “Higher levels increase global warming and climate change.”
• “The composition is for dry air because air has no water vapour in it.” Defect Wrong reason for the qualifier. Chemistry Real air does contain water vapour; the amount varies with place and weather, so it cannot be given a fixed figure and is excluded. Repair “Water vapour is excluded because its amount varies.” Transfer Measure humid air without drying it: does nitrogen come out above or below 78%? Below.
• “Carbon monoxide is produced by the complete combustion of fuels.” Defect Source and product mismatched. Chemistry Complete combustion gives carbon dioxide. Carbon monoxide and particulates come from incomplete combustion, when the oxygen supply is limited. Repair Attach “incomplete” to carbon monoxide every time you write it. Transfer A yellow flame and soot: complete or incomplete? Incomplete.
• “Carbon dioxide causes acid rain.” Defect The effect belongs to a different pollutant. Chemistry Acid rain is caused by sulfur dioxide and oxides of nitrogen. The named effect of carbon dioxide is increased global warming and climate change. Repair Learn the pairs in blocks: climate for \(\mathrm{CO_2}\) and \(\mathrm{CH_4}\); acid rain for \(\mathrm{SO_2}\) and NOx. Transfer A limestone statue is eroding. Which two pollutants? Sulfur dioxide and oxides of nitrogen.
• “Sulfur dioxide is the main cause of global warming.” Defect The mirror image of the previous error. Chemistry Sulfur dioxide causes acid rain. The greenhouse gases named in this topic are carbon dioxide and methane. Repair Two gases for climate, two for acid rain — and none in both. Transfer Which control targets sulfur dioxide? Low-sulfur fuels, or flue gas desulfurisation with calcium oxide.
• “Oxides of nitrogen come from nitrogen compounds in the petrol.” Defect Wrong origin for the nitrogen. Chemistry The nitrogen comes from the air drawn into the engine, and reacts with oxygen at the high temperature inside the cylinder. Repair “Nitrogen and oxygen from the air react at high temperature in the engine.” Transfer Which pollutant does come from the fuel itself? Sulfur dioxide.
• “Nitrogen burns in the engine to make nitrogen monoxide.” Defect Treats an unreactive gas as a fuel. Chemistry Nitrogen is very unreactive; it does not react with oxygen under ordinary conditions. The engine supplies the high temperature that makes \(\mathrm{N_2 + O_2 \rightarrow 2NO}\) possible. Repair Say “react together at high temperature”, not “burns”. Transfer Why is there no NOx from a domestic gas fire? It does not reach the temperature a car engine cylinder reaches.
• “Greenhouse gases trap the ultraviolet rays coming from the Sun.” Defect The wrong energy, travelling the wrong way. Chemistry Energy from the Sun passes through and warms the surface. It is the thermal energy radiated outwards by the warmed Earth that greenhouse gases absorb. Repair Start the answer at the surface, not at the Sun. Transfer Why is it colder on the Moon at night? No atmosphere, so all the radiated thermal energy is lost to space.
• “Greenhouse gases form a layer that reflects heat back like a mirror.” Defect A picture that predicts the wrong physics. Chemistry They are individual molecules spread thinly through the atmosphere. They absorb thermal energy and re-emit it in all directions, so less is lost to space. Repair Use the three verbs: absorb, re-emit, reduce the loss to space. Transfer If it were a mirror, why would incoming sunlight still reach the ground? It would not — which is how you know the picture is wrong.
• “Catalytic converters remove sulfur dioxide from the exhaust.” Defect The control is attached to the wrong pollutant. Chemistry Converters deal with harmful gases such as carbon monoxide and oxides of nitrogen. Sulfur dioxide is controlled by low-sulfur fuels or flue gas desulfurisation with calcium oxide. Repair Converter → vehicle; low-sulfur fuel and desulfurisation → power station. Transfer Write the desulfurisation equation. \(\mathrm{CaO + SO_2 \rightarrow CaSO_3}\).
• “A catalytic converter turns harmful gases into harmless ones.” Defect One word too strong. Chemistry Nitrogen is benign, but carbon dioxide contributes to global warming. The syllabus wording is less harmful products. Repair Write “less harmful” and, if there is room, name the residual concern. Transfer Does fitting a converter reduce carbon dioxide from a car? No — it increases it slightly, by oxidising carbon monoxide.
• “\(\mathrm{6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2}\)” Defect A condition written as a reactant. Chemistry Light is energy, not matter. It cannot be counted in an atom balance, and neither can chlorophyll, which is not used up. Repair Move “light energy” and “chlorophyll” above and below the arrow. Transfer Where does the catalyst go in the converter equation? Above the arrow, for the same reason.
• “Photosynthesis uses oxygen and gives out carbon dioxide.” Defect The equation has been written backwards. Chemistry Photosynthesis takes in carbon dioxide and water and releases oxygen. The reverse is respiration. Repair \(\mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}\). Transfer Which side of the arrow does glucose sit on? The right — it is a product.
• “Planting trees will solve climate change.” Defect A real strategy stated as a complete solution. Chemistry Photosynthesis removes carbon dioxide, but slowly, only while the trees grow, and only for as long as the carbon stays locked up. It does nothing about methane, sulfur dioxide or oxides of nitrogen. Repair Add the benefit and a limitation — both are credited. Transfer Name a strategy that addresses methane instead. Reduction in livestock farming.

• Topic 10 appears in all three written components. It rewards precision and punishes generality more sharply than almost any other topic, because almost every mark is a named substance, a named source, a named effect or an exact colour.
• State → one short fact, no reason needed. “Chlorination kills microbes.”
• Describe → the steps or the observation, in order. “Add the liquid to anhydrous copper(II) sulfate; the white solid turns blue.”
• Explain → a because is compulsory. “Distilled water is used because it contains fewer dissolved chemical impurities, which could otherwise react with the reagents and give a false result.”
• Suggest → apply what you know to a new context; the answer is not in your notes but the chemistry is.
• Shape 1 — the test. Reagent → what you do → what you see → what you may conclude. Four parts, and the last one is where marks are lost by over-claiming.