IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 10: Chemistry of the Environment -- Mock Exam 1

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0620

Instructions

Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.

Question Navigation

Question 1 -- Water: Tests and Purity

Total: 12 marks

A student in Kochi is given three unlabelled bottles containing distilled water, bottled mineral water and ethanol. She is asked to identify the liquids using chemical tests and boiling-point measurements.

(a) [4]

Two different chemical tests can show that a liquid contains water.

(i) Describe the test using anhydrous copper(II) sulfate, and give the colour change for a positive result. [2]

(ii) Describe the test using cobalt(II) chloride paper, and give the colour change for a positive result. [2]

Model Answer -- 1(a)

(i) Add a few drops of the liquid to anhydrous copper(II) sulfate powder [1]

(i) If water is present the powder turns from white to blue [1]

(ii) Touch a drop of the liquid onto dry cobalt(II) chloride paper [1]

(ii) If water is present the paper turns from blue to pink [1]

⚠ If you missed marks here: The word "anhydrous" matters — hydrated copper(II) sulfate is already blue, so it cannot show any change. Both colour changes must be given the right way round: white to blue for copper(II) sulfate and blue to pink for cobalt(II) chloride. Swapping the two colour changes is the single most common way to lose both observation marks.

1 mark -- add liquid to anhydrous copper(II) sulfate powder 1 mark -- copper(II) sulfate turns white to blue with water 1 mark -- touch liquid onto dry cobalt(II) chloride paper 1 mark -- cobalt(II) chloride paper turns blue to pink with water

(b) [4]

The tests in (a) cannot show whether a sample of water is pure.

(i) State how the student can show that a sample of water is pure. [2]

(ii) The mineral water boils at 100.8 °C at standard atmospheric pressure. Explain this observation. [2]

Model Answer -- 1(b)

(i) Measure the boiling point of the sample at standard atmospheric pressure [1]

(i) Pure water boils at exactly 100 °C; any other value, or boiling over a range, shows the water is impure [1]

(ii) The mineral water contains dissolved solids / dissolved mineral salts [1]

(ii) Dissolved impurities raise the boiling point above 100 °C [1]

⚠ If you missed marks here: Saying only "measure the boiling point" earns one mark, not two — you must also quote the expected value, exactly 100 °C at standard atmospheric pressure. In (ii) the examiner wants cause and effect: the water contains dissolved solids, and dissolved impurities raise the boiling point. An answer of "it is not pure" merely restates the data without explaining it.

1 mark -- measure boiling point at standard atmospheric pressure 1 mark -- pure water boils at exactly 100 degrees C 1 mark -- mineral water contains dissolved solids or salts 1 mark -- dissolved impurities raise the boiling point

The student tests all three liquids. Liquid 1 turns anhydrous copper(II) sulfate blue and boils at exactly 100 °C. Liquid 2 turns anhydrous copper(II) sulfate blue and boils at 100.8 °C. Liquid 3 leaves anhydrous copper(II) sulfate white and boils at 78 °C.

Identify each liquid and explain your reasoning.

Model Answer -- 1(c)

Liquid 1 is the distilled water: it contains water and its sharp boiling point of exactly 100 °C shows it is pure [1]

Liquid 2 is the mineral water: it contains water but the boiling point above 100 °C shows dissolved solids are present [1]

Liquid 3 is the ethanol: the copper(II) sulfate stays white, showing it contains no water [1]

The boiling point of 78 °C is a fixed, sharp value, consistent with pure ethanol rather than a mixture [1]

⚠ If you missed marks here: Each identification must be tied to evidence — naming the liquids without giving the reasoning scores at most one mark in total. Take the two tests in order for each liquid: first what the copper(II) sulfate result says about whether water is present, then what the boiling point says about purity. Remember that a liquid which leaves the powder white contains no water at all, so it cannot be either of the water samples.

1 mark -- liquid 1 distilled water pure because boils exactly 100 1 mark -- liquid 2 mineral water dissolved solids raise boiling point 1 mark -- liquid 3 ethanol contains no water powder stays white 1 mark -- sharp 78 degree boiling point consistent with pure ethanol

Question 2 -- Water Treatment and Substances in Water

Total: 12 marks

A new water treatment works is built beside a river in Tamil Nadu to supply drinking water to a town. The raw river water is cloudy, has an earthy smell and contains microbes.

(a) [4]

The first two stages of treatment remove insoluble material from the water.

(i) Name the first stage and describe what happens to the water during it. [2]

(ii) Name the second stage and state what it removes that the first stage cannot. [2]

Model Answer -- 2(a)

(i) The first stage is sedimentation [1]

(i) The water is allowed to stand so that the larger, heavier insoluble particles settle to the bottom of the tank [1]

(ii) The second stage is filtration, in which the water passes through beds of sand [1]

(ii) It traps the finer insoluble particles that are too light to settle out during sedimentation [1]

⚠ If you missed marks here: The two stages are easy to muddle: sedimentation is settling under gravity and must come first, while filtration through sand catches what is left. Answers that say either stage removes "dissolved" substances lose the mark — both stages remove only insoluble solids, and dissolved material passes straight through them.

1 mark -- first stage named sedimentation 1 mark -- water stands larger heavier particles settle out 1 mark -- second stage filtration through sand 1 mark -- traps finer insoluble particles that do not settle

(b) [4]

The water then passes through beds of carbon before chlorine is added as the final stage.

(i) State the purpose of the carbon beds. [1]

(ii) State the purpose of adding chlorine. [1]

(iii) Explain why chlorination is carried out as the last stage, and why a very small amount of chlorine remains in the water leaving the works. [2]

Model Answer -- 2(b)

(i) The carbon removes dissolved substances with unpleasant tastes and odours [1]

(ii) The chlorine kills harmful microbes in the water [1]

(iii) Disinfecting last means no microbes are picked up in later treatment stages before the water leaves the works [1]

(iii) The residual chlorine keeps killing any microbes that enter the water in the pipes between the works and people's taps [1]

⚠ If you missed marks here: "Carbon filters the water" is not enough — carbon deals with dissolved tastes and odours, which no filter can remove. For chlorine, "cleans the water" is too vague; the marking point is that it kills microbes (bacteria). The reason for the residual chlorine is protection in the distribution pipes — an answer that stops at "to be safe" does not identify where the re-infection risk arises.

1 mark -- carbon removes dissolved tastes and odours 1 mark -- chlorine kills harmful microbes 1 mark -- chlorinating last means water leaves works free of microbes 1 mark -- residual chlorine protects water in pipes to homes

Untreated river water contains many dissolved and suspended substances.

(i) Name one substance found in natural water that is beneficial, and explain why it is beneficial. [2]

(ii) Name two different types of harmful substance found in natural water and state one problem caused by each. [2]

Model Answer -- 2(c)

(i) Dissolved oxygen is beneficial [1]

(i) It is essential for the respiration of fish and other aquatic life [1]

(ii) Any two from: microbes from sewage, plastics, dissolved metal compounds, nitrates or phosphates from fertilisers [1]

(ii) Matching problems: microbes cause diseases such as cholera; plastics persist and harm animals that swallow them; metal compounds are toxic; nitrates and phosphates cause excessive algal growth that removes oxygen [1]

⚠ If you missed marks here: The beneficial substance is dissolved oxygen — do not write "minerals are good for you", which is not the syllabus point. In (ii) each harmful substance must be paired with its own specific problem; writing "they are bad for fish" twice for two different substances counts as one problem, not two.

1 mark -- dissolved oxygen named as beneficial 1 mark -- oxygen needed for respiration of aquatic life 1 mark -- two harmful substances named such as sewage microbes plastics metal compounds nitrates 1 mark -- correct matching problem stated for each harmful substance

Question 3 -- Fertilisers

Total: 12 marks

A fertiliser factory near Ludhiana manufactures ammonium sulfate for wheat farmers. The label on each bag lists the NPK content of the fertiliser.

(a) [4]

(i) Explain why farmers need to add fertilisers to soil that has been used for growing crops. [1]

(ii) State the effect on plant growth of each of the three essential elements nitrogen, phosphorus and potassium. [3]

Model Answer -- 3(a)

(i) Growing crops remove nutrient elements from the soil, and fertilisers replace them so that yields stay high [1]

(ii) Nitrogen promotes the growth of leaves [1]

(ii) Phosphorus promotes strong root growth [1]

(ii) Potassium promotes the development of flowers and fruit [1]

⚠ If you missed marks here: The three NPK roles must be matched to the right element — a common error is swapping phosphorus and potassium. A memory aid: N for leaves at the top of the plant, P for roots at the bottom, K for the crop you pick (flowers and fruit). Vague answers such as "helps the plant grow" score nothing because every nutrient does that.

1 mark -- fertilisers replace nutrients removed by previous crops 1 mark -- nitrogen promotes leaf growth 1 mark -- phosphorus promotes root growth 1 mark -- potassium promotes flower and fruit development

(b) [4]

Ammonium sulfate is made in the factory by reacting ammonia with dilute sulfuric acid.

(i) Write the balanced symbol equation for this reaction. [2]

(ii) Name the type of reaction taking place. [1]

(iii) Suggest one property of ammonium sulfate, other than its nitrogen content, that makes it suitable for use as a fertiliser. [1]

Model Answer -- 3(b)

(i) 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ — correct formulae [1]

(i) Equation correctly balanced with 2NH₃ [1]

(ii) Neutralisation (an acid–base reaction) [1]

(iii) It is soluble in water, so plants can absorb the nutrient ions through their roots [1]

⚠ If you missed marks here: The formula of ammonium sulfate needs brackets: (NH₄)₂SO₄, because the sulfate ion carries a 2− charge and needs two ammonium ions. Once the formula is right, the 2 in front of NH₃ follows automatically. In (iii) solubility is the property examiners expect — an insoluble compound would be useless as a fertiliser however much nitrogen it contained.

1 mark -- correct formulae NH3 H2SO4 and (NH4)2SO4 1 mark -- equation balanced with 2NH3 1 mark -- reaction identified as neutralisation 1 mark -- soluble in water so roots can absorb ions

(i) Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH₄)₂SO₄. (Aᵣ: H = 1, N = 14, O = 16, S = 32) [3]

(ii) The percentage of nitrogen in ammonium nitrate, NH₄NO₃, is 35%. State which of the two compounds supplies more nitrogen per kilogram of fertiliser. [1]

Model Answer -- 3(c)

(i) Mᵣ of (NH₄)₂SO₄ = 2(14 + 4) + 32 + (4 × 16) = 132 [1]

(i) Mass of nitrogen per formula unit = 2 × 14 = 28 [1]

(i) Percentage of nitrogen = 28 ÷ 132 × 100 = 21.2% [1]

(ii) Ammonium nitrate supplies more nitrogen per kilogram, because 35% is greater than 21.2% [1]

⚠ If you missed marks here: The bracket in (NH₄)₂SO₄ doubles everything inside it, so there are two nitrogen and eight hydrogen atoms — forgetting this gives Mᵣ = 114 and ruins the whole calculation. Show Mᵣ, the nitrogen mass and the final percentage as three separate lines, because each carries its own mark even if a later step goes wrong.

1 mark -- relative formula mass 132 1 mark -- nitrogen mass 28 from two atoms 1 mark -- percentage 21.2 1 mark -- ammonium nitrate supplies more nitrogen per kilogram

Question 4 -- Air Composition and Pollutants

Total: 12 marks

An environmental scientist in Delhi compares the composition of clean, dry air with samples of polluted air collected beside a busy road during the winter smog season.

(a) [4]

(i) State the approximate percentage by volume of nitrogen and of oxygen in clean, dry air. [2]

(ii) Name the two gases that make up most of the remainder of clean, dry air. [2]

Model Answer -- 4(a)

(i) Nitrogen makes up approximately 78% of clean, dry air [1]

(i) Oxygen makes up approximately 21% [1]

(ii) Most of the remainder is argon [1]

(ii) The rest includes a small amount of carbon dioxide (about 0.04%) [1]

⚠ If you missed marks here: The figures 78% and 21% must be attached to the right gases — writing them the other way round is a surprisingly common slip under time pressure. For the remainder, argon must be named as the major component; answers offering "carbon dioxide and water vapour" miss that the question specifies dry air and that argon outweighs carbon dioxide more than twenty-fold.

1 mark -- nitrogen 78 percent 1 mark -- oxygen 21 percent 1 mark -- remainder mostly argon 1 mark -- small amount of carbon dioxide

(b) [4]

The polluted air contains carbon monoxide.

(i) Explain how carbon monoxide is formed in vehicle engines. [1]

(ii) Explain why carbon monoxide is toxic to humans. [2]

(iii) Suggest why people are not aware when they are breathing in carbon monoxide. [1]

Model Answer -- 4(b)

(i) It forms by the incomplete combustion of the fuel when the supply of oxygen is limited [1]

(ii) Carbon monoxide binds to the haemoglobin in red blood cells [1]

(ii) This prevents the haemoglobin carrying oxygen, so the body's tissues are starved of oxygen [1]

(iii) Carbon monoxide is colourless and odourless, so it cannot be seen or smelt [1]

⚠ If you missed marks here: "Incomplete combustion" needs its cause — a limited or insufficient supply of oxygen — to earn the mark. The toxicity explanation is worth two marks and needs both halves: the binding to haemoglobin and the consequence that oxygen can no longer be transported. Writing "it is poisonous" simply restates the question.

1 mark -- incomplete combustion in limited oxygen supply 1 mark -- binds to haemoglobin in red blood cells 1 mark -- prevents oxygen being carried to body tissues 1 mark -- colourless and odourless so undetectable by senses

The winter smog also contains particulates and methane.

(i) State how particulates are produced and give two harmful effects they have on human health. [2]

(ii) Name two sources of methane in the atmosphere. [2]

Model Answer -- 4(c)

(i) Particulates are tiny solid carbon particles produced by the incomplete combustion of fuels such as diesel [1]

(i) They cause respiratory problems and increase the risk of cancer [1]

(ii) Digestion in livestock such as cattle releases methane [1]

(ii) The decomposition of vegetation, for example in flooded rice paddies or landfill, also releases methane [1]

⚠ If you missed marks here: Both health effects are needed for the second mark — respiratory problems alone is only half the answer; the increased risk of cancer completes it. For methane, the syllabus sources are biological: livestock digestion and decomposing vegetation. Car engines are not an accepted source of methane, however tempting it is to blame them for everything.

1 mark -- particulates from incomplete combustion of fuels 1 mark -- respiratory problems and increased cancer risk 1 mark -- methane from digestion in livestock 1 mark -- methane from decomposition of vegetation

Question 5 -- Sulfur Dioxide and Acid Rain

Total: 10 marks

A coal-fired power station in northern England burns coal that contains sulfur compounds. Environmental groups have complained about acid rain damaging forests and lakes downwind of the station.

(a) [3]

(i) Explain how burning the coal produces sulfur dioxide. [1]

(ii) State two harmful effects of the acid rain that forms from sulfur dioxide. [2]

Model Answer -- 5(a)

(i) The sulfur compounds in the coal are oxidised when the coal burns, forming sulfur dioxide gas [1]

(ii) Acid rain corrodes buildings made of limestone or marble and corrodes metal structures [1]

(ii) It acidifies lakes and rivers, killing fish, and damages trees and crops [1]

⚠ If you missed marks here: The key idea in (i) is that the sulfur is already inside the coal — sulfur dioxide does not come from the air. For the effects, name two genuinely different categories, such as damage to buildings and harm to living things; "kills fish" and "kills plants" from the same category may be treated as one marking point.

1 mark -- sulfur compounds in the coal oxidised on burning 1 mark -- corrodes limestone buildings and metal structures 1 mark -- acidifies lakes killing fish and damages trees

(b) [4]

The power station installs flue gas desulfurisation equipment that sprays the waste gases with calcium oxide.

(i) Explain, in terms of the types of oxide involved, why calcium oxide removes sulfur dioxide. [2]

(ii) Write a symbol equation for the reaction between calcium oxide and sulfur dioxide, and name the product. [2]

Model Answer -- 5(b)

(i) Sulfur dioxide is an acidic oxide and calcium oxide is a basic oxide [1]

(i) The base neutralises the acidic gas, forming a solid salt that can be collected [1]

(ii) CaO + SO₂ → CaSO₃ [1]

(ii) The product is calcium sulfite [1]

⚠ If you missed marks here: The explanation mark needs both classifications: acidic oxide for sulfur dioxide and basic oxide for calcium oxide — one on its own does not explain why they react. The product is calcium sulfite, CaSO₃, with three oxygen atoms; writing calcium sulfate, CaSO₄, is wrong here because no extra oxygen is involved in this reaction.

1 mark -- sulfur dioxide acidic oxide calcium oxide basic oxide 1 mark -- base neutralises the acidic gas forming a salt 1 mark -- equation CaO plus SO2 gives CaSO3 1 mark -- product named calcium sulfite

Oxides of nitrogen also contribute to acid rain.

(i) Explain how oxides of nitrogen are formed in car engines. [2]

(ii) State one other environmental or health problem, apart from acid rain, caused by oxides of nitrogen. [1]

Model Answer -- 5(c)

(i) Nitrogen and oxygen from the air are drawn into the engine and react with each other [1]

(i) The reaction only occurs because of the very high temperature inside the engine [1]

(ii) They cause photochemical smog / respiratory problems in humans [1]

⚠ If you missed marks here: Two marks means two ideas: the reacting gases both come from the air, and the high temperature of the engine is what forces the normally unreactive nitrogen to combine. Answers claiming the nitrogen comes from the fuel lose the first mark — petrol contains almost no nitrogen.

1 mark -- nitrogen and oxygen from the air react together 1 mark -- reaction caused by high temperature in engine 1 mark -- photochemical smog or respiratory problems stated

Question 6 -- Greenhouse Gases and Climate Change

Total: 12 marks

World leaders meet at a climate conference to discuss rising global temperatures. Scientists at the conference explain the role of carbon dioxide and methane in the atmosphere.

(a) [4]

Describe how greenhouse gases such as carbon dioxide and methane cause an increase in the Earth's surface temperature. Your answer should refer to thermal energy (infrared radiation).

Model Answer -- 6(a)

The Earth's surface is warmed by the Sun and radiates thermal energy (infrared radiation) back towards space [1]

Molecules of carbon dioxide and methane absorb this thermal energy [1]

They re-emit the energy in all directions, so some of it is returned towards the Earth's surface [1]

Less thermal energy escapes into space, so the surface temperature of the Earth rises [1]

⚠ If you missed marks here: The full chain of four steps is needed: the surface radiates, the gases absorb, the gases re-emit in all directions, and less energy escapes to space. Answers saying the gases "trap heat" or "form a blanket" without mentioning absorption and re-emission of radiation stop at one mark. Note it is thermal (infrared) radiation leaving the Earth that matters, not the Sun's incoming light being blocked.

1 mark -- Earth's surface radiates thermal infrared energy 1 mark -- carbon dioxide and methane absorb this energy 1 mark -- energy re-emitted in all directions some back to surface 1 mark -- less energy escapes to space so temperature rises

(b) [4]

(i) State the main human activity that increases the amount of carbon dioxide in the atmosphere. [1]

(ii) Name two sources of the greenhouse gas methane. [2]

(iii) State one strategy, other than planting trees, that a country can use to reduce its emissions of carbon dioxide. [1]

Model Answer -- 6(b)

(i) The complete combustion of fossil fuels (coal, oil and natural gas) releases carbon dioxide [1]

(ii) Digestion in livestock such as cattle produces methane [1]

(ii) Decomposition of vegetation, for example in rice paddies or landfill sites, produces methane [1]

(iii) Any one of: generating electricity from renewable sources such as wind, solar or hydroelectric power; using electric vehicles; improving energy efficiency [1]

⚠ If you missed marks here: Both methane sources must be biological — livestock digestion and decomposing vegetation — because burning fuels releases carbon dioxide, not methane. In (iii) the question rules out tree planting, so an answer must offer a way of emitting less, such as switching to renewable energy, rather than a way of absorbing what has already been emitted.

1 mark -- burning fossil fuels releases carbon dioxide 1 mark -- methane from livestock digestion 1 mark -- methane from decomposing vegetation 1 mark -- valid reduction strategy such as renewable energy

Planting forests helps to reduce climate change because trees remove carbon dioxide by photosynthesis.

(i) Write the balanced symbol equation for photosynthesis. [2]

(ii) State the two conditions essential for photosynthesis. [2]

Model Answer -- 6(c)

(i) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — correct formulae [1]

(i) Equation correctly balanced with 6, 6, 1, 6 [1]

(ii) Light (energy) is essential [1]

(ii) Chlorophyll is essential [1]

⚠ If you missed marks here: The glucose formula C₆H₁₂O₆ must be exactly right, and the equation balances with six of everything except the single glucose. The two conditions are light and chlorophyll — answers offering "sunlight and water" miss that water is a reactant in the equation, not a condition, while chlorophyll is the pigment that makes the process possible.

1 mark -- correct formulae CO2 H2O C6H12O6 O2 1 mark -- equation balanced six six one six 1 mark -- light stated as essential condition 1 mark -- chlorophyll stated as essential condition

Question 7 -- Investigating Water Samples

Total: 10 marks

A factory discharges waste water into a river. Students collect water samples from the river upstream and downstream of the factory to investigate whether the discharge is polluting the river.

(a) [3]

Describe how the students should collect and test the two water samples to compare the amounts of dissolved solids they contain. Include one precaution that makes the comparison fair.

Model Answer -- 7(a)

Collect the two samples on the same day, at the same depth, in clean containers [1]

Evaporate each sample to dryness and weigh the solid residue left behind (or measure and compare the boiling points) [1]

Use the same volume of each sample (and the same apparatus) so the comparison is fair [1]

⚠ If you missed marks here: An experiment-design answer needs a measurable outcome: evaporating a known volume and weighing the residue gives a number that can be compared between the two sites. The fairness precaution must be specific — "same volume of water evaporated in the same apparatus" scores, but "do it fairly" does not.

1 mark -- collect both samples same day same depth clean containers 1 mark -- evaporate to dryness and weigh residue or compare boiling points 1 mark -- same volume and same apparatus for fair comparison

(b) [4]

The students' results are shown below.

Upstream: boiling point 100.1 °C; residue from 100 cm³ = 0.02 g; dissolved oxygen high; many fish seen.
Downstream: boiling point 101.4 °C; residue from 100 cm³ = 0.85 g; dissolved oxygen low; few fish seen.

(i) What do the boiling points and residues show about the downstream water? Explain your answer. [2]

(ii) Suggest an explanation for the low dissolved oxygen and the small number of fish downstream. [2]

Model Answer -- 7(b)

(i) The downstream water contains far more dissolved solids: its residue is over forty times larger [1]

(i) The raised boiling point of 101.4 °C confirms this, because dissolved impurities raise the boiling point above 100 °C [1]

(ii) The discharge may contain organic waste or nutrients that let bacteria or algae multiply, and their respiration or decomposition uses up dissolved oxygen [1]

(ii) Fish need dissolved oxygen for respiration, so few fish can survive where the oxygen level is low [1]

⚠ If you missed marks here: Interpretation marks come from linking each measurement to a conclusion: the big residue means more dissolved solids, and the raised boiling point independently confirms it. In (ii) the chain must be complete — something in the discharge feeds microbes or algae, they consume the oxygen, and the fish that depend on that oxygen disappear. Jumping straight to "pollution kills fish" earns little credit.

1 mark -- downstream contains much more dissolved solid larger residue 1 mark -- raised boiling point confirms dissolved impurities 1 mark -- discharge feeds bacteria or algae which use up oxygen 1 mark -- fish need dissolved oxygen for respiration so numbers fall

(i) Suggest two ways the students could improve the reliability of their investigation. [2]

(ii) The factory owner claims the downstream water is safe because it turns cobalt(II) chloride paper pink. Explain why this claim is not valid. [1]

Model Answer -- 7(c)

(i) Repeat the tests with several samples from each site and compare or average the results [1]

(i) Sample on different days / at several points across the river to check the results are representative [1]

(ii) The cobalt(II) chloride test only shows the liquid contains water; it gives no information about dissolved impurities or microbes, so it cannot show the water is safe [1]

⚠ If you missed marks here: Reliability improvements must generate more data — repeats, more sites, more days. Using "more accurate equipment" addresses accuracy, not reliability, and usually fails to score. In (ii) the key sentence is that the test detects only the presence of water; every river, however polluted, would pass it.

1 mark -- repeat tests with several samples and average 1 mark -- sample different days or points for representative data 1 mark -- cobalt chloride only shows water present not purity or safety

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