Water & Water Pollution

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Chemical Tests for Water

Anhydrous cobalt(II) chloride (blue) turns pink in the presence of water, forming hydrated cobalt(II) chloride.
Cobalt chloride paper is commonly used: blue → pink indicates water.
Anhydrous copper(II) sulfate (white) turns blue in the presence of water, forming hydrated copper(II) sulfate.
Both tests are reversible: hydration on adding water, dehydration on heating.

Cobalt chloride paper changes from blue to pink in the presence of water

Purity of Water

Pure water has a sharp boiling point of 100 °C and melting point of 0 °C.
Impurities raise the boiling point and lower the melting point, giving a range.
Distilled water is produced by boiling water and condensing the steam, removing most dissolved impurities.
Distilled water is used in chemistry to avoid interference from dissolved ions in tap water.

Substances in Natural Water Sources

Natural water comes from lakes, rivers, and groundwater stored in aquifers.
Beneficial substances: dissolved oxygen (essential for aquatic life) and metal compounds (e.g., Ca²⁺, Mg²⁺ provide essential minerals).
Harmful substances: toxic metals (e.g., lead, aluminium), plastics, sewage (harmful microbes), and nitrates/phosphates from fertilisers.
Nitrates and phosphates can cause eutrophication – excessive plant growth leading to deoxygenation of water.

Water Treatment

Untreated water contains insoluble (soil, plant matter) and soluble (dissolved ions) impurities.
Step 1 – Sedimentation: Water stands in tanks; heavy particles settle by gravity.
Step 1 – Filtration: Water passes through sand and gravel to remove smaller particles.
Step 2 – Carbon filtration: Removes unpleasant tastes and odours.
Step 3 – Chlorination: Chlorine is added to kill bacteria and microbes (prevents diseases like cholera and typhoid).

The stages of treating water are sedimentation, filtration with carbon and chlorination

Fertilisers

NPK fertilisers supply nitrogen (N), phosphorus (P), and potassium (K) for healthy plant growth.
Nitrogen promotes leaves (chlorophyll and protein); phosphorus promotes roots; potassium promotes fruit and flowers.
Water-soluble ions: ammonium (NH₄⁺) and nitrate (NO₃⁻) for nitrogen; phosphate (PO₄³⁻) for phosphorus; potassium (K⁺) from soluble potassium compounds.
Common fertiliser compounds: ammonium nitrate (NH₄NO₃), ammonium phosphate ((NH₄)₃PO₄), potassium nitrate (KNO₃).
Fertilisers must be water-soluble so plants can absorb ions through their roots.

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Practice questions

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1.Which colour change indicates the presence of water when using anhydrous cobalt(II) chloride?
Easy
- Ablue to pink
- Bpink to blue
- Cwhite to blue
- Dblue to white
2.Which colour change indicates the presence of water when using anhydrous copper(II) sulfate?
Easy
- Awhite to blue
- Bblue to white
- Cpink to blue
- Dwhite to pink
3.What is the term for a rock that stores water?
Easy
- Aaquifer
- Breservoir
- Csediment
- Daqueduct
4.Which of the following is a beneficial substance that can be found in natural water sources?
Easy
- Adissolved oxygen
- Blead compounds
- Csewage
- Dnitrates from fertilisers
5.What is the main purpose of chlorination in water treatment?
Easy
- Ato kill bacteria and other microbes
- Bto remove insoluble particles
- Cto remove unpleasant tastes and odours
- Dto add essential minerals
6.A sample of water boils at 101 °C at standard pressure. What does this indicate about the water?
Easy
- Ait is pure water
- Bit contains dissolved impurities
- Cit is distilled water
- Dit has a lower boiling point than pure water
7.Which of the following is NOT a stage in the treatment of water for domestic supply?
Easy
- Asedimentation
- Bfiltration through carbon
- Cdistillation
- Dchlorination
8.Fertilisers contain soluble ions. Which ion provides phosphorus for plant growth?
Easy
- Aphosphate, PO₄³⁻
- Bnitrate, NO₃⁻
- Cammonium, NH₄⁺
- Dpotassium, K⁺

More questions

9.Why is distilled water preferred over tap water for use in chemical reactions?
Easy
- Ait has a higher boiling point
- Bit contains fewer dissolved impurities
- Cit has a lower pH
- Dit is more readily available
10.Which of the following substances in natural water is potentially harmful to aquatic life?
Easy
- Adissolved oxygen
- Bcalcium ions
- Clead compounds
- Dmagnesium ions
11.In the Haber process, ammonia is produced. Which of the following is a fertiliser that can be made from ammonia?
Easy
- Aammonium nitrate
- Bpotassium nitrate
- Ccalcium phosphate
- Dsodium chloride
12.A student tests a colourless liquid with anhydrous copper(II) sulfate; it turns blue. Then the liquid is boiled and the vapour is condensed; the condensed liquid is tested again with anhydrous copper(II) sulfate and it remains white. What is the most likely identity of the original liquid?
Hard
- Apure water
- Bsea water
- Cethanol
- Ddilute hydrochloric acid
13.A student measures the boiling point of a water sample and finds it to be 102 °C at standard pressure. Which conclusion about the sample is correct?
Medium
- AThe water is pure because it boils above 100 °C.
- BThe water contains dissolved impurities that raise the boiling point.
- CThe water is distilled water.
- DThe thermometer is faulty; pure water always boils at exactly 100 °C.
14.A water sample is tested with anhydrous copper(II) sulfate. It turns blue. What does this indicate?
Easy
- AThe sample contains copper(II) ions.
- BThe sample contains water.
- CThe sample is pure water.
- DThe sample contains sulfate ions.
15.A student uses anhydrous cobalt(II) chloride paper to test three colourless liquids. The paper turns pink with liquid X, remains blue with liquid Y, and turns pink with liquid Z. Which liquid(s) contain water?
Medium
- AOnly X
- BOnly Y
- CX and Z
- DY and Z
16.A water sample contains dissolved calcium ions and nitrate ions from fertiliser runoff. Which statement about this water is correct?
Medium
- AThe calcium ions are harmful to aquatic life.
- BThe nitrate ions can cause deoxygenation of water.
- CBoth calcium and nitrate ions are beneficial to all aquatic life.
- DThe water is safe to drink without treatment.
17.Which of the following is a beneficial substance found in natural water?
Easy
- ALead ions
- BDissolved oxygen
- CNitrate ions from fertilisers
- DPlastic debris
18.In water treatment, what is the purpose of filtration through carbon (charcoal)?
Easy
- ATo kill bacteria and other microorganisms
- BTo remove insoluble particles such as sand and mud
- CTo remove unpleasant tastes and odours
- DTo add chlorine to the water
19.Which of the following ions is a source of soluble phosphorus for fertilisers?
Easy
- ANH₄⁺
- BNO₃⁻
- CPO₄³⁻
- DK⁺
20.A fertiliser contains ammonium nitrate and potassium nitrate. Which elements for plant growth does this fertiliser provide?
Medium
- ANitrogen and potassium only
- BNitrogen and phosphorus only
- CNitrogen, phosphorus, and potassium
- DPotassium and phosphorus only
21.The percentage by mass of nitrogen in calcium nitrate, Ca(NO₃)₂, is (Ar: Ca=40, N=14, O=16):
Medium
- A17.1%
- B8.5%
- C34.1%
- D23.7%
22.A student adds anhydrous copper(II) sulfate to a colourless liquid and observes a white colour. What can be concluded?
Easy
- AThe liquid contains water.
- BThe liquid does not contain water.
- CThe liquid is pure water.
- DThe test is inconclusive.
23.Which of the following is a potential harmful effect of phosphate fertilisers entering water sources?
Medium
- AThey increase dissolved oxygen levels.
- BThey cause deoxygenation of water due to excessive plant growth.
- CThey directly poison fish.
- DThey make water harder.
24.In water treatment, why is chlorination carried out after filtration?
Medium
- ATo remove insoluble impurities
- BTo remove tastes and odours
- CTo kill microorganisms that are too small to be filtered
- DTo add minerals to the water
25.A student adds a few drops of a colourless liquid to anhydrous copper(II) sulfate. The solid turns blue. Which statement about the liquid is correct?
Medium
- AIt contains water.
- BIt is pure water.
- CIt is an aqueous solution.
- DIt is distilled water.
26.A sample of water has a melting point of -2 °C and a boiling point of 102 °C at standard pressure. Which conclusion is correct?
Medium
- AThe water is pure.
- BThe water contains dissolved impurities.
- CThe water is distilled water.
- DThe water is from a natural source.
27.A student tests four colourless liquids with anhydrous cobalt(II) chloride paper. The paper turns pink with liquid W, remains blue with liquid X, turns pink slowly with liquid Y, and turns pink immediately with liquid Z. Which liquid is most likely to be pure water?
Medium
- AW
- BX
- CY
- DZ
28.In the treatment of water for domestic supply, what is the purpose of filtration through sand and gravel?
Easy
- ATo remove dissolved salts
- BTo remove unpleasant tastes and odours
- CTo kill harmful microbes
- DTo remove smaller insoluble particles
29.A student is given a mixture of sand, salt, and water. Which sequence of steps would separate the sand and obtain pure water?
Medium
- AFiltration, then distillation
- BDistillation, then filtration
- CEvaporation, then condensation
- DSedimentation, then chlorination
30.Which of the following is a beneficial substance found in natural water sources?
Easy
- ALead ions
- BDissolved oxygen
- CNitrates from fertilisers
- DPlastics
31.Fertilisers contain soluble ions to provide nutrients to plants. Which ion provides phosphorus?
Easy
- ANO₃⁻
- BNH₄⁺
- CPO₄³⁻
- DK⁺
32.A fertiliser contains ammonium phosphate, (NH₄)₃PO₄, and potassium nitrate, KNO₃. Which elements for plant growth does this fertiliser provide?
Medium
- ANitrogen and phosphorus only
- BNitrogen and potassium only
- CNitrogen, phosphorus, and potassium
- DPhosphorus and potassium only
33.The percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃, is (Ar: H=1, N=14, O=16):
Medium
- A17.5%
- B35.0%
- C28.0%
- D22.4%
34.Which of the following is a harmful effect of nitrate fertilisers entering water sources?
Medium
- AThey provide essential minerals for aquatic life.
- BThey cause deoxygenation of water by promoting plant growth.
- CThey kill microbes in the water.
- DThey increase the dissolved oxygen content.
35.What colour change does anhydrous cobalt(II) chloride undergo when it comes into contact with water?
Easy
- Ablue to pink
- Bpink to blue
- Cwhite to blue
- Dblue to white
36.A student measures the boiling point of a water sample and finds it to be 101.5 °C at standard pressure. Which conclusion about the sample is correct?
Medium
- Athe water is pure
- Bthe water contains dissolved impurities
- Cthe water has a lower boiling point than pure water
- Dthe water is distilled water
37.In the treatment of water for domestic supply, what is the purpose of the sedimentation stage?
Easy
- Ato remove dissolved ions
- Bto kill bacteria
- Cto allow mud and sand to settle out
- Dto remove unpleasant tastes
38.A sample of water is tested with anhydrous copper(II) sulfate and turns blue. Its boiling point is measured as 101 °C at standard pressure. Which statement about the water is correct?
Medium
- AIt is pure water.
- BIt contains dissolved impurities.
- CIt is distilled water.
- DIt has a melting point of 0 °C.
39.Anhydrous cobalt(II) chloride is blue and turns pink in the presence of water.
Easy
True or false?
40.Pure water boils at exactly 100 °C and melts at exactly 0 °C.
Easy
True or false?
41.Which of the following is the correct colour change when anhydrous copper(II) sulfate is added to water?
Easy
- Ablue to white
- Bwhite to blue
- Cwhite to pink
- Dblue to pink
42.NPK fertilisers contain nitrogen, phosphorus, and potassium.
Easy
True or false?
43.Which of the following substances in natural water is essential for aquatic life?
Easy
- Anitrates from fertilisers
- Bdissolved oxygen
- Clead compounds
- Dplastics
44.In water treatment, what is the purpose of the sedimentation stage?
Easy
- Ato kill microbes
- Bto remove unpleasant tastes
- Cto allow mud and sand to settle
- Dto remove dissolved ions
45.Which ion provides phosphorus for plant growth in fertilisers?
Easy
- Aammonium ion
- Bnitrate ion
- Cphosphate ion
- Dpotassium ion
46.Impurities in water decrease its boiling point and increase its melting point.
Medium
True or false?
47.A sample of water boils at 103 °C at standard pressure. Which statement is correct?
Medium
- AThe water is pure.
- BThe water contains dissolved impurities.
- CThe water has a high concentration of dissolved oxygen.
- DThe water has been distilled.
48.Which of the following are potentially harmful substances that can be found in natural water? (Select all that apply.)
Hard
- Adissolved oxygen
- Blead compounds
- Ccalcium ions
- Dsewage
- Enitrates from fertilisers

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