Water

1. Overview

Water is the most important solvent in chemistry and is essential for all known forms of life. This topic covers how to chemically identify water, how to ensure its purity for scientific use, and the processes involved in making natural water safe for human consumption.

Key Definitions

• Anhydrous: A substance that contains no water (often used as a test reagent).
• Hydrated: A substance that contains water of crystallisation.
• Distilled Water: Water that has been purified by boiling it into steam and condensing it back into a liquid, leaving impurities behind.
• Sedimentation: The process where large, insoluble particles settle to the bottom of a tank.
• Filtration: The process of passing water through layers of sand and gravel to remove smaller insoluble solids.
• Chlorination: The addition of chlorine to water supplies to kill harmful microbes and bacteria.

Core Content

Chemical Tests for the Presence of Water

These tests confirm the presence of water molecules but do not prove that the water is pure.

1. Anhydrous Cobalt(II) Chloride

   • Test: Add the liquid to blue anhydrous cobalt(II) chloride paper.
   • Observation: The paper changes color from blue to pink.
   • Word Equation: anhydrous cobalt(II) chloride + water → hydrated cobalt(II) chloride
   • Symbol Equation: $\text{CoCl}_2 (\text{s}) + 6\text{H}_2\text{O} (\text{l}) \rightarrow \text{CoCl}_2\cdot6\text{H}_2\text{O} (\text{s})$

2. Anhydrous Copper(II) Sulfate

   • Test: Add the liquid to white anhydrous copper(II) sulfate powder.
   • Observation: The powder changes color from white to blue.
   • Word Equation: anhydrous copper(II) sulfate + water → hydrated copper(II) sulfate
   • Symbol Equation: $\text{CuSO}_4 (\text{s}) + 5\text{H}_2\text{O} (\text{l}) \rightarrow \text{CuSO}_4\cdot5\text{H}_2\text{O} (\text{s})$

Testing for Purity

A chemical test only tells you if water is present. To check for purity, you must measure physical properties:

• Boiling Point: Pure water boils at exactly 100 °C at standard atmospheric pressure.
• Melting Point: Pure ice melts at exactly 0 °C.
• Effect of Impurities: If water is impure, the boiling point will be higher than 100 °C and the melting point will be lower than 0 °C.

Distilled Water vs. Tap Water

In practical chemistry, distilled water is preferred over tap water.

• Tap water contains dissolved chemical impurities (such as calcium or magnesium ions) which can react with reagents and interfere with experimental results.
• Distilled water has been purified, meaning it contains fewer chemical impurities and provides more reliable data in the lab.

Substances in Natural Water Sources

Water from rivers, lakes, and oceans contains various substances. Some are helpful, while others are harmful.

Substance	Status	Effect
Dissolved Oxygen	Beneficial	Essential for the respiration of aquatic life (fish).
Metal Compounds	Beneficial	Some (like Ca²⁺ or Mg²⁺) provide essential minerals for life.
Metal Compounds	Harmful	Some (like lead or mercury) are toxic to humans and animals.
Plastics	Harmful	Can trap or be ingested by aquatic life, causing physical harm.
Sewage	Harmful	Contains harmful microbes (bacteria/viruses) which cause diseases like cholera.
Nitrates & Phosphates	Harmful	From fertilisers/detergents; lead to deoxygenation (eutrophication) of water.

Domestic Water Treatment

To make water safe for drinking (potable), it undergoes several stages:

1. Sedimentation: Water is stored in large tanks where heavy insoluble particles sink to the bottom.
2. Filtration: Water is passed through layers of fine sand and gravel to filter out remaining small insoluble solids.
3. Carbon (Activated Charcoal): Used to remove unpleasant tastes and odours from the water.
4. Chlorination: Chlorine gas is bubbled through the water to kill harmful microbes and bacteria.

Extended Content (Extended Only)

There is no supplement-only content for this specific sub-topic (10.1).

Key Equations

Reaction	Equation	Notes
Cobalt Chloride Test	$\text{CoCl}_2 (\text{s}) + 6\text{H}_2\text{O} (\text{l}) \rightarrow \text{CoCl}_2\cdot6\text{H}_2\text{O} (\text{s})$	Blue to Pink
Copper Sulfate Test	$\text{CuSO}_4 (\text{s}) + 5\text{H}_2\text{O} (\text{l}) \rightarrow \text{CuSO}_4\cdot5\text{H}_2\text{O} (\text{s})$	White to Blue

• Symbols: (s) = solid, (l) = liquid.
• Units: Temperature is measured in degrees Celsius (°C).

Common Mistakes to Avoid

• ❌ Wrong: Saying that cobalt(II) chloride turns blue to pink to prove water is pure.
• ✅ Right: Cobalt(II) chloride only proves the presence of water. Only boiling point or melting point proves purity.
• ❌ Wrong: Forgetting the word "anhydrous" when describing the chemical tests.
• ✅ Right: Always specify anhydrous copper(II) sulfate or anhydrous cobalt(II) chloride.
• ❌ Wrong: Thinking chlorination removes solids from water.
• ✅ Right: Filtration removes solids; chlorination only kills microbes.

Exam Tips

• Command Words: If the question asks you to "State" a test for water, keep it brief: "Anhydrous copper(II) sulfate turns from white to blue."
• Application Questions: You may be given an "unknown" liquid. If it boils at 102 °C, you must conclude it is water (because it is near 100 °C) but it is impure (because it is not exactly 100 °C).
• Contexts: Expect questions about why farmers shouldn't use too much fertiliser near rivers (mention nitrates leading to deoxygenation).
• Typical Values: Be prepared to interpret data where nitrate levels are given in mg/dm³ (e.g., 0.14 or 3.0). Higher values usually indicate higher pollution levels.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State the chemical formula of anhydrous copper(II) sulfate and describe the colour change observed when water is added to it. [3]

(b) Explain why distilled water is preferred over tap water in chemistry experiments. [2]

Worked Solution:

(a)

1. The chemical formula of anhydrous copper(II) sulfate is $\text{CuSO}_4$. Anhydrous copper(II) sulfate is copper(II) sulfate without any water molecules attached.

2. Anhydrous copper(II) sulfate is white. This is the initial colour before adding water.

3. When water is added, the colour changes from white to blue. The anhydrous copper(II) sulfate becomes hydrated copper(II) sulfate.

How to earn full marks:

• State the correct formula: $\boxed{\text{CuSO}_4}$ [1 mark]
• State the initial colour is white [1 mark]
• State the final colour is blue [1 mark]

(b)

1. Tap water contains dissolved impurities. Tap water has minerals and chemicals.

2. These impurities can interfere with the reaction or introduce unwanted substances, affecting the accuracy of results. These substances will affect the accuracy of the results.

How to earn full marks:

• State that tap water contains impurities [1 mark]
• State that impurities interfere with the reaction [1 mark]

Common Pitfall: Remember to state the initial and final colors of the anhydrous copper(II) sulfate. Many students forget to mention the initial white color, losing a mark. Also, be precise: "impurities" is better than "stuff" when explaining why distilled water is preferred.

Exam-Style Question 2 — Extended Response [9 marks]

Question:

A farmer notices that a nearby stream is experiencing excessive algae growth. He suspects fertiliser runoff containing nitrate ions is the cause. The following tests are performed:

(a) Describe how you would test a sample of the stream water for the presence of nitrate ions. Include any reagents needed and the expected observation for a positive result. [4]

(b) State two potential negative impacts of nitrate ions in water sources on the environment. [2]

(c) Suggest three methods for reducing nitrate pollution from agricultural sources. [3]

Worked Solution:

(a)

1. Add aqueous sodium hydroxide to the water sample, then add aluminium foil. These reagents react to reduce the nitrate ions.

2. Warm the mixture gently. Heating increases the rate of reaction.

3. Test any gas produced with damp red litmus paper. Ammonia is produced if nitrate ions are present.

4. If nitrate ions are present, the damp red litmus paper will turn blue. Ammonia is alkaline and turns red litmus paper blue.

How to earn full marks:

• State that sodium hydroxide and aluminium foil are needed [1 mark]
• State the mixture needs to be warmed [1 mark]
• State that damp red litmus paper is used to test the gas [1 mark]
• State that red litmus paper turning blue indicates nitrate ions are present [1 mark]

(b)

1. Nitrate ions can cause eutrophication. Eutrophication is the excessive enrichment of a body of water by mineral nutrients.

2. This leads to algal blooms, which block sunlight and reduce oxygen levels, harming aquatic life. The algae die and decompose, using up oxygen.

How to earn full marks:

• State that nitrate ions cause eutrophication [1 mark]
• State that this leads to algal blooms and deoxygenation [1 mark]

(c)

1. Use fertilisers more efficiently, such as applying fertilisers at the correct time and in the correct amounts. This ensures that the plants use the fertilizer and less is washed away.

2. Implement buffer zones, such as planting trees or shrubs along waterways. These buffer zones absorb excess nutrients before they reach the water.

3. Promote sustainable farming practices, such as crop rotation and reduced tillage. These practices improve soil health and reduce runoff.

How to earn full marks:

• Suggest using fertilisers more efficiently [1 mark]
• Suggest implementing buffer zones [1 mark]
• Suggest promoting sustainable farming practices [1 mark]

Common Pitfall: When describing the nitrate test, be sure to mention damp red litmus paper. Dry litmus paper won't work. Also, remember that eutrophication is a process that leads to algal blooms and oxygen depletion, so mentioning both is important for full marks.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

A student wants to test the purity of a sample of rainwater collected in a clean container.

(a) Describe how the student could test the purity of the water sample using its boiling point. [3]

(b) State three substances that might be found in water from natural sources, excluding pollutants from human activity. [3]

Worked Solution:

(a)

1. Heat the water sample and measure its temperature as it boils. Boiling point is sensitive to dissolved impurities.

2. Pure water boils at exactly $100,^\circ\text{C}$ at standard atmospheric pressure. This is the reference point for pure water.

3. If the water is impure, the boiling point will be higher than $100,^\circ\text{C}$ or the boiling point will not be constant during boiling. Impurities elevate the boiling point and cause it to change during boiling.

How to earn full marks:

• State that the water should be heated and the temperature measured [1 mark]
• State that pure water boils at $\boxed{100,^\circ\text{C}}$ [1 mark]
• State that an impure sample will boil at a temperature above $100,^\circ\text{C}$ or the temperature will not be constant [1 mark]

(b)

1. Dissolved oxygen. Essential for aquatic life.

2. Metal compounds. Can be beneficial or harmful.

3. Dissolved carbon dioxide. Forms carbonic acid, affecting pH.

How to earn full marks:

• State dissolved oxygen [1 mark]
• State metal compounds [1 mark]
• State dissolved carbon dioxide [1 mark]

Common Pitfall: Students often forget that the boiling point of an impure substance isn't just higher, it also changes during boiling. Make sure to mention both for full marks. Also, pay close attention to the question's constraints – this one asks for naturally occurring substances, not pollutants.

Exam-Style Question 4 — Extended Response [8 marks]

Question:

A municipal water treatment plant processes river water to make it safe for drinking.

(a) Name and describe the two processes used to remove solid particles from the water. [4]

(b) Explain why carbon is used in the treatment process and how it works. [2]

(c) State the purpose of chlorination in water treatment and describe one potential disadvantage of using chlorine. [2]

Worked Solution:

(a)

1. The first process is sedimentation. This involves allowing the water to stand in large tanks. Sedimentation uses gravity to settle out larger particles.

2. During sedimentation, heavier solid particles settle to the bottom of the tank due to gravity. This removes most of the larger suspended solids.

3. The second process is filtration. This involves passing the water through beds of sand and gravel. Filtration removes smaller, suspended particles that did not settle during sedimentation.

4. The sand and gravel trap the remaining solid particles, resulting in clearer water. This ensures that the water is free of visible solids.

How to earn full marks:

• Name sedimentation and describe the settling of larger particles [2 marks]
• Name filtration and describe the use of sand/gravel to trap smaller particles [2 marks]

(b)

1. Carbon is used to remove tastes and odours from the water. Carbon adsorbs organic compounds.

2. The carbon has a large surface area, which allows it to adsorb organic molecules that cause unpleasant tastes and odours. Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface.

How to earn full marks:

• State that carbon removes tastes and odours [1 mark]
• Explain that carbon adsorbs organic molecules due to its large surface area [1 mark]

(c)

1. The purpose of chlorination is to kill harmful microbes in the water, disinfecting it. Chlorine is a disinfectant.

2. A potential disadvantage is that chlorine can react with organic matter to form harmful byproducts, such as trihalomethanes (THMs), which are carcinogenic. These byproducts are carcinogenic.

How to earn full marks:

• State that chlorination kills microbes [1 mark]
• State that chlorine can form harmful byproducts [1 mark]

Common Pitfall: Don't just name the processes; you need to describe how they work to get full marks. For carbon filtration, remember that it's adsorption (surface adhesion), not absorption (entering the bulk). Also, be specific about the harmful effects of chlorine byproducts.