Water

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Testing for the Presence of Water

Water can be detected using specific chemical tests that rely on colour changes. These tests use anhydrous (water-free) compounds.

Usage Note
Most common test; very distinct colour change.
Often used in humidity indicator paper.

Why this works: The anhydrous salts have a specific crystal structure that changes when water molecules bind to them (hydration), altering the way they absorb light and thus their colour. This is a reversible reaction; heating the coloured compound drives off the water, returning it to its original colour.

Anhydrous

Anhydrous means 'without water'. In chemistry, it refers to a substance that has had all its water of crystallisation removed or has never contained water. For example, anhydrous copper(II) sulfate is white powder, whereas the hydrated form (copper(II) sulfate pentahydrate) is blue crystals.

Worked Example: Identifying Water

Question: A student adds a few drops of an unknown liquid to white powder. The powder turns blue. What conclusion can be drawn?

Answer: The presence of water is confirmed because anhydrous copper(II) sulfate changes from white to blue in the presence of water.

⚠︎ Confusing Water Tests with Gas Tests

The Error: Students often confuse the test for water (anhydrous salts) with tests for gases like carbon dioxide or oxygen.

The Correction: Remember that anhydrous copper(II) sulfate and cobalt(II) chloride are solids used to detect liquid water or water vapour. They do not react with carbon dioxide or oxygen in the same way. Do not use limewater (for CO₂) or a glowing splint (for O₂) to test for the presence of water.

Describing Colour Changes Accurately

When to use: When asked to describe the test for water or the result of adding water to anhydrous salts.

Why examiners accept this: Examiners require precise description of both the initial and final states. Saying 'it changes colour' is insufficient because it lacks specificity. You must state both colours involved in the transition.

Correct Usage: Always write: 'Changes from white to blue' (for copper sulfate) or 'changes from blue to pink' (for cobalt chloride). Do not just say 'turns blue' without mentioning the starting white colour, as this implies it was already coloured.

Past Paper Style Questions

What is the colour change when water is added to anhydrous copper(II) sulfate?

White to blue.

Name a chemical test for the presence of water and state the positive result.

Anhydrous cobalt(II) chloride; changes from blue to pink. (OR Anhydrous copper(II) sulfate; changes from white to blue).

Testing the Purity of Water

Pure water is a single substance (a pure compound). Impure water contains dissolved substances (solutions). We can test for purity by measuring physical properties that are fixed for pure substances.

Impure Water Behaviour
Below 0 °C; melts/freezes over a range of temperatures.
Above 100 °C; boils over a range of temperatures.

Why this works: Dissolved impurities interfere with the formation of the crystal lattice (lowering melting point) and reduce the vapour pressure (raising boiling point). Pure water has a sharp melting/boiling point, meaning the temperature stays constant during the phase change. Impure water shows a range of temperatures.

Sharp Melting/Boiling Point

A sharp melting point means the substance melts completely at one specific temperature (e.g., 0 °C) with no gradual softening. A sharp boiling point means the liquid boils vigorously at one specific temperature (e.g., 100 °C). Impure substances melt or boil over a range of temperatures.

Worked Example: Purity Test

Question: Sample A boils at 100 °C. Sample B boils at 102 °C. Which sample is pure water?

Answer: Sample A is pure water because it has a sharp boiling point of 100 °C. Sample B is impure because its boiling point is elevated above 100 °C.

⚠︎ Confusing Melting and Boiling Point Trends

The Error: Students often think impurities lower the boiling point or raise the melting point.

The Correction: Remember the mnemonic 'Salt raises the boil, lowers the freeze.' Impurities raise the boiling point (above 100 °C) and lower the melting/freezing point (below 0 °C). Pure water boils at exactly 100 °C and freezes at exactly 0 °C.

Describing Purity Tests Correctly

When to use: When asked to describe how to test for the purity of water.

Why examiners accept this: Examiners look for the measurement of the temperature and the comparison to standard values. Simply saying 'it is pure' is not a description of the test. You must mention measuring the boiling point or melting point and stating that it matches the standard value (100 °C or 0 °C).

Correct Usage: 'Heat the water and measure its boiling point. If it boils at exactly 100 °C, it is pure.' OR 'Measure the melting point. If it melts at exactly 0 °C, it is pure.'

Past Paper Style Questions

Describe how you would test a sample of water to determine if it is pure.

Measure the boiling point (or melting point). Pure water boils at 100 °C (or melts at 0 °C).

Why is distilled water used in chemical experiments rather than tap water?

Distilled water contains fewer chemical impurities (ions/salts), preventing unwanted side reactions or contamination of results.

Distilled Water vs. Tap Water

Tap water contains dissolved minerals, salts, chlorine, and microbes. Distilled water is produced by boiling tap water and condensing the steam, leaving impurities behind.

Why use distilled water? In practical chemistry, we use distilled water to ensure that the only substances reacting are the ones we intentionally added. Tap water contains ions (like calcium, magnesium, chloride) that could react with other chemicals, leading to unwanted side reactions or inaccurate results.

Distilled Water

Distilled water is water that has been purified by distillation. It contains very few dissolved impurities, making it suitable for use as a solvent in chemical reactions where the presence of other ions would interfere.

Worked Example: Solvent Choice

Question: Why is distilled water preferred over tap water for preparing a solution of silver nitrate?

Answer: Tap water contains chloride ions (from dissolved salts). Silver nitrate reacts with chloride ions to form a white precipitate of silver chloride. Using distilled water prevents this unwanted precipitation, ensuring the solution remains clear and the concentration of silver nitrate is accurate.

⚠︎ Misunderstanding 'Pure' in Practical Contexts

The Error: Students think distilled water is 'perfectly pure H₂O' in all contexts or that tap water is 'dirty' and unusable.

The Correction: While distilled water is chemically purer, the key reason for its use in exams is to avoid interference from dissolved ions. Tap water is not 'dirty' in a visual sense (it looks clear), but it contains dissolved substances that affect chemical reactions. Focus on the chemical impurities (ions/salts) rather than visible dirt.

Justifying the Use of Distilled Water

When to use: When asked to explain why distilled water is used in practical chemistry.

Why examiners accept this: Examiners want to see that you understand the impact of impurities on chemical reactions. Phrases like 'to prevent contamination' are acceptable but vague. Better phrases include 'to prevent unwanted side reactions' or 'to ensure accurate results by removing interfering ions.'

Correct Usage: 'Distilled water is used to avoid interference from dissolved ions present in tap water, which could cause unwanted precipitates or side reactions.'

Past Paper Style Questions

State one reason why distilled water is used in chemical experiments instead of tap water.

To prevent unwanted side reactions / to avoid contamination from dissolved ions.

Substances in Natural Water Sources

Water from natural sources (rivers, lakes, groundwater) is not pure. It contains various dissolved and suspended substances. These can be beneficial or harmful.

Potentially Harmful Effects
Essential for aquatic life (respiration).
Some provide essential minerals for life.
Some are toxic (e.g., lead, mercury).
Harm aquatic life (ingestion/blockage).
Contains harmful microbes that cause disease.
Lead to deoxygenation of water (eutrophication) and damage aquatic life.

Deoxygenation

Deoxygenation is the process by which the level of dissolved oxygen in water decreases. This often happens when nitrates and phosphates from fertilisers cause excessive plant growth (algal blooms). When these plants die, bacteria decompose them, using up large amounts of oxygen in the process, leaving insufficient oxygen for fish and other aquatic life.

Worked Example: Environmental Impact

Question: Explain how phosphates from detergents can harm aquatic life.

Answer: Phosphates act as a nutrient that causes excessive growth of algae and plants (algal blooms). When these organisms die, they are decomposed by bacteria. The decomposition process uses up dissolved oxygen in the water, leading to deoxygenation. This lack of oxygen kills fish and other aquatic animals.

⚠︎ Vague Descriptions of Harmful Effects

The Error: Students state that nitrates or sewage are 'toxic' or 'poisonous' without explaining the mechanism.

The Correction: Be specific. For nitrates/phosphates, mention deoxygenation. For sewage, mention harmful microbes/disease. For plastics, mention harm to aquatic life. Do not just say 'it kills fish' without explaining why (e.g., lack of oxygen).

Linking Substances to Specific Effects

When to use: When asked to state why certain substances in water are harmful.

Why examiners accept this: Examiners look for specific cause-and-effect links. 'Nitrates' must be linked to 'deoxygenation'. 'Sewage' must be linked to 'microbes/disease'. 'Plastics' must be linked to 'harm to aquatic life'.

Correct Usage: 'Nitrates and phosphates lead to deoxygenation of water.' 'Sewage contains harmful microbes which cause disease.' Do not use vague terms like 'bad for fish'.

Past Paper Style Questions

State two substances found in polluted water that are harmful to aquatic life.

Nitrates and phosphates. (OR Plastics, Sewage)

Explain how nitrates from fertilisers can damage aquatic life.

Nitrates cause excessive plant/algae growth. Decomposition of dead plants uses up dissolved oxygen, leading to deoxygenation.

Treatment of Domestic Water Supply

Water from natural sources is treated to make it safe for drinking and domestic use. The process involves several stages.

Purpose
To remove solid particles (sediment) and large debris.
To remove tastes and odours.
To kill harmful microbes (bacteria/viruses).

Chlorination

Chlorination is the process of adding chlorine to water. Chlorine is a disinfectant that kills harmful microbes (bacteria and viruses) that cause diseases like cholera and typhoid. It does not remove solids or change the taste.

Worked Example: Water Treatment Stages

Question: Describe two stages in the treatment of domestic water supply and their purposes.

Answer:

Filtration: Removes solid particles/sediment.
Chlorination: Kills harmful microbes.

⚠︎ Confusing Water Treatment with Wastewater Treatment

The Error: Students think chlorination removes solids or that filtration kills microbes.

The Correction: Filtration/Sedimentation removes solids. Chlorination kills microbes. Carbon removes tastes/odours. Do not mix up the purposes of these stages. Filtration does NOT kill bacteria; it only removes physical particles.

Describing Water Treatment Stages

When to use: When asked to describe the treatment of domestic water supply.

Why examiners accept this: Examiners require specific links between the stage and its purpose. 'Cleaning' is too vague. You must specify what is removed or killed.

Correct Usage: 'Sedimentation/filtration removes solids.' 'Chlorination kills microbes.' 'Carbon removes tastes and odours.' Be precise about the target (solids vs microbes vs taste).

Past Paper Style Questions

State the purpose of adding chlorine to domestic water.

To kill harmful microbes.

Describe two stages in the treatment of domestic water supply.

Sedimentation/filtration to remove solids. 2. Chlorination to kill microbes.