CIE iGCSE Co-ordinated Sciences-C10.1 Water- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C10.1 Water – Study Notes
CIE iGCSE Co-ordinated Sciences-C10.1 Water – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe chemical tests for the presence of water using anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate
- Describe how to test for the purity of water using melting point and boiling point
- State that distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities
- Describe the treatment of the domestic water supply in terms of:
(a) sedimentation and filtration to remove solids
(b) use of carbon to remove tastes and odours
(c) chlorination to kill microbes (pathogens)
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Chemical Tests for the Presence of Water
Water can be detected using chemical tests that involve colour changes in certain anhydrous salts. These tests are qualitative and indicate the presence of water but not its purity or quantity.
Test with Anhydrous Cobalt(II) Chloride
- Anhydrous cobalt(II) chloride is blue in colour.
- When water is present, it changes from blue to pink because hydrated cobalt(II) chloride is formed.
- This reaction is often used in cobalt chloride paper (test paper dipped in anhydrous cobalt chloride solution and dried).
- The colour change is reversible: heating hydrated cobalt chloride removes the water and restores the blue anhydrous form.
- This test detects the presence of water, but it does not confirm that the water is pure.
Example
What colour change is observed when a few drops of water are added to anhydrous cobalt(II) chloride?
▶️Answer/Explanation
The anhydrous cobalt(II) chloride changes from blue to pink as it forms hydrated cobalt(II) chloride.
Test with Anhydrous Copper(II) Sulfate
- Anhydrous copper(II) sulfate is white in colour.
- When water is added, it changes from white to blue because hydrated copper(II) sulfate is formed.
- This is a simple and widely used laboratory test for water.
- Like cobalt chloride, the reaction is reversible on heating, which removes the water and restores the white anhydrous salt.
- This test also only confirms the presence of water and not its purity.
Example
A student adds water to a sample of anhydrous copper(II) sulfate. What colour change is observed?
▶️Answer/Explanation
The anhydrous copper(II) sulfate changes from white to blue as it forms hydrated copper(II) sulfate.
Testing the Purity of Water
Chemical tests with cobalt(II) chloride and copper(II) sulfate show the presence of water but do not indicate whether the water is pure. To test for purity, we rely on the physical properties of pure water, namely its fixed melting and boiling points.
Melting Point
- Pure water has a sharp melting point of \( 0^{\circ}\text{C} \).
- If impurities are present, the melting point decreases and occurs over a temperature range rather than sharply at \( 0^{\circ}\text{C} \).
Boiling Point
- Pure water boils at \( 100^{\circ}\text{C} \) at standard atmospheric pressure (1 atm).
- If impurities are present, the boiling point is raised above \( 100^{\circ}\text{C} \) and the boiling is not sharp.
Example
A water sample melts between \(-2^{\circ}\text{C}\) and \(0^{\circ}\text{C}\). What does this show about the water?
▶️Answer/Explanation
The sample is impure because pure water melts sharply at \( 0^{\circ}\text{C} \). The lowered and variable melting point shows that dissolved impurities are present.
Example
A water sample boils steadily at \( 102^{\circ}\text{C} \). What does this suggest about its purity?
▶️Answer/Explanation
The sample is not pure because pure water boils at exactly \( 100^{\circ}\text{C} \) at 1 atm pressure. The higher boiling point indicates impurities, likely dissolved salts.
Example
Seawater is tested for purity. It is found to boil at \( 103^{\circ}\text{C} \) and freeze at \(-2^{\circ}\text{C}\). Explain what this indicates and why seawater behaves this way.
▶️Answer/Explanation
Seawater is impure because both its boiling point and melting point are different from those of pure water. The presence of dissolved salts causes boiling point elevation (to \( 103^{\circ}\text{C} \)) and freezing point depression (to \(-2^{\circ}\text{C}\)). This explains why seawater does not freeze easily and why it requires more energy to boil compared to pure water.
Use of Distilled Water in Practical Chemistry
In chemistry laboratories, distilled water is always preferred over tap water for experiments. This is because distilled water has undergone the process of distillation, where water is boiled, and the steam is collected and condensed back into liquid. This process removes almost all dissolved salts, minerals, and impurities, leaving behind pure water.
Why distilled water is used instead of tap water:
- 1. Absence of dissolved ions: Tap water contains dissolved ions such as \( \text{Ca}^{2+} \), \( \text{Mg}^{2+} \), \( \text{Na}^+ \), and \( \text{Cl}^- \). These can interfere with experiments, particularly in reactions involving precipitation or titration.
- 2. No unwanted reactions: Dissolved salts in tap water can react with reagents to form unwanted products. For example, calcium and magnesium ions in tap water can react with soap to form scum, making it unsuitable for accurate chemical testing.
- 3. Accuracy in quantitative experiments: In titrations or preparation of standard solutions, even small amounts of impurities from tap water can change the effective concentration of solutions, leading to errors.
- 4. No contamination in tests: Some chemical tests, such as those involving flame tests, solubility, or ion tests, can give misleading results if tap water is used. Distilled water avoids contamination and ensures reliable outcomes.
- 5. Controlled laboratory conditions: Distilled water ensures uniformity in results across different experiments and laboratories, since it does not vary in composition unlike tap water, which can differ depending on the source.
Comparison between tap water and distilled water:
| Property | Tap Water | Distilled Water |
|---|---|---|
| Dissolved salts and ions | Contains ions such as \( \text{Ca}^{2+} \), \( \text{Mg}^{2+} \), \( \text{Cl}^- \) | Free from dissolved salts and ions |
| Purity | Impure, varies with location | Very pure and consistent |
| Suitability for experiments | Not suitable – may interfere with reactions | Ideal for experiments – ensures accurate results |
Example
Why should distilled water be used when preparing copper(II) sulfate solution for a crystallisation experiment?
▶️Answer/Explanation
Tap water may contain chloride ions and other impurities that could react with copper(II) ions, producing unwanted precipitates. Distilled water avoids these side reactions, ensuring that pure blue copper(II) sulfate crystals are obtained.
Example
A student uses tap water instead of distilled water in a titration between hydrochloric acid and sodium hydroxide. How might this affect the result?
▶️Answer/Explanation
Tap water may already contain dissolved ions such as \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \), which can neutralise part of the acid before the titration. This would make the volume of sodium hydroxide required inaccurate. Distilled water avoids this issue, giving a more reliable endpoint.
Treatment of Domestic Water Supply
Domestic water must be treated before use to ensure it is safe to drink. The treatment process involves several stages:
(a) Sedimentation and Filtration (removing solids)
- Large solid particles (like sand, mud, grit) are removed first.
- Sedimentation: Water is stored in large tanks where heavy particles settle at the bottom.
- Filtration: The water is passed through layers of sand and gravel to remove smaller solid particles.
(b) Use of Carbon (removing tastes and odours)
- Water is passed through filters containing activated carbon.
- Activated carbon absorbs impurities that cause bad tastes and unpleasant odours.
- This step improves the quality and acceptability of drinking water.
(c) Chlorination (killing microbes)
- A small, controlled amount of chlorine is added to the filtered water.
- Chlorine kills harmful microbes (pathogens) such as bacteria and viruses.
- This prevents the spread of waterborne diseases like cholera and typhoid.
Example :
Why is chlorine added to drinking water even after solids and odours are removed?
▶️ Answer/Explanation
Step 1: Sedimentation and filtration only remove solids, not microbes.
Step 2: Activated carbon removes odours, but does not kill pathogens.
Step 3: Chlorination ensures all harmful microbes are destroyed.
Final Answer: Chlorine is added to kill pathogens and make the water safe to drink.