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Chemistry of the Environment - Water treatment and chemical tests for water

Grade 11IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Chemical test for the presence of water: Anhydrous cobalt(II) chloride turns from blue to pink when water is added: CoCl2+6H2O→CoCl2⋅6H2OCoCl_2 + 6H_2O \rightarrow CoCl_2 \cdot 6H_2O.

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Chemical test for the presence of water: Anhydrous copper(II) sulfate turns from white to blue when water is added: CuSO4+5H2O→CuSO4⋅5H2OCuSO_4 + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O.

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Testing for purity: Pure water has a fixed melting point of 0∘C0^\circ C and a fixed boiling point of 100∘C100^\circ C at 11 atmosphere of pressure. Impurities will increase the boiling point and decrease the melting point.

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Water treatment process: 1. Filtration (removal of large insoluble particles). 2. Sedimentation (allowing smaller particles to settle). 3. Carbon filtration (removal of tastes and odors). 4. Chlorination (adding Cl2Cl_2 to kill bacteria and microbes).

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Dissolved substances in natural water: Beneficial substances include dissolved oxygen (O2O_2) for aquatic life and metal compounds (minerals like Ca2+Ca^{2+}). Harmful substances include metal compounds (e.g., lead), plastics, sewage, and nitrate/phosphate fertilizers (NO3−NO_3^- and PO43−PO_4^{3-}) which cause eutrophication.

📐Formulae

CoCl2(s)+6H2O(l)→CoCl2⋅6H2O(s)CoCl_2(s) + 6H_2O(l) \rightarrow CoCl_2 \cdot 6H_2O(s) (Blue to Pink)

CuSO4(s)+5H2O(l)→CuSO4⋅5H2O(s)CuSO_4(s) + 5H_2O(l) \rightarrow CuSO_4 \cdot 5H_2O(s) (White to Blue)

Cl2(g)+H2O(l)→HCl(aq)+HClO(aq)Cl_2(g) + H_2O(l) \rightarrow HCl(aq) + HClO(aq)

💡Examples

Problem 1:

A student is given a colorless liquid. Describe a chemical test and a physical test to prove the liquid is pure water.

Solution:

Chemical test: Add the liquid to anhydrous copper(II) sulfate. If it turns from white to blue, water is present (CuSO4+5H2O→CuSO4⋅5H2OCuSO_4 + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O). Physical test: Measure the boiling point using a thermometer. If the liquid boils at exactly 100∘C100^\circ C at standard pressure, it is pure water.

Explanation:

Chemical tests only indicate the presence of H2OH_2O molecules, whereas physical tests like boiling point or melting point determine the purity of the sample.

Problem 2:

Explain the role of chlorine (Cl2Cl_2) in the water treatment process.

Solution:

Chlorine is added during the chlorination stage to act as a disinfectant.

Explanation:

Chlorine reacts with water to form hypochlorous acid (HClOHClO), which is highly effective at killing bacteria, viruses, and other pathogenic microorganisms that can cause waterborne diseases like cholera.

Water treatment and chemical tests for water Revision - Grade 11 Chemistry IGCSE