krit.club logo

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

Grade 11A LevelChemistry

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

🔑Concepts

•

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.

•

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.

•

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.

•

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).

•

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.