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Chemistry - Water

Grade 9IGCSE

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

🔑Concepts

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Chemical Test for Presence: Anhydrous copper(II) sulfate (CuSO4CuSO_{4}) turns from white to blue in the presence of water. Alternatively, anhydrous cobalt(II) chloride (CoCl2CoCl_{2}) turns from blue to pink.

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Testing for Purity: Pure water has a definite boiling point of 100∘C100^\circ C and a melting point of 0∘C0^\circ C at standard atmospheric pressure. The presence of impurities increases the boiling point and decreases the melting point.

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Water Treatment: The process of making water safe for human consumption includes sedimentation (to remove large particles), filtration through layers of sand and gravel (to remove smaller insoluble particles), and chlorination (adding Cl2Cl_{2} to kill bacteria and microorganisms).

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Uses of Water: Water is used domestically for drinking, cooking, and washing. In industry, it is primarily used as a solvent, a coolant for machinery, and for generating steam in power plants.

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Distilled Water: This is water that has been boiled into vapor and condensed back into liquid in a separate container. It is used in laboratories because it contains fewer dissolved ions like Ca2+Ca^{2+} or Mg2+Mg^{2+} which could interfere with chemical reactions.

📐Formulae

CuSO4(s)+5H2O(l)→CuSO4⋅5H2O(s)CuSO_{4}(s) + 5H_{2}O(l) \rightarrow CuSO_{4} \cdot 5H_{2}O(s)

CoCl2(s)+6H2O(l)→CoCl2⋅6H2O(s)CoCl_{2}(s) + 6H_{2}O(l) \rightarrow CoCl_{2} \cdot 6H_{2}O(s)

💡Examples

Problem 1:

A student is given a colorless liquid and asked to prove it is chemically water. Describe the test and the observation.

Solution:

Add the liquid to anhydrous copper(II) sulfate (CuSO4CuSO_{4}). If the liquid is water, the white powder will turn blue.

Explanation:

This is a chemical test for the presence of water. The blue color is due to the formation of hydrated copper(II) sulfate: CuSO4⋅5H2OCuSO_{4} \cdot 5H_{2}O.

Problem 2:

A sample of water is found to boil at 102∘C102^\circ C. What does this indicate about the sample?

Solution:

The water is not pure and contains dissolved impurities.

Explanation:

Pure water must boil exactly at 100∘C100^\circ C. Dissolved solutes cause 'boiling point elevation', meaning the boiling point (TbT_{b}) becomes >100∘C> 100^\circ C.

Problem 3:

Explain the role of chlorine in the water treatment process.

Solution:

Chlorine (Cl2Cl_{2}) is added to the water supply to kill harmful bacteria and microbes.

Explanation:

While filtration removes physical debris, it cannot remove microscopic pathogens. Chlorination ensures the water is biologically safe for consumption.