Ch 1 · Water · O Level Chemistry 5070
Topic 1 of 3 in Chemistry of the Environment — O Level Chemistry 5070 — 48 lessons.
Introduction
Water is the most used chemical in any laboratory and the most important resource in any city. Examiners test whether you can prove that a liquid contains water, whether you can prove it is pure water, why the laboratory bottle says "distilled" and how a water company turns river water into something safe to drink. Every part of this lesson is short recall, so precision in wording is what earns the marks.
Key ideas — Chemical tests for water
Two tests show that a liquid contains water. Both use an anhydrous salt that changes colour when it takes up water of crystallisation.
🧑🏫 Teacher's explanation
Imagine you have a sponge that is bone-dry and white. As soon as it touches water, it turns bright blue. This is exactly how anhydrous salts behave. They are 'thirsty' chemicals that change their crystal structure and color when they absorb water molecules.
📖 Definitions
🔍 Worked example — Easy
A student adds a few drops of an unknown liquid to anhydrous copper(II) sulfate. The powder turns from white to blue. What conclusion can the student draw?
🔍 Worked example — Standard
A sample of liquid is tested with anhydrous cobalt(II) chloride paper. The paper turns from blue to pink. State the equation for the hydration of cobalt(II) chloride.
🔍 Worked example — Exam-style
A chemist needs to ensure a sample is pure water. They find the melting point is -0.5^°C and the boiling point is 101.2^°C. Is this water pure?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe a chemical test that would confirm the presence of water in the liquid. [2] (b) The student performs the test and the anhydrous copper(II) sulfate turns from white to blue. Does this prove the liquid is pure water? Explain your answer. [2] [4 marks]
Key ideas — Testing the purity of water
Purity is tested using melting point and boiling point. Pure water melts at exactly 0 °C and boils at exactly 100 °C at standard atmospheric pressure. Dissolved impurities lower the melting point and raise the boiling point, and cause the change of state to happen over a range of temperatures rather than sharply at one value. So a sample that boils at 101 to 103 °C is not pure.
🧑🏫 Teacher's explanation
Imagine a crowd of people trying to exit a building through a single door. In pure water, all the molecules are identical and move in a synchronized way, so they all 'exit' (change state) at once when the temperature hits a specific point. Now, imagine someone throws large, bulky obstacles like chairs and tables into that doorway. These obstacles (impurities) make it harder for the molecules to move around and break free from their liquid structure, requiring more energy to break the bonds (raising the boiling point) and making it harder for them to settle into a solid structure (lowering the melting point). Because these obstacles are scattered randomly, the 'exit' happens unevenly, meaning the substance changes state over a range of temperatures rather than at one sharp point.
📖 Definitions
🔍 Worked example — Easy
A student tests a sample of water using anhydrous copper(II) sulfate. The white powder turns blue. What does this indicate about the sample?
🔍 Worked example — Standard
A sample of water is heated. It begins to boil at 102 °C and finishes boiling at 105 °C. Is this water pure?
🔍 Worked example — Exam-style
A scientist is purifying water for a titration. They choose distilled water over tap water. Explain why, mentioning the effect of ions on concentration.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Substances in natural water
Water from rivers, lakes and underground sources is never pure. It may contain:
🧑🏫 Teacher's explanation
Think of natural water like a soup. Even if it looks clear, it is filled with 'ingredients'—some are nutrients that help life grow (like salt or oxygen), while others are contaminants that spoil the batch (like plastic or sewage). In chemistry, 'pure' means only water molecules (H_2O) are present, but nature rarely provides this.
📖 Definitions
🔍 Worked example — Easy
A sample of water is tested with anhydrous cobalt(II) chloride. The color changes from blue to pink. State what this tells you about the sample.
🔍 Worked example — Standard
A student is purifying water for an experiment. They use a process of sedimentation, filtration, and carbon treatment. Explain why chlorination is the final necessary step.
🔍 Worked example — Exam-style
A lake shows signs of eutrophication. Describe the sequence of events starting from the runoff of nitrogen-based fertilizers into the water.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Treatment of the domestic water supply
Water companies treat water in four stages before it reaches the tap:
🧑🏫 Teacher's explanation
Think of water treatment like cleaning a messy bedroom. First, you pick up the big clutter from the floor (sedimentation). Then, you sweep up the smaller dust and debris (filtration). Next, you use an air freshener to get rid of the smell (activated carbon). Finally, you use a disinfectant to kill any germs on the surfaces (chlorination).
📖 Definitions
🔍 Worked example — Easy
A student tests a sample of tap water using anhydrous copper(II) sulfate. The blue powder turns white. What does this indicate about the sample?
🔍 Worked example — Standard
A water treatment plant uses sedimentation to remove grit. If the mass of the water decreases from 1000 g to 985 g after the grit settles, calculate the mass of the impurities removed.
🔍 Worked example — Exam-style
A sample of water is tested for purity. The boiling point is measured to be 99.5^°C. Is this water pure? Explain your answer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe a chemical test to show that water is present in the sample. [2] (b) The student wants to ensure the water is pure for a laboratory experiment. Explain why tap water is not suitable for this purpose. [1] [3 marks]
Worked example
Question. A student is given a colourless liquid. (a) Describe a chemical test, with the result, to show that the liquid contains water. [2] (b) Describe how the student could show that the liquid is pure water. [2] (c) The student's teacher insists that distilled water, not tap water, is used to make up solutions for titrations. Explain why. [1]
🔍 Worked example — Easy
A student adds a few drops of an unknown liquid to a test tube containing anhydrous copper(II) sulfate. The powder turns from white to blue. Identify the liquid and state the chemical test used.
🔍 Worked example — Standard
A sample of water is heated. It begins to boil at 102 ^circC and continues to boil over a temperature range from 102 ^circC to 105 ^circC. Is this water pure?
🔍 Worked example — Exam-style
Water from a river is collected for testing. List three potential impurities found in natural river water and state whether they are generally considered harmful or beneficial to human consumption.
⚠️ Common errors
Common mistakes
Practice
Answers Blue to pink [B1]. The water is not pure [B1]; dissolved impurities raise the boiling point above 100 °C [B1]. Harmful, any two with sources: nitrates from fertilisers, phosphates from detergents, sewage or harmful microbes from untreated waste, plastics from litter, toxic metal compounds from industry [B1 each]; beneficial: dissolved oxygen (for aquatic life) and some metal compounds such as calcium (for bones and teeth) [B1 each]. Sedimentation: large particles settle out [B1]; filtration through sand: removes remaining solids [B1]; carbon: removes tastes and odours [B1]; chlorination: kills microbes [B1].