Water & Water Pollution
Understanding how to test and treat water, recognise pollutants, and identify key substances that affect water safety and aquatic life.
The Big Picture
Testing Water
Learning how to detect water using colour changes and measure purity through boiling/melting points.
Natural Water
Identifying beneficial and harmful substances found in lakes, rivers, and groundwater.
Treatment
Understanding the three-stage process: sedimentation, filtration, and chlorination.
Fertilisers
Recognising NPK nutrients and selecting the right compounds for plant growth.
Part 1: Chemical Tests for Water
Why Test for Water?
Water is such a common substance that we often take it for granted, but in chemistry practicals it's crucial to know whether water is present in a sample. Two anhydrous (water-free) compounds change colour dramatically when they come into contact with water, making them perfect detectors.
Anhydrous Cobalt(II) Chloride
What it looks like: A bright blue powder. The "anhydrous" just means it has no water molecules in its structure.
What happens: When exposed to water vapour or liquid water, the cobalt chloride grabs those water molecules and bonds with them, forming hydrated cobalt chloride, which is pink.
Why it works: The water molecules become part of the crystal structure. The "•6H₂O" means 6 water molecules have attached to every 1 cobalt chloride molecule. This structural change also changes how the compound absorbs light, shifting the colour from blue to pink.
Real-World Use: Cobalt chloride is often used as a desiccant (water-remover) in lab storage—it turns pink to warn you that humidity has entered the container.
In Practicals: Cobalt Chloride Paper
In exams, you'll likely use cobalt chloride paper rather than the powder. It's strips of paper soaked in anhydrous cobalt(II) chloride. To test for water:
- Dip the paper into the sample being tested
- If the paper changes from blue to pink, water is present
- The paper can be dried over a flame to turn blue again and reused
Anhydrous Copper(II) Sulfate
What it looks like: A white powder. Like cobalt chloride, it's anhydrous—no water attached.
What happens: When exposed to water, copper sulfate bonds with water molecules to form hydrated copper sulfate, which is blue.
Why it works: Just like cobalt chloride, the water molecules integrate into the crystal structure. Notice it's 5 water molecules per copper sulfate (not 6)—different compounds grab different amounts of water based on their atomic structure.
Comparison: Both cobalt chloride and copper sulfate work as water detectors, but they give different colours. Blue → Pink for cobalt; White → Blue for copper. Either can be used, but the exam will usually specify which one.
Testing for Purity of Water
Pure vs. Impure Water: A pure substance like water has fixed, sharp melting and boiling points. Pure water always boils at 100°C and freezes at 0°C (at sea level).