Water
Testing water
- Anhydrous cobalt(II) chloride turns blue → pink with water.
- Anhydrous copper(II) sulfate turns white → blue with water.
- These show water is present. To show it is pure, check it melts at $0\,{}^{\circ}\text{C}$ and boils at $100\,{}^{\circ}\text{C}$ — any dissolved substance changes these.
Practice
Adding water to anhydrous copper(II) sulfate turns it:
Copper(II) sulfate goes white → blue; cobalt(II) chloride goes blue → pink.
Practice
How can you show that water is pure (not just present)?
Pure water has sharp fixed melting (0 °C) and boiling (100 °C) points; dissolved substances change them.
What's in natural water
- Natural water may contain dissolved oxygen (helpful — aquatic life breathes it), useful minerals, but also toxic metal compounds, plastics, sewage microbes, and nitrates/phosphates that cause deoxygenation.
Treating drinking water
- sedimentation + filtration remove solid bits,
- carbon removes bad tastes and smells,
- chlorination kills harmful microbes.
Practice
Order the drinking-water treatment steps.
Settle/filter solids → carbon removes tastes/smells → chlorine kills microbes.
You've got it
Key idea
- water tests: cobalt chloride blue→pink, copper sulfate white→blue; pure water melts $0\,{}^{\circ}\text{C}$, boils $100\,{}^{\circ}\text{C}$
- natural water has helpful (oxygen, minerals) and harmful (toxic metals, sewage, nitrates) things
- treatment: sedimentation/filtration → carbon → chlorination