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Chemical Tests for Ions & Gases

Identify water, chloride ions, carbonate ions, and common gases
Status
Test for Water

Observe the initial state, then press Start.

Indicator
Anhydrous cobalt(II) chloride paper
H₂O

Test Notes

  1. 1
    Use forceps to place a piece of blue anhydrous cobalt(II) chloride paper on a watch glass.
  2. 2
    Use a dropper to add a few drops of the liquid onto the paper.
Indicator

Anhydrous cobalt(II) chloride paper

Observation

Select a Test

Liquid
Ions
Gases

HKDSE Tip

  • Presence vs purity:
    If anhydrous paper turns pink, you can only conclude that water is present — not that the liquid is pure water. The liquid may be a mixture (e.g. a dilute acid or alcohol solution). To prove pure water, measure its boiling point (exactly 100°C) or melting point (exactly 0°C).
  • Reversible reaction:
    Pink cobalt(II) chloride paper can be turned back to blue by heating it. The reversible reaction is:
    Heating drives off water of crystallization, shifting the equilibrium to the left and regenerating blue anhydrous . The paper is therefore reusable.
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