Flame tests and Water tests

A flame test identifies the metal cation (positive metal ion) in a compound. When a metal compound is held in a hot flame, the metal ions give the flame a characteristic colour.

The method, in order, is:

1. Clean the wire. Dip a clean nichrome wire into dilute hydrochloric acid (HCl), then hold it in the flame. Repeat until the wire produces no colour of its own — this removes any contamination (especially sodium, which is everywhere).
2. Pick up the sample. Dip the cleaned wire back into the HCl, then into the sample (solid or solution) so a little sticks to the wire.
3. Hold it in the flame. Place the wire in the edge of a blue (roaring) Bunsen flame (air hole open).
4. Observe the colour. Record the colour the flame turns.

Why clean with HCl? The acid removes earlier samples so each test starts colour-free, and it helps turn the metal compound into a metal chloride that vaporises easily and gives a brighter colour.

Each metal ion gives its own flame colour. These five must be memorised exactly — examiners do not accept "reddish" for crimson or "purple-ish" for lilac when a precise colour is expected.

Metal cation	Flame colour
Lithium, Li⁺	Red (crimson)
Sodium, Na⁺	Yellow (orange-yellow)
Potassium, K⁺	Lilac (purple)
Calcium, Ca²⁺	Orange-red (brick red)
Copper, Cu²⁺	Blue-green

Watch the easy-to-confuse pairs:

• Li⁺ (red/crimson) vs Ca²⁺ (orange-red/brick red) — Li⁺ is a purer red; Ca²⁺ is more orange ("brick").
• Na⁺ (yellow) is the most common contaminant — a faint yellow flame from another sample is usually just sodium dirt on the wire, which is why you clean first.

A mnemonic that helps: "Lithium Loves Red", King Kilauea is lilac, Cu is Cool blue-green.

Two simple chemical tests show that a liquid contains water. Both rely on a substance changing colour as it absorbs water.

Test 1 — anhydrous copper(II) sulfate.

• What you do: add a few drops of the liquid to white anhydrous copper(II) sulfate powder.
• Observation: the white powder turns BLUE.
• Why: the water joins the copper(II) sulfate to form the hydrated (blue) form: CuSO₄ + 5H₂O → CuSO₄·5H₂O.

Test 2 — cobalt(II) chloride paper.

• What you do: dip a strip of blue cobalt(II) chloride paper into the liquid.
• Observation: the blue paper turns PINK.
• Why: the dry (blue) cobalt chloride takes in water to form the hydrated (pink) form.

Test	Starting colour	Colour with water
Anhydrous copper(II) sulfate	white	blue
Cobalt(II) chloride paper	blue	pink

The key limitation — presence is NOT purity.

Both tests tell you that water is present in a liquid. They do NOT prove the liquid is pure water. Salt water, sugar solution and pond water all contain water, so they would all turn anhydrous CuSO₄ blue and cobalt paper pink — even though none of them is pure.

To prove a liquid is pure water — and not just water with something dissolved in it — you use a physical test: measure its boiling point and freezing (melting) point.

For pure water at normal (atmospheric) pressure:

• it boils at exactly 100 °C, and
• it freezes at exactly 0 °C.

Property	Pure water	Impure water (e.g. salt water)
Boiling point	exactly 100 °C	higher than 100 °C
Freezing point	exactly 0 °C	lower than 0 °C

Why this works. A pure substance has sharp, fixed boiling and freezing points. Dissolved impurities change these values: they raise the boiling point and lower the freezing point. So:

• If your sample boils at exactly 100 °C and freezes at exactly 0 °C, it is pure water.
• If it boils above 100 °C or freezes below 0 °C, it contains dissolved impurities — it is not pure water.

Putting it together — two different questions:

• "Does this liquid contain any water?" → use a chemical test (anhydrous CuSO₄ or cobalt chloride paper).
• "Is this liquid PURE water?" → use the physical test (boils at exactly 100 °C, freezes at exactly 0 °C).