ReviseHive

Test for Carbonates (CO₃²⁻):

- Acid + Metal carbonate → Salt, Water + Carbon dioxide

- e.g., 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂

- Observation: Effervescence

- Test for carbonates:

1) Add hydrochloric/nitric acid to the compound.

2) If effervescence is seen, collect gas and bubble through limewater.

3) If limewater goes from clear to cloudy, then CO₂ was given off; you can infer the compound contained a CO₃²⁻ ion.

Flame Tests:

Method:

1) Dip nichrome wire (unreactive metal) in conc. HCl acid (to remove any impurities that may contaminate the colour), and then heat in a roaring flame until it gives no colour when you know it's clean.
2) Repeatedly dip the nichrome wire in HCl acid and then in the substance for testing.
3) Heat wire in a non-luminous Bunsen burner flame and note colour (use roaring flame to see colour clearly).

How Does it Work?

- When you heat it, the electron absorbs energy and moves to the excited state, but it's unstable there. Eventually, the electron falls to the ground state, where it releases energy in the form of visible light—colour.

Colours:

Li⁺ → Red *
Na⁺ → Yellow *
K⁺ → Lilac *
Ca²⁺ → Orange/Red *

Sr²⁺ → Red
Ba²⁺ → Green
Cu²⁺ → Blue/Green *
Mg²⁺ → No colour (like many others)

Test for Halides:

to HCl//KBr/KI (can vary)

Method:

1) Add a few drops of nitric acid (get rid of CO₃^2-)
2) Add a few drops of silver nitrate (which can form ppt. in HNO₃).
3) Observe the colour of the precipitate.

- We don't use HCl acid; we're testing for Cl^- ions.

Colours (for ppt.):

AgCl = Cl^- = White (milk)
AgBr = Br^- = Cream (cream)
AgI = I^- = Yellow (butter)

- Silver nitrate (added) is soluble; however, Ag₂CO₃ is insoluble; we add nitric acid as it can form a ppt.
- Silver halides are insoluble, so they form ppt.

Equations:

- NaCl _(aq) + AgNO₃ → AgCl + NaNO₃

- Ag⁺ _(aq) + Cl^- _(aq) → AgCl_(s)

- NaI + AgNO₃ → NaNO₃ + AgI

- Ag⁺_(aq) + I^-_(aq) → AgI_(s)

Test for Water:

1) The colour of cobalt chloride paper changes from blue to pink.
2) Adding anhydrous copper (II) sulfate: goes from white to blue.
→ Equation for this: CuSO₄ + 5H₂O → CuSO₄·5H₂O.

- A physical test to show water is pure: it boils at exactly 100^oC and freezes at exactly 0^oC.

Test for Gases:

Ammonia: pH > 7 (alkaline) (NH₃)
Test: Use damp red litmus paper (so gas can dissolve).
Result: Will turn blue is present.

Carbon dioxide: pH < 7 (acidic)
Test: Bubble gas through limewater.
Result: Goes from clear to cloudy is present.

Chlorine: pH < 7 (acidic)
Test: Use damp universal indicator paper.
Result: Turns red, and the bleach (goes white) is present.

Hydrogen: pH = 7 (neutral)
Test: Tie a lighted splint in a test tube with H₂ gas.
Result: Squeaky pop is present.

Oxygen: pH = 7 (neutral)
Test: Take a glowing splint.
Result: Relights are present.

- Hydrogen and oxygen react to form water. Use damp paper with a pH of less than 7, as the gas will dissolve and react with the water on it, creating a solution.

→ 2H₂ + O₂ → 2H₂O (Hydrogen is also very flammable.)

Note: Use damp paper to allow the gas to dissolve or react with the water on it, creating a solution.

Precipitate Tests:

- Another way of identifying metal cations in solutions by adding NaOH _(aq)
- Solubility rule: KOH + NaOH are both soluble; all other metal hydroxides are insoluble.

Method:

Add sodium hydroxide to a solution containing metal ions. The metal and hydroxide ions combine to form a precipitate.

E.g., Fe³⁺
- Iron (III) nitrate + Sodium hydroxide → Iron (III) hydroxide + Sodium nitrate
- Fe(NO₃)₃ + 3NaOH → Fe(OH)₃ + 3NaNO₃
- Fe³⁺_(aq) + OH^-_(aq) → Fe(OH)_{3 (s)}

ion	Colour of ppt	Formula of ppt
Fe2+	Green	Fe(OH)2 (s)
Fe3+	Brown	Fe(OH)3 (s)
Cu2+	Blue	Cu(OH)2 (s)

- The Fe²⁺ darkens on standing as oxidation turns it into Fe³⁺.

Testing for ammonium ion (NH₄⁺):

Method:

1) Add NaOH to a solution with NH₄⁺.
2) Heat gently.
3) Use damp red litmus paper to produce the test gas.
→ Turns blue NH₃ present.
- Since gas is NH₃ no longer NH₄⁺ was present in the compound (no ppt).

- e.g., NH₄Cl _(aq) + NaOH _(aq) → NH_3(g) + H₂O_(l) + NaCl _(aq)

Test for sulfates (SO₄²⁻):

Method:

1) Add a few drops of HCl_(aq) to the solution in the test tube.
2) Then add a few drops of barium chloride solution.
3) If sulfate ions are present, a white ppt is formed.

- We add HCl first to remove any carbonates present that could give a BaCO₃—also a white ppt.
- We cannot use sulphuric acid, as you can’t add sulfate ions when testing for sulfate.

- BaCl₂ and Ba(NO₃)₂ are soluble barium compounds.
- BaCO₃ and BaSO₄ are insoluble barium compounds.
- For the SO₄²⁻ ppt, we need 2 soluble solutions, we can only use the top 2 compounds, but we obtain the last two.
- e.g., BaCl₂(aq) + CuSO₄(aq) → BaSO_4(s) + CuCl₂
- e.g., Ba²⁺_(aq) + SO₄^2-_(aq) → BaSO_{4 (s)}

Recap of Solubility Rules:

Soluble	Insoluble
all common acids	-
All group hydroxides	all other hydroxides + Oxides
All group oxides + ammonia	-
All nitrates	-
All chlorides except →	Lead + Silver chloride
All sulfates except →	Barium, Lead + Calcium sulfate
Sodium + Potassium carbonattes	← All carbonates except
All ammonium salts	-