PREPARING AND TESTING GASES
Hydrogen Gas (H₂)
-colourless + odorless gas
-test; pop test
2H₂+ O₂ --> 2H₂O
-litmus-no change
-preparation:
ACID + ACTIVE METAL
  Mg + 2HCl --> MgCl₂ + H₂
WATER + REACTIVE METAL
  2Na + 2H₂O --> 2NaOH + H₂
ELECTROLYSIS OF SOLUTIONS

Oxygen Gas (O₂)
-colourless + odorless gas
-test; re-light a glowing splint
C + O₂ --> CO₂
-litmus-no change
-preparation:
HEATING SOLIDS
  2 KClO₃ --> 2KCl + 3O₂
HEATING & REACTING PEROXIDES
  2H₂O₂ --> (MnO₂) --> 2H₂O + O₂

Carbon Dioxide (CO₂)
-colourless + odorless gas
-test; turns limewater milky, limewater test
  Ca(OH) 2 [limewater] + CO₂ --> CaCO₃ + H₂O
-litmus-red
-preparation:
HEATING CARBONATES
  CaCO₃ --> CaO + CO₂
ACID + CARBONATES
  CuCO₃ + 2HCl --> CuCl₂ + H₂O + CO₂

Ammonia (NH₃)
-colourless + pungent gas
-test; litmus > blue
-preparation:
HEATING + AMMONIUM SALTS
  NH₄Cl --> NH₃ + HCl
BASE + AMMONIUM SALTS
  Ca(OH)₂ + 2NH₄Cl --> CaCl₂ + 2H₂O + 2NH₃

Sulphur Dioxide (SO₂)
-colourless + choking gas
-test; dichromate =K₂Cr₂O₇ - orange > green
-preparation:
HEATING SULPHUR
  S + O₂ --> SO₂
ACID + THIOSULPHATE
  2HCl + Na₂S₂O₃ --> 2 NaCl + S + SO₂ + H₂O
ACID + SULPHITE
  2HCl + Na₂SO₃ --> 2NaCl + H₂O + SO₂

Chlorine Gas (Cl₂)
-yellowish-green + pungent gas
-test; bleaches litmus white
-preparation:
  2KMnO₄ + 16HCl --> 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O


CATIONS

cation	NaOH (aq)	NH3 (aq)
Al3+	white ppt. excess-soluble	white ppt. excess-insoluble
NH4+	no ppt. NH3 on warming	no ppt.
Ca2+	white ppt. excess-insoluble	no / very slightly white ppt.
Cu2+	light blue ppt. excess-insoluble	light blue ppt. excess-soluble > dark blue sol.
Fe2+	green ppt. excess-insoluble (exposure to air > brown)	green ppt. excess-insoluble (exposure to air > brown)
Fe3+	red-brown ppt. excess-insoluble	red-brown ppt. excess-insoluble
Zn2+	white ppt. excess-soluble	white ppt. excess-soluble

ANIONS

anion	Test	result
CO32-	Acidify	effervescence, CO2
Cl-	Acidify, + AgNO3(aq)	white ppt.
I-	Acidify, + Pb(NO3)2(aq)	yellow ppt.
NO3-	Add NaOH, + Al/Zn / Devarda's alloy, heat	ammonia produced
SO42-	Acidify, + BaCl2 / Ba(NO3)2(aq)	white ppt.

Test	CO3-	SO42-	Cl-	I-	NO3-
Add HNO3	Bubbles	-	-	-	-
+ BaCl2(aq)	-	White ppt. acid-insoluble	-	-	-
+ AgNO3(aq)	White ppt. acid-soluble	(small amount of white ppt.)	White ppt., acid-insoluble	Yellow ppt., acid-insoluble	-
+ Pb(NO3)2(aq)	-	White ppt., acid & warming -insoluble	White ppt., acid-insoluble, warming-soluble, cooling-white crystals	Yellow ppt., acid-insoluble, warming-soluble, cooling-golden-yellow crystals	-
NaOH, + Al/Zn/ Devarda's Alloy, heat	-	-	-	-	NH3

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