Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 10.23 CORE PRACTICAL 8: Unknown analysis-Study Notes - New Syllabus

10.23 Core Practical 8: Analysis of Inorganic and Organic Unknowns

This practical involves identifying unknown inorganic ions and organic functional groups using qualitative tests. A systematic approach is required, combining observations with known chemical behaviour.

Inorganic Analysis

Test for acids and bases

• Blue litmus → red = acid
• Red litmus → blue = base
• Universal indicator used to estimate pH

Test for carbonate ions \( \mathrm{CO_3^{2-}} \)

• Add dilute acid → effervescence
• Gas = \( \mathrm{CO_2} \)
• Turns limewater milky (formation of \( \mathrm{CaCO_3} \))

Test for sulfate ions \( \mathrm{SO_4^{2-}} \)

• Add acidified \( \mathrm{Ba^{2+}} \) solution
• White precipitate = \( \mathrm{BaSO_4} \)
• Insoluble in acid

Test for halide ions

• Add dilute \( \mathrm{HNO_3} \) then \( \mathrm{AgNO_3} \)
• \( \mathrm{Cl^-} \): white ppt, soluble in dilute \( \mathrm{NH_3} \)
• \( \mathrm{Br^-} \): cream ppt, soluble in concentrated \( \mathrm{NH_3} \)
• \( \mathrm{I^-} \): yellow ppt, insoluble in \( \mathrm{NH_3} \)

Test for ammonium ions \( \mathrm{NH_4^+} \)

• Add aqueous \( \mathrm{NaOH} \), warm gently
• Ammonia gas released
• Turns damp red litmus paper blue

Test for metal cations (using \( \mathrm{NaOH} \))

• \( \mathrm{Cu^{2+}} \): blue ppt
• \( \mathrm{Fe^{2+}} \): green ppt (turns brown)
• \( \mathrm{Fe^{3+}} \): brown ppt
• \( \mathrm{Al^{3+}} \), \( \mathrm{Zn^{2+}} \): white ppt (dissolve in excess NaOH)

Organic Analysis

Test for alkenes

• Add bromine water
• Orange → colourless

Test for alcohols

• Add acidified \( \mathrm{K_2Cr_2O_7} \), heat
• Orange → green = primary/secondary alcohol
• No change = tertiary alcohol

Test for carboxylic acids

• Add \( \mathrm{Na_2CO_3} \) or \( \mathrm{NaHCO_3} \)
• Effervescence (CO₂ produced)

Test for aldehydes

•  
• Tollens’ reagent → silver mirror
• 
• Fehling’s solution → brick-red ppt

Test for ketones

• No reaction with Tollens’ or Fehling’s

Iodoform test

• Warm with iodine + \( \mathrm{NaOH} \)
• Yellow precipitate = \( \mathrm{CHI_3} \)
• Indicates \( \mathrm{CH_3CO-} \) or \( \mathrm{CH_3CH(OH)-} \) group

Solubility behaviour

• Alcohols, amines, acids → may dissolve in water (H-bonding)
• Carboxylic acids dissolve in alkali (form \( \mathrm{RCOO^-} \))
• Amines dissolve in acid (form \( \mathrm{RNH_3^+} \))