Topic 4: Predicting and Identifying Reactions and Products - Subtopic 2: Identify the Products of Chemical Reactions
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What is the correct test for carbon dioxide?
Bubble gas through limewater (calcium hydroxide solution). Positive result: clear solution turns milky/cloudy due to calcium carbonate formation.
What common mistake do students make with CO₂ testing?
Mistaking extinguishing a splint as a positive test - nitrogen also does this. Only limewater test is definitive.
Describe the hydrogen test procedure.
Hold burning splint at test tube mouth. Positive result: loud ‘pop’ from H₂ + O₂ → H₂O combustion. Keep splint at mouth for air mixing.
Why is oxygen tested with a glowing splint?
Glowing splint relights in oxygen (O₂ supports combustion). Contrast with H₂ test which uses burning splint.
What happens in the chlorine test?
Damp blue litmus: 1) Turns red briefly (acidic), 2) Bleaches white (Cl₂ oxidizes dye). MUST use fume cupboard.
Why can’t you rely on chlorine’s smell for testing?
Though distinctive (‘swimming pool’), smell is subjective and unsafe. Exam requires litmus bleaching observation.
How do you test for metal cations with NaOH?
Add NaOH dropwise: 1) Observe initial precipitate color 2) Add excess - some redissolve (Al³⁺, Zn²⁺), others don’t (Mg²⁺, Ca²⁺).
What are the colored precipitates from NaOH tests?
Cu²⁺: blue; Fe²⁺: green; Fe³⁺: red-brown; Al³⁺/Mg²⁺/Ca²⁺: white (Al³⁺/Zn²⁺ dissolve in excess NaOH).
Describe the carbonate ion test.
Add dilute acid → effervescence (CO₂ gas). Confirm by bubbling gas through limewater → milky precipitate (CaCO₃).
Explain the sulfate test procedure.
1) Acidify with HCl (removes CO₃²⁻ interference) 2) Add BaCl₂ → white BaSO₄ precipitate forms if SO₄²⁻ present.
Why must HNO₃ be used for halide tests?
HCl would add Cl⁻ ions, causing false AgCl white precipitate. HNO₃ acidifies without contamination.
What colors identify silver halides?
AgCl: white; AgBr: cream; AgI: yellow. Colors distinguish Cl⁻, Br⁻, I⁻ in qualitative analysis.
List flame test colors for cations.
Li⁺: red; Na⁺: yellow; K⁺: lilac; Ca²⁺: orange-red; Cu²⁺: blue-green. Use clean wire in roaring blue flame.
Why clean flame test wire with acid?
Removes previous metal ions to prevent contamination. Nichrome/platinum wires are chemically inert.
What does a mass spectrum’s molecular ion peak show?
Rightmost peak (highest m/z) = molecular mass (M⁺). E.g., peak at 142 → C₁₀H₂₂ (10×12 + 22×1 = 142).
How are isotopes identified via mass spec?
Peak heights show relative abundance
List 4 advantages of instrumental analysis.
1) Higher sensitivity (detects tiny amounts) 2) Faster results 3) Automated multi-sample processing 4) Objective digital data
Why is instrumental analysis more accurate than chemical tests?
1) Eliminates human error in color interpretation 2) Precise measurements 3) Computerized pattern matching
What’s the difference between ‘colorless’ and ‘clear’?
Colorless = no pigment (e.g. water). Clear = transparent (may be colored like blue CuSO₄ solution).
Why acidify before SO₄²⁻/halide tests?
Removes CO₃²⁻ interference which also forms precipitates (e.g., BaCO₃ could mimic BaSO₄).
How are calibration curves used?
Plot known concentrations vs instrument readings (e.g., light intensity). Compare unknown sample to curve.
How to distinguish Ca²⁺ from Al³⁺ in NaOH tests?
Both form white precipitates but: Ca(OH)₂ insoluble in excess NaOH, Al(OH)₃ dissolves forming [Al(OH)₄]⁻ complex.
What safety precautions are needed for Cl₂ testing?
Use fume cupboard - chlorine is toxic. Avoid inhalation and skin contact with gas.
Why use roaring blue flame for flame tests?
Hotter flame excites electrons more, giving brighter colors. Yellow safety flame masks test colors.