40 questions for the chemistry practical. Print a salt writeup with this.
What is qualitative salt analysis?
Identification of the acidic radical (anion) and basic radical (cation) in an inorganic salt using systematic tests.
Why is ammonium tested first (Group 0)?
NH₄⁺ would interfere later because NH₄OH is a group reagent. NaOH + Nessler confirms it; then stop.
Why is Nessler’s reagent used?
It gives a brown ppt of basic mercury(II) amidoiodide with ammonia, confirming NH₄⁺.
Why add AgNO₃ after acidifying with HNO₃?
HNO₃ prevents phosphates, carbonates and oxalates of silver from precipitating. Only AgCl, AgBr, AgI ppt.
Why is AgCl soluble in NH₄OH?
It forms the diamminesilver(I) complex [Ag(NH₃)₂]⁺. AgI does not.
What is the chromyl chloride test?
Salt + K₂Cr₂O₇ + conc. H₂SO₄ → CrO₂Cl₂ vapours, which with lead acetate give yellow PbCrO₄. Confirms chloride.
Why does lime water turn milky with CO₂?
Ca(OH)₂ + CO₂ → CaCO₃ (white ppt). Excess CO₂ gives soluble Ca(HCO₃)₂ and the milkiness goes.
How do you distinguish CO₂ from SO₂?
SO₂ smells of burning sulphur and decolourises acidified KMnO₄ / K₂Cr₂O₇. CO₂ does not.
What is the brown ring test?
FeSO₄ + nitrate + conc. H₂SO₄ forms [Fe(H₂O)₅NO]SO₄ at the junction of two layers.
Why must FeSO₄ be freshly prepared?
Fe²⁺ oxidises to Fe³⁺ on standing and the ring fails.
How is nitrate distinguished from nitrite?
Nitrite gives brown fumes / a ring with dilute acid; nitrate needs conc. H₂SO₄ for the brown ring.
Why is BaSO₄ insoluble in conc. HCl?
It is a very insoluble sulphate. BaSO₃ dissolves in acid with evolution of SO₂.
Why acidify before adding BaCl₂?
To destroy carbonate and sulphite which would also give a white barium ppt.
Why NH₄Cl before NH₄OH in Group III?
Common-ion effect lowers [OH⁻] so Mg(OH)₂ does not ppt; only Al(OH)₃ / Fe(OH)₃ do.
What is the blue lake test?
Al(OH)₃ adsorbs blue litmus and floats as a blue lake, confirming Al³⁺.
Why is Al(OH)₃ soluble in excess NaOH?
It is amphoteric and forms [Al(OH)₄]⁻. Fe(OH)₃ is not amphoteric in this way.
Why is H₂S passed in acid for Group II?
[S²⁻] is low in acid, enough to ppt CuS (very low Ksp) but not ZnS.
Why is H₂S passed in alkali for Group IV?
[S²⁻] is higher, enough to ppt ZnS, MnS, NiS, CoS.
Why is CuS black and ZnS white?
Different sulphides; colour is a quick group hint. Cu is Group II, Zn is Group IV.
Why deep blue with excess NH₄OH for copper?
Tetraamminecopper(II), [Cu(NH₃)₄]²⁺.
Why chocolate-brown ppt with K₄[Fe(CN)₆]?
Copper ferrocyanide. Zinc ferrocyanide is bluish-white.
Why is hydrated CuSO₄ blue?
[Cu(H₂O)₆]²⁺. Anhydrous CuSO₄ is white.
Why does PbCl₂ dissolve in hot water?
Its solubility increases sharply with temperature; AgCl does not. Used to separate lead in Group I.
What are golden spangles?
Shining yellow crystals of PbI₂ that appear on cooling.
Why does lead nitrate give brown fumes on heating?
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. The residue is yellow PbO.
Flame colour of Ba, Sr, Ca, Cu, Na?
Ba apple-green, Sr crimson, Ca brick-red, Cu bluish-green, Na persistent yellow.
Why is the flame wire moistened with conc. HCl?
Chlorides are more volatile in the flame and traces of previous salts are removed.
Why Group V after I-IV are absent?
(NH₄)₂CO₃ would also ppt carbonates of earlier groups if they were still in solution.
How is Ba distinguished from Sr and Ca?
Apple-green flame; yellow BaCrO₄ in acetic acid. Sr crimson + SrSO₄; Ca brick-red + oxalate.
Why is Mg not precipitated in Group V?
Ammonium salts keep MgCO₃ in solution. Mg is Group VI with Na₂HPO₄.
Why scratch the test tube for magnesium?
MgNH₄PO₄ crystallises slowly; scratching starts precipitation.
Why does oxalate decolourise KMnO₄?
Oxalate reduces MnO₄⁻ to Mn²⁺ in warm dilute H₂SO₄.
Why a vinegar smell with acetates?
Acid liberates acetic acid, which is volatile.
Why the ester test?
Acetate + ethanol + conc. H₂SO₄ → ethyl acetate (fruity odour).
Why must FeCl₃ be neutral for acetate?
Free acid prevents the deep-red ferric acetate complex.
Why does NH₄Cl sublime?
It dissociates to NH₃ and HCl which recombine on cooler parts of the tube.
Why keep the mouth of the tube away from the face?
HCl, Cl₂, Br₂, NO₂ and H₂S are harmful gases.
What is soda extract?
Salt fused / boiled with Na₂CO₃ so anions go into solution as sodium salts, free of heavy-metal cations.
Why is original solution often made in dilute HCl?
To dissolve the salt and to have the medium ready for Group I / II. Boil off CO₂ if a carbonate was present.
Why not use excess of reagents?
Excess NaOH / NH₄OH can redissolve amphoteric hydroxides or mask later tests. School instruction: small quantities.