Analysis: Determination of chemical components in a given sample. It is divided into:
Reagent: A chemical substance that reacts with another substance to produce a characteristic observation (colour change, gas evolution, or precipitate formation) used for identification.
Salts of Representative Elements (Groups 1, 2, 13 to 17) are generally colourless / white, whereas salts of Transition Elements (Groups 3 to 12) are generally coloured due to incomplete inner d-orbitals.
| Ion Type | Cation / Anion | Symbol | Colour in Solution |
|---|---|---|---|
| Colourless Cations | Ammonium ion | $NH_4^+$ | Colourless |
| Sodium ion | $Na^+$ | Colourless | |
| Potassium ion | $K^+$ | Colourless | |
| Calcium ion | $Ca^{2+}$ | Colourless | |
| Magnesium ion | $Mg^{2+}$ | Colourless | |
| Aluminium ion | $Al^{3+}$ | Colourless | |
| Lead ion | $Pb^{2+}$ | Colourless | |
| Zinc ion | $Zn^{2+}$ | Colourless | |
| Coloured Cations | Cupric ion | $Cu^{2+}$ | Blue |
| Ferrous ion | $Fe^{2+}$ | Light / Pale Green | |
| Ferric ion | $Fe^{3+}$ | Yellowish Brown / Reddish Brown | |
| Nickel ion | $Ni^{2+}$ | Green | |
| Chromium ion | $Cr^{3+}$ | Green | |
| Manganese ion | $Mn^{2+}$ | Pink | |
| Cobalt ion | $Co^{2+}$ | Pink / Red | |
| Coloured Anions | Permanganate ion | $MnO_4^-$ | Pink / Purple |
| Dichromate ion | $Cr_2O_7^{2-}$ | Orange | |
| Chromate ion | $CrO_4^{2-}$ | Yellow | |
| Other Anions ($Cl^-, SO_4^{2-}, CO_3^{2-}, NO_3^-$) | Various | Colourless |
Metal Salt Solution + Sodium Hydroxide ($NaOH$)
$\rightarrow$ Metal Hydroxide $\downarrow$ (Precipitate) + Sodium Salt
Procedure: Add $NaOH$ solution drop by drop to 2-3 mL of salt solution in a test tube. Observe precipitate colour. Then add $NaOH$ in EXCESS and shake.
| Salt Cation | Precipitate Formed & Colour | Solubility in Excess NaOH | Balanced Chemical Equations |
|---|---|---|---|
| Calcium ($Ca^{2+}$) $Ca(NO_3)_2$ |
$Ca(OH)_2\downarrow$ White precipitate |
Slightly Soluble / Insoluble | $Ca(NO_3)_2 + 2NaOH \rightarrow Ca(OH)_2\downarrow + 2NaNO_3$ Ionic: $Ca^{2+} + 2OH^- \rightarrow Ca(OH)_2\downarrow$ |
| Iron(II) ($Fe^{2+}$) $FeSO_4$ (pale green) |
$Fe(OH)_2\downarrow$ Dirty Green gelatinous ppt |
INSOLUBLE (Turns reddish-brown on standing due to $Fe(OH)_3$ oxidation) |
$FeSO_4 + 2NaOH \rightarrow Fe(OH)_2\downarrow + Na_2SO_4$ Ionic: $Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2\downarrow$ |
| Iron(III) ($Fe^{3+}$) $FeCl_3$ (yellow/brown) |
$Fe(OH)_3\downarrow$ Reddish-Brown ppt |
INSOLUBLE | $FeCl_3 + 3NaOH \rightarrow Fe(OH)_3\downarrow + 3NaCl$ Ionic: $Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3\downarrow$ |
| Copper ($Cu^{2+}$) $CuSO_4$ (blue) |
$Cu(OH)_2\downarrow$ Pale Blue ppt |
INSOLUBLE | $CuSO_4 + 2NaOH \rightarrow Cu(OH)_2\downarrow + Na_2SO_4$ Ionic: $Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2\downarrow$ |
| Zinc ($Zn^{2+}$) $ZnSO_4$ (colourless) |
$Zn(OH)_2\downarrow$ Gelatinous White ppt |
SOLUBLE in excess (Forms colourless Sodium Zincate complex) |
1. $ZnSO_4 + 2NaOH \rightarrow Zn(OH)_2\downarrow + Na_2SO_4$ 2. $Zn(OH)_2 + 2NaOH \rightarrow Na_2ZnO_2 + 2H_2O$ (Sodium zincate - colourless) |
| Lead ($Pb^{2+}$) $Pb(NO_3)_2$ (colourless) |
$Pb(OH)_2\downarrow$ Chalky White ppt |
SOLUBLE in excess (Forms colourless Sodium Plumbite complex) |
1. $Pb(NO_3)_2 + 2NaOH \rightarrow Pb(OH)_2\downarrow + 2NaNO_3$ 2. $Pb(OH)_2 + 2NaOH \rightarrow Na_2PbO_2 + 2H_2O$ (Sodium plumbite - colourless) |
| Aluminium ($Al^{3+}$) $AlCl_3$ (colourless) |
$Al(OH)_3\downarrow$ Gelatinous White ppt |
SOLUBLE in excess (Forms colourless Sodium Meta-aluminate complex) |
1. $AlCl_3 + 3NaOH \rightarrow Al(OH)_3\downarrow + 3NaCl$ 2. $Al(OH)_3 + NaOH \rightarrow NaAlO_2 + 2H_2O$ (Sodium Meta-Aluminate - colourless) |
Reaction: When any ammonium salt ($NH_4Cl, (NH_4)_2SO_4, NH_4NO_3$) is heated with a strong alkali like $NaOH$ or $KOH$, Ammonia gas ($NH_3$) is evolved.
$$NH_4Cl + NaOH \xrightarrow{\Delta} NaCl + H_2O + NH_3\uparrow$$ $$(NH_4)_2SO_4 + 2NaOH \xrightarrow{\Delta} Na_2SO_4 + 2H_2O + 2NH_3\uparrow$$Identification Tests for Evolved Ammonia Gas:
Metal Salt Solution + Ammonium Hydroxide ($NH_4OH$)
$\rightarrow$ Metal Hydroxide $\downarrow$ (Precipitate) + Ammonium Salt
Key Principle: $NH_4OH$ is a weak alkali providing a lower concentration of $OH^-$ ions compared to strong $NaOH$.
| Salt Cation | Precipitate Formed & Colour | Solubility in Excess NH₄OH | Balanced Chemical & Complex Equations |
|---|---|---|---|
| Calcium ($Ca^{2+}$) | NO PRECIPITATE FORMED | — | $NH_4OH$ is a weak base; $[OH^-]$ concentration is too low to exceed solubility product of $Ca(OH)_2$. |
| Iron(II) ($Fe^{2+}$) | $Fe(OH)_2\downarrow$ Dirty Green ppt |
INSOLUBLE | $FeSO_4 + 2NH_4OH \rightarrow Fe(OH)_2\downarrow + (NH_4)_2SO_4$ |
| Iron(III) ($Fe^{3+}$) | $Fe(OH)_3\downarrow$ Reddish-Brown ppt |
INSOLUBLE | $FeCl_3 + 3NH_4OH \rightarrow Fe(OH)_3\downarrow + 3NH_4Cl$ |
| Copper ($Cu^{2+}$) | $Cu(OH)_2\downarrow$ Pale Blue ppt |
SOLUBLE in excess (Forms DEEP BLUE / INKY BLUE solution) |
1. $CuSO_4 + 2NH_4OH \rightarrow Cu(OH)_2\downarrow + (NH_4)_2SO_4$ 2. $Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow$ $[Cu(NH_3)_4]SO_4 + 4H_2O$ (Tetraamminecopper(II) sulphate - Deep Blue) |
| Zinc ($Zn^{2+}$) | $Zn(OH)_2\downarrow$ Gelatinous White ppt |
SOLUBLE in excess (Forms COLOURLESS soluble Tetraamminezinc complex) |
1. $ZnSO_4 + 2NH_4OH \rightarrow Zn(OH)_2\downarrow + (NH_4)_2SO_4$ 2. $Zn(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow$ $[Zn(NH_3)_4]SO_4 + 4H_2O$ (Tetraamminezinc(II) sulphate - Colourless) |
| Lead ($Pb^{2+}$) | $Pb(OH)_2\downarrow$ Chalky White ppt |
INSOLUBLE | $Pb(NO_3)_2 + 2NH_4OH \rightarrow Pb(OH)_2\downarrow + 2NH_4NO_3$ |
| Aluminium ($Al^{3+}$) | $Al(OH)_3\downarrow$ Gelatinous White ppt |
INSOLUBLE | $AlCl_3 + 3NH_4OH \rightarrow Al(OH)_3\downarrow + 3NH_4Cl$ |
Crucial Distinguishing Test (Zinc vs Lead vs Aluminium):
• White ppt of $Zn(OH)_2$ is soluble in BOTH excess $NaOH$ AND excess $NH_4OH$.
• White ppt of $Pb(OH)_2$ and $Al(OH)_3$ is soluble in excess $NaOH$, but INSOLUBLE in excess $NH_4OH$!
Metals like Zinc ($Zn$), Aluminium ($Al$), and Lead ($Pb$) react with hot concentrated alkalis ($NaOH, KOH$) to form soluble complex salts and liberate Hydrogen gas ($H_2$).
| Metal | Reagents Used | Soluble Salt Formed | Balanced Equation |
|---|---|---|---|
| Zinc ($Zn$) | Hot conc. $NaOH$ | Sodium Zincate (colourless) | $Zn + 2NaOH \xrightarrow{\Delta} Na_2ZnO_2 + H_2\uparrow$ |
| Hot conc. $KOH$ | Potassium Zincate (colourless) | $Zn + 2KOH \xrightarrow{\Delta} K_2ZnO_2 + H_2\uparrow$ | |
| Aluminium ($Al$) | Boiling conc. $NaOH + H_2O$ | Sodium Meta-aluminate | $2Al + 2NaOH + 2H_2O \xrightarrow{\Delta} 2NaAlO_2 + 3H_2\uparrow$ |
| Boiling conc. $KOH + H_2O$ | Potassium Meta-aluminate | $2Al + 2KOH + 2H_2O \xrightarrow{\Delta} 2KAlO_2 + 3H_2\uparrow$ | |
| Lead ($Pb$) | Hot conc. $NaOH$ | Sodium Plumbite | $Pb + 2NaOH \xrightarrow{\Delta} Na_2PbO_2 + H_2\uparrow$ |
| Hot conc. $KOH$ | Potassium Plumbite | $Pb + 2KOH \xrightarrow{\Delta} K_2PbO_2 + H_2\uparrow$ |
Amphoteric Oxides / Hydroxides: Oxides and hydroxides of metals ($Zn, Al, Pb$) that show dual character — reacting with both acids and strong bases (alkalis) to form salt and water.
1. Zinc Oxide ($ZnO$) & Zinc Hydroxide ($Zn(OH)_2$)
(a) Action of Acid ($HCl$):
(b) Action of Caustic Soda ($NaOH$):
(c) Action of Caustic Potash ($KOH$):
2. Aluminium Oxide ($Al_2O_3$) & Aluminium Hydroxide ($Al(OH)_3$)
(a) Action of Acid ($HCl$):
(b) Action of Caustic Soda ($NaOH$):
(c) Action of Caustic Potash ($KOH$):
3. Lead Oxide ($PbO$) & Lead Hydroxide ($Pb(OH)_2$)
(a) Action of Acid ($HNO_3$):
(b) Action of Caustic Soda ($NaOH$):
(c) Action of Caustic Potash ($KOH$):
| Cation | Action of NaOH (Limited) | Action of NaOH (Excess) | Action of NH₄OH (Limited) | Action of NH₄OH (Excess) |
|---|---|---|---|---|
| $Ca^{2+}$ | White ppt | Insoluble | No ppt | No ppt |
| $Fe^{2+}$ | Dirty green ppt | Insoluble | Dirty green ppt | Insoluble |
| $Fe^{3+}$ | Reddish-brown ppt | Insoluble | Reddish-brown ppt | Insoluble |
| $Cu^{2+}$ | Pale blue ppt | Insoluble | Pale blue ppt | Deep Blue Solution |
| $Zn^{2+}$ | Gelatinous white ppt | Soluble (Colourless) | Gelatinous white ppt | Soluble (Colourless) |
| $Pb^{2+}$ | Chalky white ppt | Soluble (Colourless) | Chalky white ppt | Insoluble |
| $Al^{3+}$ | Gelatinous white ppt | Soluble (Colourless) | Gelatinous white ppt | Insoluble |
