Dry Tests for Basic Radicals
Before making a solution, burn it! Master the preliminary dry tests: the physics of Flame Tests, the chemistry of the Borax Bead, and the hot/cold color anomalies of metal oxides.
Module Focus: Preliminary Identification
Dry tests are performed directly on the solid salt. They do not give confirmatory evidence but provide extremely strong clues about the presence of specific transition metals or alkaline earth metals. Because they rely on fundamental physical properties (like electron excitation in flames or glass-forming properties of borates), NEET examiners use them heavily in match-the-column questions.
1. Physical Appearance & Dry Heating
The color of a solid salt often instantly identifies the presence of transition metal ions (due to d-d transitions). Heating the dry salt in a test tube can reveal trapped gases or unique metal oxide properties.
- Blue: Usually Copper ($Cu^{2+}$) salts (e.g., Hydrated $CuSO_4$).
- Light Green: Iron(II) ($Fe^{2+}$) salts.
- Dark Green: Nickel ($Ni^{2+}$) salts.
- Yellow / Brown: Iron(III) ($Fe^{3+}$) salts.
- Pink: Cobalt ($Co^{2+}$) or Manganese ($Mn^{2+}$).
Some metal oxides change color reversibly when heated.
Yellow when HOT $\rightleftharpoons$ White when COLD
Reddish-Brown when HOT $\rightleftharpoons$ Yellow when COLD
- Decrepitation: A crackling sound upon heating. Caused by the expansion of trapped water or crystal breaking (Classic for $Pb(NO_3)_2$ or $NaCl$).
- Sublimation: Solid vaporizes to form a white sublimate on the cooler upper parts of the tube. Indicates Ammonium ($NH_4^+$) salts.
2. The Flame Test
A paste of the salt is made with Concentrated $HCl$ and introduced into a non-luminous Bunsen flame using a platinum wire.
Metal salts (like sulfates or carbonates) are often non-volatile. Concentrated $HCl$ is added to convert the metal salts into Metal Chlorides. Metal chlorides are highly volatile, allowing them to vaporize easily in the flame, exciting the electrons.
3. The Borax Bead Test
Used to identify certain transition metals. Borax ($Na_2B_4O_7 \cdot 10H_2O$) is heated on a platinum wire loop. It swells (loses water) and melts into a transparent, glassy bead.
The glassy bead is a mixture of Sodium metaborate ($NaBO_2$) and Boric anhydride ($B_2O_3$).
When touched to a transition metal salt and heated, the basic metal oxide reacts with the acidic $B_2O_3$ to form colored Metal Metaborates.
| Metal Ion | Color in Oxidizing Flame (Hot/Cold) | Color in Reducing Flame (Hot/Cold) |
|---|---|---|
| Cobalt ($Co^{2+}$) | Blue / Blue | Blue / Blue |
| Copper ($Cu^{2+}$) | Green (Hot) / Blue (Cold) | Colorless (Hot) / Red Opaque (Cold) |
| Chromium ($Cr^{3+}$) | Green / Green | Green / Green |
4. Charcoal Cavity & Cobalt Nitrate Test
Used specifically for salts that leave a white residue in the charcoal cavity test (typically Zn, Al, Mg). A drop of Cobalt nitrate solution is added to the white residue and heated. Mixed oxides are formed with characteristic colors.
(Rinmann's Green)
(Thenard's Blue)
Magnesium ($Mg^{2+}$) yields a pink mass ($MgO \cdot CoO$).
NEET Grand Test: Dry Tests
15 High-Yield Questions testing Flame test physics, Borax formulas, and Cobalt nitrate mixed oxides.
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