Dry Heating Test, Charcoal Cavity & Cobalt Nitrate Test

What the Dry Heating Test Is

The dry heating test is a preliminary examination in which about 0.1 g of the dry salt is placed in a clean, dry test tube and heated for roughly a minute. No reagent is added; the salt is heated alone, so the only chemistry at play is thermal decomposition, sublimation and dehydration. The observer watches four channels at once — any gas or vapour driven off, whether the salt sublimes, the colour of the residue while hot, and its colour again after it has cooled.

The NCERT Laboratory Manual is emphatic that this is a preliminary test. The colour change of a residue on heating and cooling "gives indications about the presence of cations, which may not be taken as conclusive evidence." Dry tests speed up analysis and rule families in or out, but the cation is reported only after the systematic wet group separation confirms it. A new student should learn to treat every dry-heating observation as a hypothesis, not a verdict.

The test sits within a wider set of dry tests in the manual — colour test, dry heating, flame test, borax bead test, charcoal cavity test and the cobalt nitrate test — all performed before wet analysis. This article concentrates on dry heating itself and the two cavity-based tests that extend it.

Gases and Vapours Evolved

The first diagnostic channel is whatever leaves the salt. Each gas has a colour, an odour and a chemical test that together fingerprint an anion or, in the case of ammonia, a cation. The single most discriminating step is the glowing-splinter test: a smouldering wooden splinter held at the mouth of the tube relights only in oxygen, immediately separating a nitrate of potassium or sodium from every other gas.

A common pairing must be read carefully. Brown $\ce{NO2}$ is given by nitrates of heavy metals, whereas nitrates of the very electropositive metals (K, Na) give only colourless $\ce{O2}$ because they stop at the nitrite stage. A "colourless gas now, brown later" report usually means $\ce{NO2}$ was masked momentarily or that the brown ring was missed — brown fumes that intensify on standing point firmly to nitrate.

Decomposition Equations

Every gas observation traces back to a thermal decomposition. Writing these reactions cleanly is what converts an observation into an inference, and several appear directly in equation-completion questions.

A carbonate breaks down to the metal oxide and carbon dioxide:

$$\ce{CaCO3 ->[\Delta] CaO + CO2 ^}$$

A nitrate of an alkali metal stops at the nitrite, releasing oxygen — the source of the relighting splinter:

$$\ce{2KNO3 ->[\Delta] 2KNO2 + O2 ^}$$

A heavy-metal nitrate goes further, giving the oxide together with brown nitrogen dioxide and oxygen:

$$\ce{2Pb(NO3)2 ->[\Delta] 2PbO + 4NO2 ^ + O2 ^}$$

An ammonium salt such as ammonium chloride dissociates into two gases that recombine on the cool wall (covered below); a sulphite gives off sulphur dioxide:

$$\ce{ZnSO3 ->[\Delta] ZnO + SO2 ^}$$

A hydrated salt simply loses its water of crystallisation, the change exploited in the classic copper sulphate observation:

$$\ce{CuSO4.5H2O ->[\Delta] CuSO4 + 5H2O ^}$$

Residue Colours Hot and Cold

The second diagnostic channel is the solid left behind. Several metal oxides are thermochromic — their colour while hot differs from their colour once cooled — and this reversible change is, per NCERT Table 7.7, a strong indicator of the cation. The student must record both colours, because the change itself, not the static colour, is the clue.

The copper sulphate observation is worth isolating. Blue hydrated copper sulphate loses water on heating to give white anhydrous $\ce{CuSO4}$; adding a drop of water reverses the change and the blue returns. This white-to-blue rehydration is a standard demonstration of the role of water of crystallisation and a favourite distractor when set against the hot/cold colour pairs of true oxides.

Sublimation of the Salt

Some salts leave no residue at all: they pass directly from solid to vapour and re-deposit as a white solid on the cooler upper wall of the tube. The textbook example is ammonium chloride, which dissociates on heating and recombines on cooling:

$$\ce{NH4Cl ->[\Delta] NH3 ^ + HCl ^}$$

$$\ce{NH3 + HCl ->[cool] NH4Cl v}$$

A white sublimate accompanied by the pungent smell of ammonia confirms an ammonium salt. Other sublimates exist — iodine deposits as violet-black crystals, and some mercury and arsenic compounds sublime — but ammonium chloride is the case NEET expects.

The Charcoal Cavity Test

When dry heating points to a metal that forms a reducible oxide, the charcoal cavity test extends the inference by trying to reduce that oxide to the free metal. A small cavity is bored into a charcoal block, filled with about 0.2 g of salt and 0.5 g of anhydrous sodium carbonate, moistened with a drop of water so the powder does not blow away, and heated with a blowpipe in the luminous (reducing) flame.

Sodium carbonate first converts the salt to its carbonate, which decomposes to the oxide; the carbon of the charcoal then reduces the oxide. For copper sulphate the full chain is:

$$\ce{CuSO4 + Na2CO3 ->[\Delta] CuCO3 + Na2SO4}$$

$$\ce{CuCO3 ->[\Delta] CuO + CO2 ^} \qquad \ce{CuO + C ->[\Delta] Cu + CO ^}$$

Zinc behaves similarly but its oxide is not reduced to the metal under these conditions — it stays as the thermochromic oxide:

$$\ce{ZnCO3 ->[\Delta] ZnO + CO2 ^}$$

The cavity is read both hot and cold, for either a coloured residue or a metallic bead.

The Cobalt Nitrate Test

The cobalt nitrate test is performed only when the charcoal cavity residue is white, to resolve the cations whose oxides are colourless. The white residue is moistened with two or three drops of cobalt nitrate solution and heated strongly in the non-luminous (oxidising) flame. Cobalt nitrate first decomposes to cobalt(II) oxide:

$$\ce{2Co(NO3)2 ->[\Delta] 2CoO + 4NO2 ^ + O2 ^}$$

The $\ce{CoO}$ then combines with the metal oxide present to form a coloured double oxide whose colour names the cation:

$$\ce{CoO + Al2O3 ->[\Delta] CoO.Al2O3} \quad(\text{blue})$$

$$\ce{CoO + ZnO ->[\Delta] CoO.ZnO} \quad(\text{green})$$

$$\ce{CoO + MgO ->[\Delta] CoO.MgO} \quad(\text{pink})$$

The blue colour for aluminium is the classic result: a white, infusible $\ce{Al2O3}$ residue moistened with cobalt nitrate and reheated turns a deep blue. The green for zinc neatly complements the dry-heating yellow-hot/white-cold observation, while the pink for magnesium handles a cation that gives no informative dry-heating colour of its own.

Reading the Observations Together

No single dry observation is decisive; the power of the method is in combining channels. A disciplined reading runs: note the gas, then the sublimate, then the residue colour hot and cold, and only escalate to the charcoal cavity and cobalt nitrate when the residue is ambiguous or white. The schematic below traces that flow.