In this test, a little of the substance is heated in a dry test-tube. As a result, the substance undergoes decomposition, or oxidation with evolution of gases along with some characteristic changes which throw light on the cation or anion present as described below.

(a)    Evolution of gases

(i)    Non-alkali metal carbonates and all bicarbonates decompose on heating with the evolution of CO₂ gas.

BaCO₃     →     BaO + CO₂
                      Heat
2NaHCO₃     →     Na₂CO₃ + CO₂ + H₂O

(ii)    Hydrated sulphides such as Na₂.S.2H₂O give H₂S

Na₂S . 2H₂O    →    2NaOH + H₂S

(iii)    Hydrated chlorides such as CuCI₂.H₂O produce HCI gas

CuCI₂ . H₂O   →    CuO + 2HCI

(iv)    Bromides and iodides get oxidized to produce bromine or iodine gases as

2CdBr₂ + O₂    →     2CdO + 2Br₂
2CdI₂ + O₂      →      2CdO + 2I₂

(v)    Heavy metal nitrates such as Pb(NO₃)₂ produce NO₂

2Pb(NO₃)₂    →    2PbO + 4NO₂ + O₂

(vi)    Ammonium salts decomposes liberating NH₃ gas

(NH₄)₂SO₄   →    NH₄HSO₄ + NH₃

(b) Condensation of water vapours. Hydrated salts such as CuSO₄. 5H₂O or  7H₂O on heating lose water of crystallization and form anhydrous salt. The water vapours thus produced get deposited on cooler portion of the test-tube.

CuSO₄ . 5H₂O   →   CuSO₄ + 5H₂O

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