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Common gas formed in double replacement reactions are as follows:
1. H2S (rotten egg smell) - Any sulfide (salt of S2-) plus any acid form H2S(g) and a salt.
2. CO2 - Any carbonate (salt of CO32-) plus any acid form CO2 (g), HOH, and a salt.
3. SO2 - Any sulfite (salt of SO32-) plus any acid form SO2 (g), HOH, and a salt.
4. NH3 - Any ammonium salt (NH41+) plus any soluble strong hydroxide react upon heating to form NH3(g), HOH, and a salt.