Carbon oxides (monoxide, dioxide) are gases :)
H₂O + H₂O ⇌ H₃O⁺ + OH⁻
You use two single-barbed arrows pointing in opposite directions (technically rightwards harpoon over leftwards harpoon) <em>to indicate a reversible reactio</em>n.
An occluded front forms when a warm air mass is caught between two cooler air masses.
