What is the chemical formula for water?

Water = H₂O = 2 hydrogen atoms + 1 oxygen atom. Common chemical formulas to know: H₂O (water); CO₂ (carbon dioxide); O₂ (oxygen gas); N₂ (nitrogen gas); H₂ (hydrogen gas); NaCl (sodium chloride/salt); HCl (hydrochloric acid); NaOH (sodium hydroxide/lye); H₂SO₄ (sulfuric acid); NH₃ (ammonia); CH₄ (methane); C₆H₁₂O₆ (glucose); H₂O₂ (hydrogen peroxide); O₃ (ozone); CaCO₃ (calcium carbonate/limestone). Water properties making it essential for life: (1) Polar molecule — oxygen attracts electrons more than hydrogen, creating partial negative on O and partial positive on H; this polarity allows water to dissolve many substances ('universal solvent'); (2) Hydrogen bonding — partial charges attract neighboring water molecules; gives water high surface tension, cohesion, adhesion, high specific heat, high heat of vaporization; (3) Density anomaly — water expands when freezing, so ice floats on liquid water; insulates aquatic life in winter; (4) Three states common at Earth temperatures (solid, liquid, gas).