The most important compounds of carbon
Carbon forms two oxides, CO and CO₂. Carbon(II) oxide (carbon monoxide) is a colourless, odourless, very poisonous gas that is slightly soluble in water and slightly lighter than air (M = 28 g/mol against about 29 for air); it is an indifferent oxide, forming no salts with acids or bases. It burns in air with a blue flame: 2CO + O₂ → 2CO₂, and as a reducing agent it frees metals from their oxides: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Carbon monoxide binds to hemoglobin in the blood more firmly than oxygen does, leaving the body short of oxygen. Carbon(IV) oxide is a colourless, odourless acidic oxide heavier than air (M = 44 g/mol): CO₂ + H₂O ⇄ H₂CO₃, CO₂ + 2NaOH → Na₂CO₃ + H₂O, CaO + CO₂ → CaCO₃. In the laboratory it is made by acting on marble or chalk with hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑; industrially limestone is roasted: CaCO₃ → CaO + CO₂↑ (on heating). Qualitative test: lime water turns cloudy, Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. CO₂ does not support burning, which is why it is used in fire fighting, but magnesium burns in it: 2Mg + CO₂ → 2MgO + C. Solid CO₂, “dry ice”, keeps food cold; adults handle it with gloves because it causes cold burns.
Demonstration (teacher only): CO₂ is made from chalk and dilute acid and passed through lime water; the clouding shows the qualitative test. Students do not smell the gas; they watch and write the equations. No experiments with acids or gases at home.