carbon dioxide

Colourless, odourless gas, slightly soluble in water, and denser than air. It is formed by the complete oxidation of carbon. Carbon dioxide is produced by living things during the processes of respiration and the decomposition of organic matter, and it is used up during photosynthesis. It therefore plays a vital role in the carbon cycle.

Solid carbon dioxide is called dry ice, as it changes directly from a solid to a gas (sublimes) on warming. It is used as a coolant in its solid form and in the chemical industry.

Its increasing quantity in the atmosphere contributes to the greenhouse effect and global warming. Britain has 1% of the world's population, yet it produces 3% of CO₂ emissions; the USA has 5% of the world's population and produces 25% of CO₂ emissions. Despite growing awareness of the problem, carbon dioxide levels continue to rise worldwide.

Chemistry of carbon dioxide Carbon dioxide is formed when carbon and carbon-containing compounds are fully oxidized, as when they are burnt in an excess of air.

C + O2 → CO2

C2H5OH + 3O2 → 2CO2 + 3H2O

It is also produced when acids are added to carbonates or hydrogencarbonates, and when these salts are heated.

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

CaCO3 → CaO + CO2

It is a typical acidic oxide, dissolving in water to give a solution of the weak dibasic acid carbonic acid, and forming salts with alkalis.

H2O + CO2 ⇌ H2CO3 ⇌ H+(aq) + HCO3−(aq)

NaOH + CO2 → NaHCO3

With a solution of calcium hydroxide (limewater) the gas forms a white (milky) precipitate of calcium carbonate. This reaction is used as the confirmatory test for carbon dioxide.

Ca(OH)2(aq) + CO2(g) → CaCO3 (s) + H2O(l)

The gas supports the combustion of burning magnesium, but extinguishes lower-temperature flames; it is used in fire extinguishers.

2Mg + CO2 → 2MgO + C

Respiration Carbon dioxide is always produced during aerobic respiration so is produced by animals at all times. Plants release carbon dioxide at night, but during the day the carbon dioxide produced by respiration is used inside the plant and more carbon dioxide is absorbed for the process of photosynthesis. In photosynthesis carbon dioxide and water are combined to produce glucose and oxygen.

Mammals can respire anaerobically for short periods, producing lactic acid. Anaerobic respiration can be carried out at all times by some micro-organisms, but there are others that can respire both anaerobically and aerobically if oxygen is present. An example of this kind of organism is the microscopic fungus we call ‘yeast’. Yeast respires aerobically when used in bread dough. The carbon dioxide it produces makes the bread rise. However, yeast used in beer production and winemaking is respiring anaerobically, although carbon dioxide is also being produced. In ‘naturally conditioned’ beer the carbon dioxide dissolves in the beer, making it fizzy. The yeast produces alcohol as well as carbon dioxide when it respires anaerobically.

The bodies of plants and mammals have special adaptations for gas exchange. In a mammal this involves efficient transport of carbon dioxide in the blood and loss at the lungs. For a plant during the day, it involves the efficient uptake of carbon dioxide. This is quite a problem for a plant, because there is little carbon dioxide in the air. Only about 0.04% of air is carbon dioxide. To help uptake, plants have many tiny holes, usually on the underside of the leaf, known as stomata. The carbon dioxide diffuses through these holes and into numerous air spaces inside the leaf, from where it diffuses to the surfaces of leaf cells around the air spaces and then into the cells to the chloroplasts, where photosynthesis takes place. Despite this, on a sunny, warm day the level of carbon dioxide may be the raw material for photosynthesis that is in the shortest supply. This is why carbon dioxide is often added to the air in commercial greenhouses. The plants grow faster as a result.