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. Annual releases of carbon dioxide reached 23 billion tonnes in 1997. According to a 1997 estimate by the World Energy Council, carbon dioxide emissions rose by 7.8% between 1986 and 1996.

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.

The carbon cycle and the ‘greenhouse effect’ The production of carbon dioxide by respiration and the absorption of carbon dioxide in photosynthesis are two aspects of the cycling of carbon compounds on Earth. The carbon cycle is of great interest at the moment because the concentration of carbon dioxide in the air is increasing. Carbon dioxide and some other gases have the effect of reducing the loss of heat from the Earth into space. Since radiation from the sun is constantly being absorbed by the Earth, this warms the Earth. This is known as the ‘greenhouse effect’. We need this warmth to live.

However, there are worries that the Earth's temperature may be rising too high because human activities are adding a lot of carbon dioxide to the air. This is known as the ‘enhanced greenhouse effect’. Carbon dioxide is being added by the burning of fossil fuels - coal, oil, diesel, petrol, and natural gas. In addition to this, the removal of large areas of forest adds to carbon dioxide in the air in two ways. Firstly, the wood is often burned and secondly, the micro-organisms of decay become more active in deforested areas and their respiration adds to the carbon dioxide. Micro-organisms that break down the dead remains of plants and animals (decomposers) help to recycle the chemicals in organisms and when they respire they also produce carbon dioxide. If the trees are allowed to remain, they have the opposite effect. They absorb carbon dioxide and it is turned into cellulose which is a component of wood. So trees are important ‘sinks’ removing carbon dioxide from the air.