Dictionary of Chemistry [6th Ed.]

A method of estimating the ages of
archaeological specimens of biologi-
cal origin. As a result of cosmic radia-
tion a small number of atmospheric
nitrogen nuclei are continuously
being transformed by neutron bom-
bardment into radioactive nuclei of
carbon–14:
14
7 N + n →

14
6 C + p
Some of these radiocarbon atoms
Ünd their way into living trees and
other plants in the form of carbon
dioxide, as a result of photosynthesis.
When the tree is cut down photosyn-
thesis stops and the ratio of radiocar-
bon atoms to stable carbon atoms
begins to fall as the radiocarbon de-
cays. The ratio^14 C/^12 C in the speci-
men can be measured and enables
the time that has elapsed since the
tree was cut down to be calculated.
The method has been shown to give
consistent results for specimens up
to some 40 000 years old, though its
accuracy depends upon assumptions
concerning the past intensity of the
cosmic radiation. The technique was

developed by Willard F. Libby

(1908–80) and his coworkers in

1946–47.

carbon dioxideA colourless

odourless gas, CO 2 , soluble in water,

ethanol, and acetone; d. 1.977 g dm–3

(0°C); m.p. –56.6°C; b.p. –78.5°C. It oc-

curs in the atmosphere (0.04% by vol-

ume) but has a short residence time

in this phase as it is both consumed

by plants during *photosynthesis and

produced by respiration and by com-

bustion. It is readily prepared in the

laboratory by the action of dilute

acids on metal carbonates or of heat

on heavy-metal carbonates. Carbon

dioxide is a by-product from the

manufacture of lime and from fer-

mentation processes.

Carbon dioxide has a small liquid

range and liquid carbon dioxide is

produced only at high pressures. The

molecule CO 2 is linear with each oxy-

gen making a double bond to the car-

bon. Chemically, it is unreactive and

will not support combustion. It dis-

101 carbon dioxide

c

carbon dioxide in

the atmosphere

combustion respiration respiration indecomposers

respiration photosynthesis

organic

compounds

in animals

carbon compounds

in dead organic

matter

death

deathdeathdeath

organic compounds

in green plants

fossilizationfossilizationfossilization feedingfeedingfeeding

carbon

in fossil

fuels

Carbon cycle