Sodium exists as more than a trace element in the stars and the sun and is the sixth most abundant element on the Earth's crust, making up an estimated 2.8%. Sodium, along with potassium, is classed as a soft metal but is not found naturally as a metal. It is found in a wide distribution as compounds with other substances, the most familiar being sodium chloride or table salt. Other salts of sodium are found in many rocks and in nearly all soil types. These include halides, silicates, carbonates, sulphates and nitrates. Sodium occurs throughout the rocky crust of the Earth as feldspar, a silicate of sodium, and in various other rocks. Along with potassium, sodium is a soft metal with a silvery white lustre. It tarnishes in the air and is among the most reactive of metals.
Early civilizations knew about the benefits of sodium, although as potassium compounds were very similar in appearance they thought they were the same substance. Records exist of saltpetre from potassium being used to make glazes for pots in Mesopotamia 17th century BC and the Egyptians used sodium carbonate 16th century BC for making glass. They also used natron for embalming and preserving.
From early times sodium carbonate was prepared by passing water through the ashes of plants which had been burned. The water was then evaporated from the solution. Soda as a term was first used for either sodium or potassium but later was used to refer to ash produced from sea plants. Potash was used for ash produced from land vegetation. The name potash, incidentally, originated from the use of large pots for evaporating the water from the solution.
Over the years soda and potash were defined as both natural and artificial products and as vegetable and mineral. Imagine playing the old favourite '20 Questions' with the ancestors - it was hard enough trying to define substances as animal, vegetable or mineral as a science game with primary school children, but at least they have a better understanding of materials through the National Curriculum. Another nugget of information that might corner your imagination for a split second: it wasn't until the beginning of the 19th century that Sir Humphry Davy decomposed both alkalis and called them sodium and potassium, which are really Latinized versions of soda and potash. The two metals cannot be isolated by normal chemical processes and were only prepared after the discovery of the electric current in 1800, when the electrolytic processes were developed. Davy's method was modified and called the Castner process and was used to prepare sodium for a long time. Most is now prepared by the'Down's process which produces chlorine as well as sodium.
Sodium chemicals today are used widely in synthetic chemistry as drying agents and reducing agents. Sodium has also been used in the manufacture of photoelectric cells. Its high heat capacity and conductivity make it useful as a heat transfer medium. It is lighter than water and can be cut with a knife at room temperature. It reacts with water to give hydrogen and sodium hydroxide, so it is an extremely active chemical which easily unites with oxygen. The metal is, therefore, usually kept immersed in an inert liquid for safety. It is used in some nuclear reactors, and sodium lights give that characteristic yellow illumination to many of our roads and towns. This works by gaseous sodium glowing in a tube which has voltage passed through it.