The following is the edited text of a winning essay.
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A Winning Essay


Silver Nitrate

by Matthew DeSouza, Lester B. Pearson Catholic High School, Gloucester, Ontario




A tourist pauses to block out all sounds around him, taking in the beauty of the Caribbean sunset. As many tourists around him do, he pulls out a camera, and captures the moment forever, blissfully unaware of the scientific marvel that has just taken place in his hands.

"In our present state of knowledge, it cannot be done," stated l9th-century chemist Jean Dumas, when asked to comment on production of permanent pictures from images produced by a lens, "but I cannot say it will always remain impossible, nor set the man down as mad who seeks to do it." Indeed, as an outstanding chemist of the period, Dumas was aware that chemists had been experimenting for many years with "light-reactive" compounds. In the 17th century, Italian Angelo Sala reported that certain compounds turn "black as ink" when exposed to the sun, and in 1727, German scientist Johann H. Schulze noticed, during an unrelated experiment, that the sun had a darkening effect on a flask of solution that he had inadvertently left outside for a few minutes. After placing the solution in a hot, dark oven, and noticing no change, he determined that light, not heat was responsible for this reaction.

The light-sensitive reactant in these early experiments is silver nitrate, (AgNO3), a very easily produced compound. One can make silver nitrate by dissolving silver in nitric acid, and evaporating the solution, resulting in a mass of transparent crystals. The compound is soluble in water, alcohol, and acetone.

The reaction that takes place in a camera is nothing short of remarkable. Although black and white film is usually only about 0.013cm thick, it is composed of several layers. There are several layers for protection and support, as well as one that prevents light from being reflected back into the film, but the most crucial layer is called the emulsion layer. This layer is composed of the light sensitive crystals, and a gelatin substance that suspends them in order to react with light.

A negative is formed when millions of exposed crystals are converted to silver metal by the developer. Thus, the areas struck by the most light are darkened by the metallic silver, while areas not struck by light remain transparent, as they have no silver content. Intermediate areas are varying shades of gray, depending on the amount and colour of the light striking the film.

The emulsion layer of most modern film is composed of silver halides, such as silver iodide, silver bromide, and silver chloride, which participate in the same type of light sensitive reaction as silver nitrate. It is fitting, however, that silver nitrate, the building block of the photography industry, is the building block in the manufacturing process of these silver halides. For example, silver bromide is made from silver nitrate and a potassium bromide solution.

Being the cornerstone of a multi-billion dollar industry such as photography, silver nitrate is certainly deserving of recognition as a truly remarkable chemical. Incredibly, the versatile compound has done much more than snap pictures. The compound has equally amazing applications in medicine. Doctors use silver nitrate to cauterize wounds, preventing bleeding or infection, as well as for removal of warts. When diluted, a mild solution of silver nitrate can be used to treat eye and skin diseases, and as an antiseptic. In fact, the eyes of millions of babies across North America every year are treated with a 1% solution of silver nitrate to destroy harmful gonococcal bacteria. This process is actually required in some U.S. states as a precautionary measure to prevent possible blindness!

Besides the wondrous uses in photography and medicine, there are other practical uses for silver nitrate. It is used to make mirrors, indelible ink, and in silver plating. It is also used along with electricity in the purification process of precious silver.

In summation, it is clear that silver nitrate is surely worthy of worldwide fame, if not for the key role it has played in the photography industry throughout the centuries, or for its impressive uses in medicine, at least for its sheer versatility.

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