Monday, September 26, 2011

Sunken Silver

In 1941, the British cargo ship SS Gairsoppa was carrying 7,000 tonnes of cargo from Calcutta. The cargo included pig iron, tea and about 200 tonnes of silver. A German U-boat torpedo sunk the SS Gairsoppa as it made its way to Ireland in stormy weather. While only one person out of the 85 crew members on board survived the attack, it is believed its cargo of silver is still at the bottom of the ocean.

Silver is unique because it has the highest electrical and thermal conductivity of any known element. It is a soft metal, just a little harder than gold, and is extremely ductile and malleable meaning it can be bent or beaten into almost any shape.
There were many uses for silver during World War II :
  • many electrical connectors and switches were silver plated
  • silver bus bars were needed for the new aluminum plants (aluminium aircraft)
  • silver replaced large amounts of tin in solder
  • silver was used in the reflectors in lights
There are a number of silver alloys:
  • Fine silver contains 99.9% by mass silver
  • Britannia silver contains 95.84% by mass silver with copper making up the remaining mass
  • Sterling silver contains 92.5% by mass silver and 7.5% by mass copper.
  • Argentium sterling silver is a modern alloy containing 92.5% silver and 7.5% by mass of copper and germanium
  • Electrum is a natually occurring alloy of gold and silver. The % by mass of gold can be within the range of 70-90%.
And, silver is present in most coloured carat gold alloys:
  • 9 carat gold contains 62.5% silver and 37.5% gold
  • 22 carat gold contains 91.7% gold with the remaining mass made up of silver and/or copper

Silver is stable in pure air and water, but tarnishes when exposed to air or water containing ozone or hydrogen sulfide. In the presence of oxygen gas and hydrogen sulfide gas, elemental silver forms the dark-coloured silver (I) sulfide and water.

Shipwreck of SS Gairsoppa reveals
£150m silver haul
BBC News Online

Further Reading
Properties of Metals and Non-metals
Percent by Mass
Writing Ionic Formula
Balancing Chemical Equations

Suggested Study Questions
  1. For the element silver, give the
    • chemical symbol
    • atomic number (Z)
    • atomic mass
  2. For each of the uses given for silver in the article above, explain which physical and/or chemical properties of silver make it ideal for that use.
  3. Using the composition of the various silver alloys provided in the article above, place the alloys in order of increasing mass of silver present in a 1kg sample.
  4. Using the information contained in the article above, describe the relationship between the mass of gold in a sample and the use of the term carat.
  5. Place the following terms in order of decreasing mass of gold: 9 carat gold, 18 carat gold, 22 carat gold and 24 carat gold.
  6. Write a word equation to describe the process of tarnishing in air that contains some hydrogen sulfide.
  7. Write a balanced chemical equation for the tarnishing of silver in air that contains some hydrogen sulfide.
  8. It is thought that the silver being carried by the SS Gairsoppa contained some gold. Which alloy of silver could this be? Explain your answer.
  9. The SS Gairsoppa is resting 4,700m below the ocean's surface. Do you expect the silver to be tarnished? Explain your answer.

No comments:

Post a Comment