Saturday, April 20, 2013

Ancient Elements

Only about a dozen elements were known to the people living in ancient civilizations.

  • Copper beads dating from about 6000 B.C. have been found in Turkey.
  • A lead statuette found in an Egyptian temple dates from around 3800 B.C. and golden artefacts have also been found in ancient Egyptian tombs.
  • Silver was used by the ancient Greeks and Romans to prevent infection, and was used as an early form of currency.
  • There is evidence of the systematic production of iron in Turkey around 2000 B.C. for use in tools and weapons.
  • Carbon, in the form of diamonds, was also known in the Ancient world, but, carbon in the form of charcoal was far more important to these early people because it could be used in the production of copper, tin, and therefore bronze (an alloy of copper and tin).
  • Sulfur was also known to the Ancient Egyptians and Greeks, who used it as a medicine.
  • The ancient Chinese, Indians and Egyptians also knew about mercury, using it in ointments and cosmetics.
  • Before 1000 B.C., Indians were extracting zinc from its ores. Ornaments containing 80% or more of zinc have been found.
  • During the Bronze Age (an earlier period than the Iron Age), arsenic was included in bronze to make the alloy harder. Ancient people understood that they could produce arsenic by heating certain substances (which we would now call arsenic sulfides and oxides).
  • Antimony was used in cosmetics in the Ancient world, notably by the Egyptians who used it around their eyes (known as kohl).
  • Chromium has been found in ancient Chinese artifacts. The weapons the Chinese Terracotta Army carry are coated in chromium oxide and date from around the 3rd century B.C.
In fact, these were really the only elements known  until the 17th century A.D.
Today we know of over a 100 elements.
Chemistry has come a long way in the last few hundred years!

Further Reading:
History of the Elements
Metal Extraction Concepts
Periodic Table
Metals and Non-metals
Pure Substances and Mixtures

Suggested Study Questions:
  1.  Find each of the elements mentioned above on the Periodic Table.
  2. Draw up a table of the name and chemical symbol for each of the elements mentioned above.
  3. Draw up a table classifying each of these elements as metals, non-metals or semi-metals (metalloids).
  4. Name a mixture mentioned in the article above.
  5. Name a compound mentioned in the article above.
  6. There are only 3 metallic elements that are not a "silvery" colour. One of these is cesium (or caesium), but if the sample of cesium is very pure it loses its golden colour. Name the other two non-silvery metallic elements.
  7. Explain why you often find weapons like swords made of iron, but you never find functional weapons made of gold.
  8. Explain why you find ornamental weapons made out of gold, but rarely out of zinc.
  9. Ancient people could have produced mercury by heating cinnabar (mercury(II) sulfide). The cinnabar decomposes, producing liquid mercury and sulfur. Write a word equation for the decomposition of cinnabar.

Wednesday, April 17, 2013

Phlogiston Theory

Before the modern chemical ideas of atoms and elements, phlogiston theory was a widely held belief.
According to phlogiston theory, matter consisted of three essential essences:
  • sulfur (terra pinguis - the essence of inflammability, which was to become known as phlogiston)
  • mercury (terra mercurialis - the essence of fluidity)
  • salt (terra lapida - the essence of fixity and inertness)
 Phlogiston theory was an early attempt to to try to explain what happened when things were burnt or combusted. In phlogiston theory, substances were made up of a "calx" (or residue) combined with phlogiston (the essence of inflammability). When a substance was burnt (combusted), phlogiston was released, and the residue (calx) was left behind. Even the rusting of iron could be explained using phlogiston theory, because the "iron" would lose its phlogiston during the rusting process and leave behind the "calx" or residue. One of the problems with the phlogiston theory is that metals should lose mass when they burn, owing to the loss of phlogiston.

In the 18th century, Antoine-Laurent de Lavoisier, the man who is considered to be the father of modern of modern chemistry, conducted a series of combustion experiments. In these experiments he carefully weighed the substances to be combusted as well as the products of combustion, and found that the weight of the products of combustion was greater than the weight of the substance before combustion. He also demonstrated that when a substance corrodes in a sealed container, the gain in weight of the substance is equal to the loss in weight of the air in the container.
This was the beginning of the downfall of the phlogiston theory, but it was the beginning of the modern chemistry when Lavoisier generalized that if the weights of all substances involved in a chemical reaction are considered then there is no overall loss or gain in weight.

Further Reading:
http://www.ausetute.com.au/elemhist.html
http://www.ausetute.com.au/elements.html
http://www.ausetute.com.au/atomichist.html
http://www.ausetute.com.au/wriiform.html
http://www.ausetute.com.au/namiform.html
http://www.ausetute.com.au/namcform.html
http://www.ausetute.com.au/balcheme.html

Suggested Study Questions
  1. Lavoisier burnt sulfur. When sulfur burns in air, it reacts with oxygen to form sulfur dioxide. Write a word equation for this reaction.
  2. Write the chemical formula for each of the following:
    • sulfur
    • oxygen gas
    • sulfur dioxide gas
  3. Write a balanced chemical equation for the combustion of sulfur to produce sulfur dioxide gas.
  4. If Lavoisier had weighed out 32 grams of sulfur and then burnt it in air, the sulfur dioxide he collected would have a mass of 64 grams. What mass of oxygen would have been reacted with the sulfur? 
  5. Explain why a substance such as sulfur appears to gain mass when it is combusted.
  6. When wood is burnt, the mass of the ashes left behind is actually less than the mass of wood you started with. How do you explain this loss of mass?
  7. Magnesium is a metal that combusts readily in oxygen gas to form magnesium oxide.
    • Write a word equation for the combustion of magnesium to form magnesium oxide,
    • Write a balanced chemical equation for this reaction.
  8. If 20.16 grams of magnesium oxide is produced as a result of the combustion of 12.16 grams of magnesium, how much oxygen gas was consumed during the reaction?
  9. Imagine you are living in the 18th century and that you are a firm believer in the phlogiston theory. How would you explain to Lavoisier how metals gain mass when they burn?

Monday, April 1, 2013

Calculations for Strong Bases

AUS-e-TUTE has just added new resources!
As part of the update of our "Acids and Bases" topic, tutorials, games, tests, and exams have been added for the following:
  • pOH Concepts
  • Strength of Bases
  • Calculating the pOH of Strong Bases (aqueous solutions)
  • Calculating the Hydroxide Ion Concentration of Strong Bases (aqueous solutions)
  • Calculating the pH of Strong Bases (aqueous solutions)
  • Calculating the Hydrogen Ion Concentration of Strong Bases (aqueous solutions)