A Historical Look at Experimentation
Historically experiments have played a major role in providing scientific information concerning the world around us. This is especially true in the field of chemistry. The experimental approach to investigating substances or phenomena dates back to the 1600’s. Scientists like Evangelista Torricelli, Otto von Guericke, and Robert Boyle proved that air has pressure through their experiments. Torricelli showed air could support a column of mercury about thirty inches. He did it by placing mercury inside a long narrow tube (longer than thirty inches) and inverting it into a bowl of mercury. The column of mercury dropped to a level of thirty inches above the level of mercury in the bowl.8 In 1654 Otto von Guericke, after inventing an air pump, sealed two metal hemispheres together. He then removed the air from between them. He demonstrated the force of air pressure by trying to pull the hemispheres apart with horses. They were unsuccessful. However when he allowed air to re-enter the hemispheres, they fell apart.9 (The outside pressure was counter acted by the air pressure inside.) Robert Boyle went one step further and showed that air could be compressed. In his apparatus, he used a long J shaped tube, closed at the short end. After trapping air in the short end of the tube, Boyle observed the effects of increasing and decreasing the amount of mercury in the tube. He discovered the volume of the trapped air would either decrease or increase depending on the addition or removal of mercury, respectfully. This led to the formation of Boyle’s law, which states that the volume of a fixed amount of gas varies inversely with the pressure of the gas.
One of the most celebrated chemists was, the French scientist Antoine Lavoisier (1743 -1794). Among his many accomplishments, he was known for his precise experimentation and making extremely careful measurements. He also gave very detailed documentation of his experiments. Examples of his writings can be seen in Great Books of the World, vol. 45 Lavoisier, Fournier, Faraday. Up until his time careful measurements were not done; sometimes resulting in erroneous conclusions. (Unfortunately because of his status in society, he met an untimely death with the guillotine.)
In this part of the unit two of the experiments of Lavoisier will be studied. They will include his disproving the theory that water was the source of earth, and the discovery of the composition of air. Following Lavoisier, the unit will end with the students researching the famous 1909 experiment by Ernest Rutherford which led to the discovery of the atomic nucleus. In each of the experiments (either Lavoisier’s or Rutherford’s) students will be looking for four things. These are the purpose of the experiment, how it was done, the results of the experiment, and the conclusions.
Experiments of Antoine Lavoisier
The first experiment students will examine is the one that showed earth does not come from water. This idea came from the earlier philosophy the all matter came from four basic elements: earth, fire, air, and water. This belief dates back to the time of the early Greeks, approximately 300 BC. The idea of water being a source for some forms of earth was reinforced by the process of distillation. Early scientists observed that after distilling a liquid, a residue was always found in the container that held the original liquid. This was true even when only water was the liquid. They concluded from this evidence that water was a source for some forms of solid material. This is the background setting for the account of Lavoisier’s experiments given in Tom McGowen’s book, Chemistry, The Birth of a Science.10 McGowen begins by saying Lavoisier didn’t believe that water was changing into this residue. Lavoisier used a specially designed glass container to boil water in this experiment. The container was constructed so that as steam rose from the boiling water to the top of the container, it would condense and return to the bottom of the container. This would prevent the water from evaporating out. In order to insure the accuracy of his experiment, Lavoisier cleaned and weighed the empty container before filling it with the “purest water he could find”.11 Next he weighed the container with the water in it. He was able to get the weight of the water by subtracting the weight of the empty container from the weight of the container with water. Once this was done, Lavoisier proceeded to boil the water until he got enough residue that he could weigh it. This took a little over three months. After allowing the container to cool, Lavoisier removed the water and the residue from the container. he weighed the water and the residue separately. he dried and cleaned the empty container; then weighed it again. His results were as follows:
1. The weight of the empty container was a little less than its original weight.
2. The water weighed the same as it did before boiling.12
3. The weight of the residue was almost equal to the weight loss of the glass container.
He concluded that the residue came from the glass and not the water, since it was the container that lost weight.
The information on this experiment will be given to the students in the form of a simulated newspaper article written in Paris, France; during the year of 1770. Students will read the article, then answer the questions. The questions will be reviewed in a discussion. The article and the lesson plans for this section are included at the end of the unit.
The second experiment of Lavoisier is the one from which he made the discovery of the composition of air. In trying to understand combustion, many of the earlier scientists would burn materials and make observations. Sometimes what would be left after the material was burned was a powdery substance called calx.. It could be formed from metals as well as nonmetals. Scientists also discovered that when the calx was heated it changed back into its original substance and a gas was released.
Lavoisier set up a two part experiment to investigate these phenomena. The following is a summary of his account from the book Great Books of the World, vol 45 Lavoisier, Fournier, Farady. During the first part he heated a certain measured amount of mercury in a special flask. The flask had a long neck bent to feed into an empty bell shaped jar. The jar was placed upside down in a bowl like container of mercury. This served as a closed container.13 As the mercury heated up a red calx appeared. At the end of twelve days no more red calx had formed. Once the flask was cool , Lavoisier measured the amount of air left in the flask. He found it to be about one sixth less than the initial amount. He also tested the air for its ability to support combustion and respiration. This was done by placing a lighted candle inside the air. The candle went out. Animals were used for respiration. When placed inside the air they “suffocated”. This experiment was repeated several times yielding the same results. Lavoisier termed the left over air from the formation of the mercury calx “noxious air”or the “non-respirable parts of air”14 Later this part of air once carbon dioxide was removed from it, would be called nitrogen.
In the second part of this experiment, Lavoisier took the red calx formed from the mercury and heated it. He used a container that was designed to receive gases that might form . It had two parts, one in which the calx was placed and the other that received the gas. As the calx was heated it disappeared and mercury formed in the receiving end of the container. A gas was also collected in this section. Lavoisier tested the gas for combustion with a lighted candle, charcoal, and phosphorous. The candle burned brighter. Instead of burning quietly without a flame, the charcoal burned noisily with a flame. The phosphorus burned with a blinding light. Animals were found to breath easier when they were place in the gas. Lavoisier called this gas “highly respirable air”. Later he changed the name to “vital air”.15 This gas was oxygen. Lavoisier concluded that air was composed of two gases which were of “different and opposite qualities”.16
As previously done for the first example, the information for this experiment can be given to the students in the form of a hand out. After reading the handout students will give a written summary of what was done in each part of the experiment. Next they will make a list of the results. Finally they will explain Lavoisier’s reasoning for his conclusion.
The historical background for Ernest Rutherford’s 1909 experiment with alpha particles and gold foil can be found earlier in this unit under the introduction section of Gaining Indirect Information by Experimentation. A detailed description of the actual experiment is also given in that section.
An excellent account of this experiment is found in Prentice Hall’s Chemistry: The Study of Matter, pages 119-121. Students are to read this account. As stated before, there are four objectives for studying each historical experiment. They are determining the purpose, procedure, results, and conclusion of each experiments. These can be met either through a discussion format or written assignment. In addition, students are to copy the drawing of the apparatus used on page 120 of this book. If this book is not accessible, another text should be consulted that would provide the same information.