Present the students with the state of knowledge of chemistry in the late 18th and early 19th century. In 1789, Antoine Lavoisier published Traité élémentaire de chimie (Elementary Treatise on Chemistry) which clearly stated the law of conservation of mass (the reactants have the same mass as the products) and denied the existence of phlogiston (fire, air, water, earth theory). Joseph Louis Proust proved and published the law of definite proportions (the masses of constituent elements always have the same proportion) in 1799. The idea that elements exist, elements cannot be decomposed, and elements combine to form compounds had just gained solid footing at this time. The list of known elements, first produced by Lavoisier, contained only 31 entries.
John Dalton considered the nature of elements by a series of his own experiments and results from other chemists. We cannot safely or quickly reproduce those experiments but we can show, in the table below, results similar to those Dalton might have obtained by 1802.
Compound | Element 1 | Element 2 | Element 3 | Mass of Element 1 (g) | Mass of Element 2 (g) | Mass of Element 3 (g) |
---|---|---|---|---|---|---|
Water | Hydrogen | Oxygen | 2.83 | 22.67 | ||
Ammonia | Nitrogen | Hydrogen | 21.00 | 4.50 | ||
Methane | Carbon | Hydrogen | 19.12 | 6.37 | ||
Prussic acid | Hydrogen | Carbon | Nitrogen | 0.94 | 11.33 | 13.22 |
Laughing Gas | Nitrogen | Oxygen | 16.22 | 9.28 | ||
Carbon Dioxide | Carbon | Oxygen | 6.95 | 18.55 | ||
Nitric Oxide | Nitrogen | Oxygen | 11.90 | 13.60 | ||
Hydrogen Sulfide | Hydrogen | Sulfur | 1.50 | 24.00 |
Ask the students to consider the table above as their "observation". Point out that Dalton probably "wallowed" in the last three columns by "normalizing" the masses of the elements in various ways, keeping in mind the "law of definite proportions". By "normalizing", I mean express the masses in different ways by multiplying every mass in a row by the same number (definite proportions). The happily wallowing students could choose to convert the masses to percentages of the total mass, make one of the masses 1 g, convert the masses to ratios, etc. Encourage the students to study their normalized values and then boldly create a hypothesis concerning the nature of elements and the way they combine with each other.
The students may have difficulty recovering Dalton's hypothesis within their own group so encourage wider, intergroup discussions. If they do not make the Dalton's giant leap, the instructor should provide strong hints about about potential normalization techniques. That is, the data should hint at expressing the masses as integer ratios as shown in the table below. Integer ratios hint at an atomic nature within elements. They also hint at different masses for different elements.
Compound | Chemical Formula | Atomic Weight of Element 1 | Atomic Weight of Element 2 | Atomic Weight of Element 3 | Ratio of Weights |
---|---|---|---|---|---|
Water | H2O | 1 X 2 = 2 | 16 X 1 = 16 | 1 : 8 | |
Ammonia | NH3 | 14 X 1 = 14 | 1 X 3 = 3 | 14 : 3 | |
Methane | CH4 | 12 X 1 = 12 | 1 X 4 = 4 | 3 : 1 | |
Prussic acid | HCN | 1 X 1 = 1 | 12 X 1 = 12 | 14 X 1 = 14 | 1 : 12 : 14 |
Laughing Gas | N2O | 14 X 2 = 28 | 16 X 1 = 16 | 7 : 4 | |
Carbon Dioxide | CO2 | 12 X 1 = 12 | 16 X 2 = 32 | 3 : 8 | |
Nitric Oxide | NO | 14 X 1 = 14 | 16 X 1 = 16 | 7 : 8 | |
Hydrogen Sulfide | H2S | 1 X 2 = 2 | 32 X 1 = 32 | 1 : 16 |
Dalton, upon studying similar data, hypothesized the following:
Once the students have Dalton's hypothesis at their disposal, ask them to devise a very general set of chemistry experiments that would clarify, extend, or disprove the hypothesis. Such experiments will not be a part of this course but might be fun as a student project.