The Periodic Table is an example of bringing order to a large amount of information that may seem chaotic. Before it was developed, trends between similar compounds were more difficult or impossible to visualize. There are a large number of properties which can be roughly predicted from trends in the periodic table including:
- How the elements react
- What the elements will react with
- How big the atoms are
- How the electrons are organized around the atoms.
There are several others properties that are also predicted well from the table. Mendeleev was not the first to notice the patterns but he was the first to bring an organizational scheme to the scientific community in a way that people would accept his work. In particular, he did several things differently from his predecessors:
- He chose different patterns on which to base his ordering scheme. Previous attempts at organizing the elements met with some failure because they were based on increasing atomic weight, and when two dissimilar compounds fell in the same column of the table, they went with the order based on weights, not based on properties .
- Mendeleev not only switched around molecular weights so that the properties of elements in the same column were similar, he also left spaces for undiscovered compounds when there was no in a reasonable weight range. In this way he was forward-thinking, which left room for later discoveries.
- He was able to convince people of the value of his scheme after several of the elements for which he had left "holes" were discovered.
This example shows that science benefits a great deal from the ability to organize information. Organizing information is necessary in order to generalize what is known and to generate new theories from the observed trends, and is a key step in hypothesis generation and testing.
Boyle, back in the 17th century, helped to prove that gasses have weight and that their density depends on how much pressure is applied to them. In particular he discovered Boyle's Law, which roughly says that if you double the amount of pressure applied to a gas (at constant temperature), its volume will be halved .
He did this by making use of a manometer, which is a U-shaped instrument that measures pressure by the height of a liquid that is displaced. He capped one end and poured some mercury into the other end, thus trapping air in the middle. Then he measured the pressure, and added enough mercury to halve the volume of air present in the tube . He then measured the pressure at that instant, and through many measurements, showed that volume and pressure are inversely related.
This example shows that sometimes a scientist must be quite clever to achieve a new discovery. Many groundbreaking experiments, including this one, involved the use of fairly new inventions or apparatuses which cleverly could be used to measure quantities. It is only through measurement that hypotheses can be proved.
Before Avagadro made a keen, unifying observation to the theory of gasses, scientists weren't sure how to usefully define atoms and molecules, and therefore had difficulties measuring the molecular weight of compounds. However, Avagadro was able to remedy this by hypothesizing that any gas occupying the same volume at the same temperature and pressure has the same number of molecules, not the same number of atoms, and that these molecules were made of combined elements . Scientists were later able to prove that this is true by using the theory to measure atomic weights more accurately than had been possible before, and then combining them to yield the weights of known molecular compounds.
Avagadro's theory was important because it reconciled a couple of other theories: Dalton's theory that everything is made of atoms, and Gay-Lussac's observation that the volume of gasses in a gas-phase reaction changes in proportion to the molecules of gas consumed or generated  . This type of unification is central to the advancement of science.
Pasteur and Enantiomers of Tartaric AcidEdit
Among his many accomplishments, Pasteur was one of the first people to discover that certain molecules have a property called chirality. A molecule is considered chiral if its mirror image or enantiomer is different from the original molecule. A typical physical analogue to this is a glove: you cannot put the right glove on your left hand because the thumb is the "wrong way" (unless they're specifically designed to fit both, in which case the glove is achiral).
Pasteur's experiment involved separating the enantiomers of tartaric acid from a mixture containing both. Now, enantiomers are not in general easy to separate from each other because they usually have identical physical and chemical properties, but tartaric acid is unusual because it forms crystals which are visibly different from one another in their direction . Therefore, Pasteur was able to visually separate the two enantiomers.
Once he separated, he drew upon the work of Jean Biot and shined light through each enantiomer. Biot had previously shown that, due to some unknown physical pheonomenon, some substances rotated light in one direction, some on another direction, and some did not at all. Pasteur hypothesized that this was due to the presence of the asymmetric enantiomers, and when he tested his theory he turned out to be right.
It is now known that many compounds are achiral due to the nature of carbon-carbon bonds. In particular, if a carbon has four different substituents attached to it, that carbon is chiral  Pasteur's experiment helped in both deducing structures of chiral compounds and in spurring experiments regarding their biological significance.
- Asimov, Isaac. Asimov's Guide to Science. New York: Basic Books, 1972, 230.
- [History of the Periodic Table]
- Boyle's Law demo
- Description of Boyle's measurement methods
- Avagadro's theory
- Guy-Lussac's Law
- Avagadro's Hypothesis
- Pasteur's Experiment
- http://www.chemguide.co.uk/basicorg/isomerism/optical.html#top Chiral molecules]