The Laws of Definite Proportions and Multiple Proportions

The Laws of Definite Proportions and of Multiple Proportions

Although, theoretically, alchemy was the pursuit of the Philosopher’s Stone, the alchemists did a great deal of science over the centuries, slowly amassing a store of knowledge that powered early chemistry and is still, occasionally, in use today. (For example, many “recipes” for home-made pigments as well as solutions for etching and non-silver photo processes, involve compounds with names like “Liver of Sulfur”. Many of these names are hold-overs from the days before chemistry became a science and long before there was any systematic way of naming compounds.)

Many of the most important contributions alchemy passed on to chemistry were founded on the careful and consistent measurements that began to dominate the work. Specifically, two laws were generated that helped John Dalton “rediscover” the atomic theory.

The Law of Definite Proportions

The first of these laws was the law of Definite Proportions (also called the Law of Constant Composition). Alchemists had found ways to decompose certain compounds. In the process, they discovered an interesting fact. When they took apart a given compound, they found that the ratio of the elements that made it up was always the same. This would be like taking apart a bunch of chocolate chip cookies and finding that they always contain 13.9g of chocolate. Given that (in the world of cooking) a cookie may end up with more or fewer chips than another, that would be surprising and odd. Now imagine that this was true of EVERY chocolate chip cookie in the world (no matter who made it or what recipe they used).

Other types of cookies would, of course, have different amounts of chocolate, but chocolate chip cookies would always be the same.

The full statement of the Law of Constant Composition is “When a compound is made of two different elements, the mass of the second element that combines with 1 g of the first element is always the same.” 

In other words, the amount of oxygen (relative to hydrogen) in water is always the same, no matter where that water came from or how it formed.

The Law of Multiple Proportions

The Law of Multiple proportions is somewhat murkier, but equally important. Let’s start with the cookie analogy used above. An alchemist decomposes three different cookies: a chocolate chip cookie, a chocolate sandwich cookie and a chocolate chocolate chip cookie. She discovers that the chocolate chip cookie contains 13.9g of chocolate, the sandwich cookie has 20.85g of chocolate and the chocolate chocolate chip cookie has 41.7g of chocolate. 

Those numbers may not seem like much to you, but our alchemist does a little thinking and decides to divide each number by the smallest (13.9). What she discovers is that they form a simple ratio

13.9 : 20.85 → 2:3

13.9 : 41.7 → 1:3

20.85 : 41.7 → 1:2

Things have suddenly gotten weird. It seems that no matter what kind of cookies you look at, the amounts of chocolate in them are related in a simple ratio.

It turns out that compounds work the same way. There are LOTS of compounds that contain nothing by hydrogen and carbon but, in all of them, the amount of C (relative to hydrogen) is always in a simple ratio (like 1:2 or 3:5, etc.)

The full statement of the Law of Multiple Proportions is “ When two elements combine to form 2 or more different compounds, the amounts of the second element that reacts with 1 gram of the first element are always in a simple ratio.”