Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Gay Lussac's Law of Gaseous Volumes
Gay Lussac's Law of Gaseous Volumes
Gay Lussac’s Law of Gaseous Volumes
In 1808, Joseph Louis Gay Lussac published a law that described a striking pattern in the way gases react. He had been studying reactions between gases — for example, the combination of hydrogen and oxygen to form water vapour — and noticed that the volumes of the reacting gases and their products, when measured at the same temperature and pressure, always stood in simple whole-number ratios.
This was not a coincidence. Gay Lussac’s experiments showed that 100 mL of hydrogen combine with 50 mL of oxygen to give 100 mL of water vapour — exactly the display the book prints:
The volumes of hydrogen and oxygen which combine (100 mL and 50 mL) bear a simple ratio of 2:1. Similar simple ratios appeared in many other gas-phase reactions. Gay Lussac's discovery of integer ratios in volume relationships is really the law of definite proportions by volume — the law of definite proportions stated earlier was with respect to mass. From these observations, Gay Lussac stated his law:
Gay Lussac’s Law of Gaseous Volumes: When gases combine or are produced in a chemical reaction, they do so in volumes that bear a simple whole-number ratio to each other, provided all volumes are measured at the same temperature and pressure.
The key condition — same temperature and pressure — is essential because the volume of a gas changes with temperature and pressure. Only when these are fixed does the volume become a direct measure of the amount of gas (as we will see later with Avogadro’s law).
The Law in Action: Examples
Consider the reaction between hydrogen and chlorine to form hydrogen chloride gas. Experiment shows that one volume of hydrogen reacts with one volume of chlorine to produce two volumes of hydrogen chloride. The volume ratio is 1:1:2 — a simple whole-number ratio.
Another example: the formation of ammonia from nitrogen and hydrogen. Three volumes of hydrogen combine with one volume of nitrogen to give two volumes of ammonia. The ratio is 3:1:2.
These results are not arbitrary. They point to a deeper relationship between the volumes of gases and the numbers of molecules they contain — a relationship that Avogadro would later make explicit.
The Limitation of Gay Lussac’s Law
Gay Lussac’s law applies only to gaseous reactants and products. It says nothing about solids or liquids. Moreover, the law is an experimental generalisation — it describes what happens, but it does not explain why the volumes combine in simple ratios. That explanation came from Avogadro’s hypothesis, which we will study next.
A common mistake is to apply Gay Lussac’s law to reactions that involve solids or liquids. The law is strictly for gases only. Also, remember that the volumes must be measured under the same conditions of temperature and pressure — otherwise the ratios will not be simple whole numbers.
The Connection to Avogadro’s Hypothesis
Gay Lussac’s law provided the experimental foundation for Avogadro’s idea. If equal volumes of gases (at the same T and P) contain equal numbers of molecules, then the volume ratios in a reaction directly reflect the ratios of molecules taking part. For example, the 2:1:2 ratio for the hydrogen–oxygen reaction means that two molecules of hydrogen react with one molecule of oxygen to give two molecules of water vapour. This is exactly what the balanced chemical equation shows:
…