1.5 Laws of Chemical Combinations

The combination of elements to form compounds is governed by the following five basic laws

• Law of Conservation of Mass: This law was put forth by Antoine Lavoisier in 1789. He performed careful experimental studies for combustion reactions and reached to the conclusion that in all physical and chemical changes, there is no net change in mass during the process. Hence, he reached to the conclusion that matter can neither be created nor destroyed. This is called ‘Law of Conservation of Mass’.
• Law of Definite Proportions: This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight. Proust worked with two samples of cupric carbonate; one of which was of natural origin and the other was synthetic. He found that the composition of elements present in it was same for both the samples as shown below: Thus, he concluded that irrespective of the source, a given compound always contains same elements combined together in the same proportion by mass. It is sometimes also referred to as Law of Definite Composition.

• Law of Multiple Proportions: This law was proposed by Dalton in \(1803 .\) According to this law, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers.

For example, hydrogen combines with oxygen to form two compounds, namely, water and hydrogen peroxide.
Hydrogen \(+\) Oxygen \(\rightarrow\) Water
\(2 \mathrm{~g}  \quad \quad 16 \mathrm{~g} \quad  \quad 18 \mathrm{~g}\)
Hydrogen \(+\) Oxygen \(\rightarrow\) Hydrogen Peroxide
\(2 \mathrm{~g}  \quad \quad 32 \mathrm{~g}  \quad \quad 34 \mathrm{~g}\)
Here, the masses of oxygen (i.e., \(16 \mathrm{~g}\) and \(32 \mathrm{~g}\) ), which combine with a fixed mass of hydrogen \((2 \mathrm{~g})\) bear a simple ratio, i.e., 16:32 or 1: 2.         

• Gay Lussac’s Law of Gaseous Volumes: This law was given by Gay Lussac in 1808. He observed that when gases combine or are produced in a chemical reaction they do so in a simple ratio by volume, provided all gases are at the same temperature and pressure.

Thus, \(100 \mathrm{~mL}\) of hydrogen combine with \(50 \mathrm{~mL}\) of oxygen to give \(100 \mathrm{~mL}\) of water vapor.
\(
\begin{aligned}
&\text { Hydrogen + Oxygen } \rightarrow \text { Water }\\
&100 \mathrm{~mL} \quad \quad 50 \mathrm{~mL} \quad \quad 100 \mathrm{~mL}
\end{aligned}
\)
Thus, the volumes of hydrogen and oxygen which combine (i.e., \(100 \mathrm{~mL}\) and \(50 \mathrm{~mL}\) )  and bear a simple ratio of \(2: 1\).

Gay Lussac’s discovery of integer ratio in volume relationship is actually the law of definite proportions by volume. The law of definite proportions, stated earlier, was with respect to mass.

• Avogadro’s Law: In 1811, Avogadro proposed that equal volumes of all gases at the same temperature and pressure should contain equal number of molecules. Avogadro made a distinction between atoms and molecules which is quite understandable in present times. We see that two volumes of hydrogen combine with one volume of oxygen to give two volumes of water (Fig. 1.9) without leaving any unreacted oxygen.