Law of Chemical Combination

Chemistry is considered to be an opening to the hidden interior of the world. Everything we know and perceive today, be they the rocks or the plants, consists of various components. All these components have been explained with the application of chemistry. Sometimes, chemistry is said to be ‘halfway’ between physics and biology.

This is because it holds a strong presence in explaining various aspects of both subjects. As we know, chemistry has its laws revolving around atoms, ions, compounds, and metals. One of the important laws that define chemical compounds and their reactions is the law of constant composition.

The definition of the law of constant composition lays the basis for the existence of compounds in chemistry. Understanding the law of constant composition gives us an in-depth look at how we can combine compounds. Let us understand this law concerning compounds in detail. Along with this, we will also look at an example of the law of constant composition. 

What is the Law of Constant Composition?

The definition of the law of constant composition was formulated by Joseph Proust. He was a French chemist and had discovered the law of constant composition after a string of experiments that he led between 1798 and 1904. After the experiments, he deduced that compounds have particular compositions.

During the time of Proust’s discovery of the law of constant composition, several other theories were emerging too. For instance, John Dalton was initiating his theory, wherein he concluded that a particular element has one type of atom. 

As per the law of constant composition, pure compounds consist of the same elements in identical mass proportion. In a generic sense, a compound has combinations of the same elements with the same propionate mass. So, regardless of how a compound is extracted or made, it will always have elements in the same mass and ratio.

The law of constant composition is also referred to as the law of definite proportions. Here, definite proportions are of the elements that are mixed to formulate the compound. 

The law of constant composition forms the foundation for stoichiometry, along with the law of multiple proportions. Stoichiometry refers to the quantitative relation of products and reactants in a particular chemical reaction. This again is done in terms of the masses, ratios and volumes. 

According to this law of constant composition, the aim is to look at the nearest ratios of elements in the compound. Further, this law is also used to find out whether the same elements are combined in making the specific compound. 

Law of Constant Composition: an example

To know the practicality of the law of constant composition, we can consider examples. Here is a law of constant composition example to help us understand the subject better. 

For instance, pure water that is obtained from a river or a pond is taken to the laboratory to illustrate the law of constant composition. While proving the law, pure water (H2O) is found to be composed of the same elements as water. Here, the ratio between hydrogen and oxygen is 1:8. Therefore, this implies that the mass ratio between elements of pure water (hydrogen and oxygen) is the same. 

Let us look at another example involving the extraction of carbon dioxide.

This can be processed by burning carbon. Further, it can also be done through heating of limestone (CaCO3) or applying HCL (diluted) to marble. By practising any of these, carbon dioxide is extracted. Again, by applying the law of constant composition definition, all the methods would give in CO2. The elements to give CO2 (carbon and oxygen) will be in the same ratio, that is, 3:8. 

Thus, the law of constant composition implies that regardless of compounds being made or extracted, the ratio of their elements stay unchanged.

Exceptions to the Law of Constant Composition

The law of constant composition does not apply to all the compounds. For instance, wustite is a type of iron oxide where the law isn’t applicable. This is because it is a non-stoichiometric compound. This means that this compound has a varied composition when combined differently. Similarly, any kind of non-stoichiometric compound is not subject to the law of constant composition. 

There might also be chances of less reliable compounds that are usually explainable through the law. This is because atoms of elements in a compound consist of varied isotopes. This would result in variation of the compound that is formed. It also relies on what isotopes of the atoms are active. 

Again, the law of constant composition does not work if the isotopes of the elements formulate the compound. Hence, this law of constant composition is inapplicable for isotopes and non-stoichiometric compounds. 

Conclusion

The field of chemistry has given us several laws to understand the inner workings of the world. One of these laws is the law of constant composition. This law was pioneered by Joseph Proust. A French chemist, Proust came up with the law after experimenting for years. 

The law of constant composition definition implies that pure compounds consist of identical elements in identical mass proportion. That is, the elements used to make the compound or are naturally in it, are of the same mass or ratio. This law of constant composition is also known as the law of definite proportions. This law and the law of multiple proportions are the basics of stoichiometry. 

There are, however, exceptions to the law of constant composition. It is not applicable when isotopes of an element constitute the compound. Further, it is also not applicable for the non-stoichiometric compounds.