The key difference between law of conservation of mass and law of constant proportion is that the law of conservation of mass explains that matter cannot be created or destroyed but converted into something else, whereas the law of constant proportion explains that a given chemical compounds always contain the same element in the exact same proportions by mass.
Law of conservation of mass and law of constant proportion are both laws in chemistry that are related to mass.
CONTENTS
1. Overview and Key Difference
2. What is the Law of Conservation of Mass
3. What is the Law of Constant Proportion
4. Law of Conservation of Mass vs Law of Constant Proportion in Tabular Form
5. Summary – Law of Conservation of Mass vs Law of Constant Proportion
What is the Law of Conservation of Mass?
The law of conservation of mass is a law that states that in a closed system, which is closed to all the transfers of matter and energy, the mass is constant over time. This means that the mass of the system cannot change over time, so the quantity can neither be added nor removed. In other words, the mass is conserved over time. It is also known as the principle of mass conservation.
According to this law, the mass can neither be created nor destroyed though it can be rearranged in space. Moreover, the entities associated with it can change forms. E.g. mass of the reactants in a chemical reaction is equivalent to the mass of the products. Therefore, in chemical reactions and low-energy thermodynamic processes that occur in isolated systems, the total mass of reactants is equivalent to the mass of products.
We can formulate the law of conservation of mass in classical mechanics where the energy scales associated with an isolated system are comparatively smaller than mc2 (m is the mass of an object, and c is the speed of light). This has to be measured in the frame of reference where the object is resting.
What is the Law of Constant Proportion?
The law of constant proportion asserts that a given chemical compound can always be composed of its component elements in a fixed ratio by mass. This does not depend on its source and method of preparation. E.g. by mass, water has about 8/9 oxygen and 1/9 hydrogen.
However, this law is not universally true. For example, there are non-stoichiometric compounds having an elemental composition that can vary from sample to sample. These compounds tend to follow the law of multiple proportions. For example, iron oxide wustite can contain iron atoms in the range of 0.83 to 0.95 for every oxygen atom (this means it is non-stoichiometric). Although its ideal chemical formula is FeO, the crystallographic experiments give it as Fe0.95O.
Additionally, the isotopic composition of an element varies depending on the source; therefore, it even contributes to the mass of a pure stoichiometric compound that can vary. This variation is useful in radiometric dating.
What is the Difference Between Law of Conservation of Mass and Law of Constant Proportion?
Law of conservation of mass and law of constant proportion are both laws in chemistry that are related to mass. The key difference between law of conservation of mass and law of constant proportion is that the law of conservation of mass asserts that matter cannot be created or destroyed but converted into something else, whereas the law of constant proportion asserts that a given chemical compounds always contain the same element in the exact same proportions by mass.
Below is a summary of the difference between law of conservation of mass and law of constant proportion in tabular form for side by side comparison.
Summary – Law of Conservation of Mass vs Law of Constant Proportion
The key difference between law of conservation of mass and law of constant proportion is law of conservation of mass explains that matter cannot be created or destroyed but converted into something else, whereas the law of constant proportion explains that a given chemical compounds always contain the same element in the exact same proportions by mass.
Reference:
1. “Proust’s Law of Constant Proportion.” Chemistry LibreTexts, Libretexts, 11 Sept. 2022.
Image Courtesy:
1. “Combustion reaction of methane” By JyntoRobert A. RohdeJacek FHJynto – Methane-3D-space-filling.svgOxygen Molecule VdW.pngCarbon-dioxide-3D-vdW.svgWater molecule 3D.svg (Public Domain) via Commons Wikimedia