Law of Conservation of Mass - Examples, FAQs
Imagine a burning candle made of wax and a wick. As the candle burns, the amount of solid wax becomes less. At first glance, it may seem that some mass is lost. However, the wax does not disappear. Instead, it changes into gases such as carbon dioxide and water vapour, which mix with the air. Thus, the total mass remains the same before and after burning. This idea is explained by the Law of Conservation of Mass, which states that matter is neither created nor destroyed, only transformed. Although ancient Greek scientists discussed similar ideas, the law was clearly stated and proved by the French chemist Antoine Lavoisier. Hence, it is also called Lavoisier’s Law.
Law of Conservation of Mass
The Law of Conservation of Mass states that "mass can neither be created nor be destroyed in a reaction." The total mass shall always be retained from the start of the reaction till the end.
According to this law, matter cannot be created nor destroyed, also known as the law of indestructibility of matter.
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Law of Conservation of Mass Formula
The formula for the law of conservation of mass is given by,
$
\begin{aligned}
& \frac{\delta \rho}{\delta t}+\nabla \cdot(\rho v)=0 \\
& \rho \text { is density } \\
& \mathrm{t}=\text { time }
\end{aligned}
$
$\nabla$ = divergence
$v=$ flow velocity
Law of Conservation of Mass Examples
- Combustion process: Wood burning is a mass conservation process because it contains oxygen, carbon dioxide, water vapor, and ashes.
- Chemical reactions: To make one molecule of H2O (water) with a molecular weight of 10, hydrogen with a molecular weight of 2 is combined with oxygen with a weight of 8 preserving mass.
Law of Conservation of Mass – Solved Examples
Example 1: Hydrogen reacts with oxygen to form water.
If $\mathbf{2}$ g of hydrogen reacts with $\mathbf{1 6}$ g of oxygen, find the mass of water formed.
Solution:
According to the law of conservation of mass,
Total mass of reactants = Total mass of products
Mass of reactants $=2 \mathrm{~g}+16 \mathrm{~g}=18 \mathrm{~g}$
Answer:
Mass of water formed = $\mathbf{1 8 ~ g}$
Example 2: When $\mathbf{3 ~ g}$ of magnesium burns completely in oxygen, it forms magnesium oxide.
Find the mass of oxygen used if $\mathbf{5}$ g of magnesium oxide is formed.
Solution:
Mass of reactants = Mass of products
Oxygen used $=5 \mathrm{~g}-3 \mathrm{~g}=2 \mathrm{~g}$
Answer:
Mass of oxygen used = $\mathbf{2}$ g
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Frequently Asked Questions (FAQs)
Atoms are neither generated nor destroyed during a chemical reaction. To generate products, the atoms of the reactants are simply rearranged. As a result, in a chemical reaction, there is no change in mass.
The Big Bang is the ultimate source of energy in our current cosmos. At the beginning of time, all of the energy was created, and as the cosmos expanded, numerous stages of particulate matter were produced from that energy.By the time of the Modern Universe, energy had been dispersed as mass, kinetic energy, chemical energy in lumps of matter, or radiant energy. Galaxies and stars inside them are used to classify the masses. The sun is one of those stars, and it received its energy from the Big Bang.
Because mass is changed into energy or vice versa, nuclear reactions appear to violate both the Laws of Conservation of Mass and Energy.
The law of conservation of mass holds that no mass is generated or lost during a chemical process. The mass of a carbon atom does not change as it transitions from a solid to a gas. The law of conservation of energy, on the other hand, maintains that energy cannot be created or destroyed.