Mass is conserved because, in a closed system, matter is neither created nor destroyed, only transformed.
Here's a more detailed explanation:
The Law of Conservation of Mass
The law of conservation of mass is a fundamental principle in physics and chemistry. It states that for any closed system, the mass must remain constant over time. A closed system means that no matter can enter or leave.
Explanation Based on Atoms
Mass conservation fundamentally boils down to the behavior of atoms:
- Atoms are the building blocks of matter: Everything around us is made of atoms.
- Atoms are not created or destroyed in ordinary chemical reactions: Chemical reactions involve the rearrangement of atoms into different molecules.
- Because the same number and type of atoms are present before and after a reaction, the total mass remains constant. The "reactants" (starting materials) might change into "products" (new substances), but the underlying atoms stay the same.
Example: A Simple Chemical Reaction
Consider the reaction of hydrogen gas (H2) and oxygen gas (O2) to form water (H2O):
2H2 + O2 → 2H2O
- Reactants: 2 molecules of hydrogen (each with 2 hydrogen atoms) and 1 molecule of oxygen (with 2 oxygen atoms).
- Products: 2 molecules of water (each with 2 hydrogen atoms and 1 oxygen atom).
Notice that:
- We start with 4 hydrogen atoms and 2 oxygen atoms.
- We end with 4 hydrogen atoms and 2 oxygen atoms.
Since the number and type of atoms are the same on both sides of the equation, the mass must be conserved.
Exceptions and Clarifications
- Nuclear Reactions: The law of conservation of mass, as described above, applies to chemical reactions. In nuclear reactions, mass can be converted into energy (and vice-versa), as described by Einstein's famous equation E=mc2. Therefore, mass is not strictly conserved in nuclear reactions, but the total mass-energy is conserved.
- Closed Systems: The conservation of mass only holds true in closed systems where no matter enters or leaves. If you were to burn wood in an open fireplace, the mass of the ash would appear to be less than the mass of the original wood. However, this is because some of the matter has escaped as gases (carbon dioxide, water vapor, etc.). If you could capture all of these gases, you would find that the total mass of the ash and gases equals the original mass of the wood.
In Summary
The conservation of mass is a direct consequence of the fact that atoms are neither created nor destroyed in chemical reactions; they are only rearranged. This fundamental principle is a cornerstone of our understanding of chemistry and physics.
