Law of Mass conservation and Law of Definite Proportions

Let's say that you've built a house out of Legos. After completing your masterpiece using blocks of different sizes, shapes and colors, you found that you're whole house weighs 1 pound. After marveling at your creation for a bit, you decide you want a bridge instead of a house, so you use all the pieces from the house and make yourself a bridge. How much would this bridge weigh? It seems intuitive to say that the bridge would weigh 1 pound also. Since you used the same blocks used in the house, their weights would have to be equal.

This is he basic idea that describes the Law of Mass Conservation.



Back in the late 1700s, a man named Antoine Lavoisier discovered, after discovering that oxygen was key in combustion, that in chemical reactions the sum of the mass of the reactants equals the sum of the mass of the products. That is, to say, that the weight of what goes in equals the weight of what comes out. Let's take a look at an example.

2H₂ + O₂ --> 2H₂O

If you take 2g of hydrogen and react it with 16g of oxygen, then if you don't let anything escape the reaction chamber you will get 18g of water.

Observations like these led to the formation of the Law of Mass Conservation, which states:

Mass is neither created nor destroyed in chemical reactions

Further observation of reactions led to the formation of the Law of Definite proportions, which says

Different samples of a pure chemical substance always contain the same proportion of elements by mass 

Taking a look back at our equation earlier with this in mind, H₂O always has 1 part H to 8 parts O by mass. CO₂ always has 3 parts C to 8 parts O by mass.


These two laws led to some of the early theories about what atoms were.