Why those two hold
Count the atoms before a change. Count them after. The counts match.
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The rule
Atoms separate, then join in new groups.
Matter is made of tiny atoms. A chemical change only rearranges atoms. Each compound is built from the same atoms in the same count. So its ratio stays fixed.
This is Dalton's atomic theory. One idea explains two laws: conservation of mass and constant proportions.
- Eight hydrogen atoms and four oxygen atoms, scattered and unjoined.
- Every two hydrogen join one oxygen. Four water units form.
- Count the atoms again. The same eight and four are there.
One run, two laws
Atom masses use a tiny unit called u. One hydrogen atom is 1 u. One oxygen atom is 16 u.
One water unit has 2 hydrogen and 1 oxygen. So it is 2 × 1 + 1 × 16 = 18 u.
| Atoms | Mass | |
|---|---|---|
| Before | 8 hydrogen, 4 oxygen | 8 × 1 + 4 × 16 = 72 u |
| After | 4 water units | 4 × 18 = 72 u |
Law 1 is conservation of mass. It says the mass stays 72 u. No atom is made or destroyed.
Law 2 is constant proportions. It says the ratio stays fixed. A water unit has 2 u of hydrogen, 16 u of oxygen. Divide both by 2 to get 1 : 8.
Take 4 g of hydrogen and 32 g of oxygen. They make 36 g of water, as 4 + 32 = 36. That total is law 1. The split 4 : 32 is 1 : 8, which is law 2.