Atoms only change partners
Why the two laws hold.
Where do the atoms go?
Hydrogen and oxygen combine to make water. Dalton said that the atoms taking part are indivisible.
What do you think happens to the atoms in this reaction?
What this lesson covers
The idea
Dalton's theory explains both mass laws: during a chemical reaction atoms are indivisible and merely rearrange rather than being created or destroyed, so mass is conserved and elements combine in fixed proportions.
Where do the atoms go?
Hydrogen and oxygen combine to make water. Dalton said that the atoms taking part are indivisible.
What do you think happens to the atoms in this reaction?
- They join in a new way
- They are destroyed and new atoms are made
- They split into smaller atoms
Count the atoms
Press Next step to follow the atoms.
One idea explains both laws
Atoms are not made or lost, so the total mass cannot change. And each compound is built from the same atoms in the same numbers, so its elements are always in the same mass ratio.
Dalton's theory explains both laws. During a chemical reaction atoms are indivisible and merely rearrange, rather than being created or destroyed, so mass is conserved, and elements combine in fixed proportions.
Notes
In a reaction atoms merely rearrange, they are not created or destroyed. So mass is conserved and elements combine in fixed proportions.
Check yourself
Why is mass conserved in a chemical reaction, according to Dalton's theory?
A water molecule has 2 hydrogen atoms and 1 oxygen atom. Hydrogen atoms have mass 1 u and an oxygen atom has 16 u. What is the mass of one water molecule, in u?
Answer: 18 u
2 × 1 u + 1 × 16 u = 18 u.
Which two laws does Dalton's atomic theory explain?
2 hydrogen atoms and 1 oxygen atom combine to form water. How many atoms are there in the water formed?
Answer: 3
No atom is created or destroyed, so there are still 2 + 1 = 3 atoms.
- The atoms are broken into smaller pieces and then rejoined. Dalton said atoms are indivisible, so they are not broken into smaller pieces.
- The atoms only rearrange, and none is created or destroyed — correct. Yes!
- New atoms are made to replace the used ones. Atoms are not created in a chemical reaction, so nothing is replaced.
- Conservation of mass and the law of gravity. The theory explains the two laws about mass in reactions, not the law of gravity.
- Constant proportions and the boiling of water. The theory explains the two laws about mass in reactions, not boiling.
- Conservation of mass and constant proportions — correct. Yes!