Change partners
Atoms never change or vanish. They swap partners.
Hydrogen burns in oxygen
Two hydrogen molecules (H₂) and one oxygen molecule (O₂) react and make two water molecules (H₂O). Count: 4 hydrogen atoms and 2 oxygen atoms to start with.
After the reaction, how many hydrogen and oxygen atoms are there in total?
What this lesson covers
The idea
In a chemical reaction atoms of one element never change into another element, disappear or appear; instead, bonds between atoms are broken and made to produce new substances.
Hydrogen burns in oxygen
Two hydrogen molecules (H₂) and one oxygen molecule (O₂) react and make two water molecules (H₂O). Count: 4 hydrogen atoms and 2 oxygen atoms to start with.
After the reaction, how many hydrogen and oxygen atoms are there in total?
- 4 hydrogen and 2 oxygen: the same
- Fewer, because some are used up
- More, because water is new
Snap and join
First snap every bond (tap it). The atoms drift to new places. Then make the new bonds (tap the dotted lines).
Same atoms, new partners
In a chemical reaction, atoms of one element never change into another element, and atoms never disappear or appear. Bonds between atoms are broken and made to produce new substances.
Think of children holding hands. In a game they let go, change partners and hold hands again. Nobody joins or leaves the game.
You will study the kinds of bonds in Chapters 3 and 4.
Notes
In a chemical reaction atoms of one element never change into another element, disappear or appear; instead, bonds between atoms are broken and made to produce new substances.
Check yourself
In 2H₂ + O₂ → 2H₂O, what happens to the hydrogen atoms?
In CH₄ + 2O₂ → CO₂ + 2H₂O, how many hydrogen atoms are there after the reaction (in 2H₂O)?
Answer: 4
2 × 2 = 4 hydrogen atoms: the same 4 that were in CH₄.
Which sentence describes a chemical reaction correctly?
- They change into oxygen atoms. Atoms of one element never change into another element.
- They stay hydrogen atoms and join oxygen atoms — correct. Yes! Only the partners change.
- They disappear when water forms. Atoms never disappear. The 4 hydrogen atoms are inside the 2 water molecules.
- Atoms of one element change into atoms of another. This never happens in a chemical reaction.
- Some atoms disappear and some new atoms appear. No atoms disappear or appear. They are rearranged.
- Old bonds are broken and new bonds are made — correct. Yes! That is how new substances are produced.