Synthesis and Decomposition: Atoms Rearrange

0:00When substances react, what happens to their atoms?

0:04In this model, each ball is one atom: white is hydrogen and red is oxygen.

0:11Atoms joined by chemical bonds form a molecule.

0:16Reactants are the substances we start with: here, two hydrogen molecules and one oxygen molecule.

0:24Count the atoms: four hydrogen and two oxygen.

0:29In the reaction, the bonds between the atoms break.

0:33The atoms rearrange and form new bonds, making two water molecules.

0:39Water is the product, the new substance made.

0:43Count again: still four hydrogen and two oxygen.

0:48Joining two or more substances into one new substance is a synthesis reaction: A plus B gives AB.

0:57Sodium and chlorine also join this way, making sodium chloride.

1:02Now reverse it. An electric current splits water: this is called electrolysis.

1:09The bonds break, and the atoms rearrange into hydrogen and oxygen.

1:16Breaking one substance down into two or more simpler ones is a decomposition reaction: AB gives A plus B.

1:25Hydrogen peroxide also decomposes, into water and oxygen.

1:30A common mistake is to think atoms are destroyed when a substance breaks down.

1:36But all four hydrogen and two oxygen atoms are still there, just rearranged.

1:42In both synthesis and decomposition, atoms are only rearranged, never created or destroyed.