What's the difference between an aldehyde and a ketone?

Both aldehydes and ketones contain a double bond between carbon and oxygen.

Aldehydes have the double bond at the end of the molecule. That means the carbon at the end of the chain has a double bond to an oxygen atom.

Ketones have the double bond anywhere in the molecule except for the end. That means you will see a double bond to oxygen from one of the carbon atoms in the middle of the chain.

If you've got a solution and you don't know if it's an aldehyde or a ketone, you can use Tollen's Reagent to help. You can add some of the reagent to your solution and if you see a silver colour, there is aldehyde present. Tollen's Reagent has the formula [Ag(NH₃)₂]NO₃ and it can oxidise aldehydes but not ketones! If you add Tollen's Reagent to a ketone, nothing will happen.