Aldehydes

a second hydrogen atom or, more commonly, a hydrocarbon group which might be an alkyl group or one containing a benzene ring

a third hydrogen atom with another R group

one hydrogen atom with two R groups

by oxidising primary alcohols

by a reflux experiment

By heating up a ketone

Yes

No

Only short chain aldehydes

An aldehyde differs from a ketone by having a hydrogen atom attached to the carbonyl group. This makes the aldehydes very easy to oxidise.

An aldehyde and a ketone are very similar and act in the same way when you try to oxidise them

Ketones have that hydrogen atom and are not resistant to oxidation.

usually more reactive toward nucleophilic substitutions than ketones because of both steric and electronic effects.

atomically extremely heavy

do not oxidise