Sheesh! What a mess of attempts of explaining! Former teacher of chemistry here.
Organic chemistry definition:
It was believed a few hundred years ago ago that certain chemicals that contained carbon had a 'life force' that came from the living things they had come from. So chemistry was split into 2 fields: organic chemistry which looked at these chemicals that had a 'life force', and inorganic chemistry which looked at the other chemicals. As not all carbon containing chemicals appeared to have come from living things, not all carbon containing compounds were seen as 'organic' and therefore must be inorganic. For a long time these were seen as 2 completely different branches until a series of experiments by Wohler showed that chemicals that are 'organic' can be formed from reactions of purely 'inorganic' chemicals. Even so, the name has stuck (and there's a lot of rivalry between organic & inorganic chemists!)
Biology is the study of living organisms and their habitats. Biochemisty is the study of the roles of chemicals in living systems. Organic chemistry is the study of chemicals primarily consisting of Carbon and Hydrogen, although there may be a fair overlap between the latter two.
Homologous series:
A group of compounds that have
- the same general formula (CnH2n+2 etc etc)
- similar structural formulae,
- show similar chemical properties,
- and show a trend in physical properties (rising melting + boiling pts, decreasing flammability, increasing viscosity etc)
Saturated:
The word in all its meanings means 'full', eg a saturated towel is so full of water it can't hold any more, or a saturated salt solution can't dissolve any more sugar.
When it comes to hydrocarbons, it means 'full of hydrogen'. An alkane is 'full of hydrogen' as there are no more positions for a hydrogen to be placed, ie all carbons are already making 4 single bonds. (For saturation, it is purely the C-C bonds that are looked at. Any C=O or similar present is ignored) If a hydrocarbon is 'unsaturated' it means it isn't 'full of hydrogen', ie more hydrogen atoms could be added. Google an image of ethene and ethane to see the difference for those unsure. Ethane has no more positions to place a hydrogen (ie saturated). Ethene has a double bond, which, if broken, would allow another pair of hydrogen atoms to be placed; one hydrogen added to each carbon. The same is true of ethyne which is H-C=C-H where the '=' sign here should have 3 lines, ie a triple bond. It too can have more hydrogen added, and is therefore 'unsaturated'.
So, to summarise:
- Organic: chemicals containing mainly C and H, found primarily in living things (or their breakdown products: crude oil and coal, for example)
- Saturated: an organic chemical where all carbon to carbon bonds are _single_ bonds (the compound is 'full')
- Unsaturated: an organic compound that contains at least one carbon-to-carbon multiple bond. It may be any number of double or triple bonds. This compound is not 'full' of Hydrogen
For the person who mentioned 'methylene', it's probably not the best thing to mention, as for that matter you would have been just as well pointing out to the person that made the point about single bond hydrogen that Hydrogen is capable of more than one bond (in B2H6, 2 of the hydrogens are making 2 bonds each and are quite happy to do so) so even though interesting things exist, mentioning them is not always the best idea. It more often than not serves to confuse people.
I hope that this has helped.