[martainthebox]

we use H+/CrO7(-2 ion) and it reduces to Cr3+. to show a colour change from orange to green. But i don't understand why it becomes Cr3+ and why there's a hydrogen infront of the potssium dichromate. Is it because it's acidified potassium dichromate? Also.. why won't a tertiary alcohol oxidise with potassium dichromate?
and finally... when we have a primary alcohol it oxidises to an aldehyde but then it can be oxidised again to form a carboxylic acid.. why does this happen?

could you be so kind to answer all my questions? thank you!!

[nj_bartel]

If you were to oxidize a tertiary alcohol, what would your product be?

Aldehydes continue to oxidize because they're a very reactive species - moreso than the alcohol.

[Borek]

Is it because it's acidified potassium dichromate?

Yes. You need to acidify it to increase the redox potential.

[UG]

Also.. why won't a tertiary alcohol oxidise with potassium dichromate?

Now, just think about this for a minute, a tertiary alcohol has an -OH group attched to a carbon atom which has three other carbon atoms attached to it. Aldehydes and ketones have the carbonyl group C=O, carbon can only form four covalent bonds, so if a tertiary alcohol was oxidised, you will end up with 5 bonds (3 single, 1 double) around the carbon atom.

[UG]

when we have a primary alcohol it oxidises to an aldehyde but then it can be oxidised again to form a carboxylic acid.. why does this happen?

This is because aldehydes will react with even mild oxidising agents. To prevent further oxidation of an aldehyde, for example ethanal to ethanoic acid, the ethanal is removed from the reaction vessel as it forms, this is done by warming the mixture to ~35 degrees and collecting the condensed liquid (fractional distillation).