I was wondering if someone can further explain to me what the "Purity" skill for Alchemy does. It says All negative effects are removed from created potions and all positive effects are removed from created poisons. Can someone give me an example or explain this more in depth?

Say you make a poison with a damage health affect, but a restore magicka. If you have the purity perk, the restore magicka gets removed because it would buff the enemy, not poison him.

It does exactly what the description says.
Most ingredients have both positive and negative effects. Two or more positive effects of the same kind make a potion. Negative effects make poison. It's always the strongest effect that determines if it is a poison or a potion.
With Purity you no longer get the negative effects when mixing potions and the positive effects when mixing poisons. But you could just get the same result by picking better combinations of ingredients.

Not a good perk if you use alchemy as a source of revenue, as quite bizarrely the unwanted effects add to the value, so Purity makes certain combinations worth less. Great if you only take one path, playing as a poison using assassin or healer, however.

Yeah, that confused me too. I would think people would pay less for a potion that cures your disease, heals you 50, and causes 40 points of lingering poison damage than they would for a potion that cures your disease and only heals you 20.

Has been this way since Morrowind, it had no poisons so negative effects was always unwanted.
However value is determined by effect not usefulness.

If you get alchemy high enough for this perk you probably has enough money or to many potions
The other question is if its an useful perk: is it many good potions with common ingredients with unwanted effects?
Note an poison who add non combat benefit like fortify smithing work just as well.

thanks everyone