What happens when you put bleach and toothpaste in a envelope together?

My dentist recommended this today, specifically to whiten teeth, he said that a mixture of bleach and ground pumice must whiten teeth as well as any over-the-counter potion as (presumably for liability reasons) the Clorox must be far stronger than anything anyone dares to sell in the US AND the pumice must be far more abrasive. Personally I don't dare try bunging this sort of mixture up my gob until someone tells me generally what it tastes like.