The following way your tooth can whiten is by drying out the tooth
. To help understand, I compare this to wet concrete. When concrete is moist, it is darker, and when concrete is dry, it is whiter.
While not as dramatic of a color change, your tooth is whiter when it is dry. When the tooth is wet, it is darker.
It may seem silly for me to cover tooth dehydration because the whitening results reverse within a few minutes. However, this point is essential to understand to avoid teeth whitening gimmicks. Acids will dry out your tooth. So if you put a supposed whitening agent that a blogger or YouTuber told you to put on your tooth, which is acidic like a strawberry or lemon, it will dry out your tooth and make it look whiter. This whiter tooth shade will last for however long it takes your tooth to rehydrate. It usually takes a few minutes to an hour.
It doesn’t stop there. Sometimes the gimmick tooth whitening agent will cause you to stay open for a while, so if saliva isn’t touching your teeth and you are breathing, this will dry out the tooth. Almost any whitening technique will whiten your teeth more than it appears at first because you keep your mouth open, and your saliva stays away from your teeth during the process.
The temporary shift in color is a problem for us dentists too. For example, we will do a crown on a tooth, but if the patient has been open a long time, we may not notice the surrounding teeth are dry and whiter. So if we note the shade for the new tooth while the surrounding teeth are drier and whiter and don’t realize the change, then when we get the crown back, it will be much whiter than the actual color of the surrounding teeth.