If i had 3 plates 2 metals and 1 glass .I put them together to form a basic parallel plates capacitor. After charging it and approaching it to an electroscope nothing happened that's because electric field outside a capacitor =0 . But what happens if i have separated the 3 plates , will the metal plates keep its charge?why? What happens when i approach them to the electroscope?
It will depend on how you separate the plates and whether you have disconnected the charger too.
If you handle the plates with non conducting gloves, separated they will keep their charge until it dissipates to the air due to the existing small conductivity of the air. With bare hands prepare yourself for a shock. Until the charge dissipates the electroscope will see the field they create over the plates. The charges will distribute themselves on both sides of the plate.
This link shows the field for single metal plate and when in a capacitor.