Is Zakat obligatory upon selling a residential apartment before purchasing another, and is Zakat obligatory on an apartment that was bought to preserve wealth instead of depositing it in a bank?

The apartment bought for residential purposes is not subject to Zakat, unless it is sold and a hawl (lunar year) passes while its price constitutes a nisab (minimum amount subject to Zakat). As for the second apartment, as long as there was no intention of trading with it at the time of purchase, it is not subject to Zakat, because Zakat on trade goods requires the intention of trading at the time of acquisition.