Final answer:
Hydrates easily lose water when heated due to the increased kinetic energy of the water molecules that breaks the hydrogen bonds holding them, known as dehydration. Conversely, they regain water when exposed to moisture because water's adhesive properties facilitate the absorption of water molecules back into the hydrate structure.
Step-by-step explanation:
Hydrates are compounds that have waters of hydration, which are loosely bound water molecules integrated into their structure. These water molecules are held in place through weak interactions such as hydrogen bonds and electrostatic attractions. When hydrates are heated, the added thermal energy increases the kinetic energy of the water molecules. This increase in kinetic energy overcomes the strength of the hydrogen bonds keeping the water molecules in place, leading to the loss of water in a process called dehydration.
The high heat of vaporization of water means that it requires a lot of energy to break these bonds, but, once they break at a certain temperature, the water molecules can easily escape from the hydrate structure. Conversely, when exposed to moisture, the hydrate can regain water because of water's adhesive properties, where it 'crawls' along hydrophilic surfaces, and its high heat capacity, which allows it to absorb heat energy gradually, pulling water molecules back into the structure through capillary action or direct absorption.
The variation in colors between hydrated and anhydrous forms of compounds is often a visual indicator of this property. The phenomenon is particularly noticeable in transition metal hydrates, which can change color upon loss or gain of water.