A salt which loses mass when exposed to air is

The correct answer is "efflorescent". Efflorescence is a process where a salt that contains water molecules loses its water content when exposed to air. The loss of water causes the salt to crumble or turn into a powder, which can result in a decrease in its mass. This occurs because the salt is more soluble in water than in air, and the water molecules in the salt are attracted to the water molecules in the air. As a result, the water molecules in the salt are released into the air, leaving behind the dry salt. This process is commonly observed in some household items like baking soda or washing soda, which can form a white powdery substance on their surface when left exposed to air.