Efflorescence

Efflorescence is the loss of water of crystallization from a hydrated solid exposed to air. It can leave a powdery deposit as the solid becomes anhydrous or less hydrated.

Water of crystallization: Water molecules incorporated into a crystal’s structure in a definite proportion. Efflorescence releases this structurally bound water from hydrated crystals.
Crystal water: Water molecules held within a crystalline solid’s structure. Its departure is the defining material change in efflorescence.
Sodium carbonate decahydrate: The hydrated sodium carbonate Na₂CO₃·10H₂O, commonly known as washing soda. It loses water in air and can crumble into a powdery, less hydrated solid.
Gravimetric analysis: A quantitative chemical analysis method that determines an amount from mass measurements. Mass loss can reveal how much water a hydrate releases.
Deliquescence: The absorption of atmospheric moisture by a solid until it dissolves in the absorbed water. It reverses the direction of moisture exchange: the solid takes up water instead of losing it.
Anhydrous compound: A compound containing no water as part of its chemical composition. Some efflorescing hydrates become anhydrous compounds after losing their water.
Epsom salt: Magnesium sulfate heptahydrate, MgSO₄·7H₂O, commonly used in baths and gardening. Its hydration state can change as it exchanges water with its surroundings.
Thermogravimetric analysis: A technique that measures a sample’s mass as it is heated or cooled over time. It measures staged water loss and helps distinguish hydrate transitions.
Relative humidity: The ratio of water vapor present in air to the maximum it can hold at that temperature. It helps determine whether a hydrate loses water to the surrounding air.
Stoichiometry: The quantitative relationships between substances in a chemical reaction or compound. Hydrates contain characteristic ratios of water that can change during efflorescence.

Linked from 12 pages