Decahydraatti

Decahydraatti, or decahydrate, is a chemical hydrate in which ten water molecules are incorporated into the crystal structure per formula unit. The water molecules act as water of crystallization and are part of the solid’s lattice; the compound is typically written as salt·10H2O. The hydration number reflects how many water molecules accompany the anhydrous salt.

The best-known decahydrate is sodium sulfate decahydrate, Na2SO4·10H2O, commonly called Glauber’s salt. It forms colorless crystals at ordinary temperatures and can lose water upon heating, yielding the anhydrous salt or lower hydrates. Other inorganic salts can also form decahydrates under suitable crystallization conditions, making decahydrates a broader, though less common, category of hydrates.

In terms of properties, decahydrates are generally thermodynamically reversible hydrates: dehydration occurs with heating, and rehydration occurs when exposed to moisture. Stability is strongly influenced by temperature, humidity, and pressure. Because of their high water content, decahydrates are of interest in crystallography and materials science, where hydration state affects crystal structure and properties. Some decahydrates have historical or niche industrial applications, including desiccation and phase-change related uses, depending on the specific chemical involved. The hydration number of ten is a key criterion used to classify hydrates alongside other stoichiometries such as mono-, di-, and nonahydrates.

Decahydraatti is the Finnish term for this class of compounds, conveying the same chemical concept as the English decahydrate.