Which statement best describes hydrated salts?

• A. They contain water of crystallisation within their crystal structure
• B. They never contain water
• C. They produce water on heating only from the salt solution
• D. They lose water when dissolved in water

Answer: (A)

Hydrated salts have water molecules bonded within their crystal structure, called water of crystallisation. This is why they are written with a dot and a number, like a salt with a specific number of water molecules attached. Those water molecules are not just moisture; they are part of the solid’s lattice.

When you gently heat a hydrated salt, the water molecules are driven off as steam, leaving behind the dry, anhydrous salt. This dehydration is the key idea: the water is integral to the solid form and is removed by heating.

That’s why describing hydrated salts as containing water of crystallisation within their crystal structure is the best answer. The other statements don’t fit because hydrated salts do contain water, so claiming they never contain water is incorrect; the release of water happens upon heating, not just from heating in a solution; and dissolving the hydrated salt in water doesn’t define the concept—the essential point is the water is part of the crystal and can be removed by heating.