定义Definition
结晶水是以一定比例结合在晶体中的水分子，含结晶水的化合物称为水合物，化学式中用点连接，如 CuSO₄·5H₂O、Na₂CO₃·10H₂O。加热可使水合物失去结晶水，常伴随颜色或晶形变化。
Water of crystallisation is water bound in a crystal in a fixed ratio; compounds containing it are hydrates, written with a dot, as in CuSO₄·5H₂O and Na₂CO₃·10H₂O. Heating drives the water off, often with a change of colour or crystal form.
硫酸铜的例子Copper(II) sulfate
蓝色的 CuSO₄·5H₂O 加热后变为白色无水 CuSO₄；白色无水硫酸铜遇水又变蓝，因此可用来检验水的存在：CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O。
Blue CuSO₄·5H₂O turns into white anhydrous CuSO₄ on heating, and the white powder turns blue again with water, which makes it a test for water: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O.
测定结晶水的数目Finding the number of waters
加热至恒重，由失去的质量求水的物质的量。例：2.50 g 水合硫酸铜加热后剩 1.60 g 无水 CuSO₄，失水 0.90 g = 0.050 mol；n(CuSO₄) = 1.60 ÷ 159.6 ≈ 0.0100 mol，比值为 5，化学式为 CuSO₄·5H₂O。
Heat to constant mass and work out the moles of water from the mass lost. 2.50 g of hydrated copper(II) sulfate leaves 1.60 g anhydrous CuSO₄, so 0.90 g = 0.050 mol of water is lost; n(CuSO₄) = 1.60 ÷ 159.6 ≈ 0.0100 mol, a ratio of 5, giving CuSO₄·5H₂O.
风化Efflorescence
某些水合物在干燥空气中自发失去部分结晶水，晶体表面变成粉末，称为风化，如 Na₂CO₃·10H₂O。
Some hydrates lose part of their water spontaneously in dry air, the crystal surface turning powdery; this is efflorescence, shown by Na₂CO₃·10H₂O.