Hydrated & Anhydrous Salts
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
What is Water of Crystallization? Water of crystallization is water that is chemically bonded into the crystal structure of a salt. This water is part of the compound and is included in the chemical formula.
When some salts crystallize from solution, water molecules become trapped in the crystal lattice and become part of the structure. Not the Same as Wet! Water of crystallization is not the same as surface water or moisture.
It is chemically bonded within the crystal structure and can only be removed by heating. Hydrated Salts Hydrated salts are salts that contain water of crystallization in their crystal structure. The water molecules are bonded to the metal ions.
Common Hydrated Salts Salt Formula with Water Color Common Name Copper(II) sulfate CuSO₄·5H₂O Blue crystals Blue vitriol Cobalt(II) chloride CoCl₂·6H₂O Pink crystals - Iron(II) sulfate FeSO₄·7H₂O Green crystals Green vitriol Magnesium sulfate MgSO₄·7H₂O White crystals Epsom salt Sodium carbonate Na₂CO₃·10H₂O White crystals Washing soda Calcium sulfate CaSO₄·2H₂O White powder Gypsum Dot Notation The dot (·) in the formula shows that water is loosely bonded to the salt.
It's not the same as covalent or ionic bonding in the salt itself. Example: Copper(II) Sulfate Pentahydrate CuSO₄·5H₂O copper(II) sulfate pentahydrate Contains 5 molecules of water of crystallization per formula unit Forms bright blue crystals The water is essential to the crystal structure and color "Penta" means five Anhydrous Salts Anhydrous salts are salts that contain no water of crystallization .
"Anhydrous" means "without water". Anhydrous salts are formed when hydrated salts are heated strongly, driving off the water of crystallization. Common Anhydrous Salts Salt Anhydrous Formula Color Formed From Copper(II) sulfate CuSO₄ White powder CuSO₄·5H₂O (blue) Cobalt(II) chloride CoCl₂ Blue powder CoCl₂·6H₂O (pink) Sodium carbonate Na₂CO₃ White powder Na₂CO₃·10H₂O (white crystals) Calcium sulfate CaSO₄ White powder CaSO₄·2H₂O (gypsum) Color Change Many hydrated salts change color when they become anhydrous.
This color change can be used as a test for water. Heating Hydrated Salts The Dehydration Reaction When a hydrated salt is heated, the water of crystallization is driven off, leaving the anhydrous salt: General equation: Hydrated Salt → Anhydrous Salt + Water Example: Heating Copper(II) Sulfate CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g) blue crystals → white powder + water vapor Observations: Blue crystals turn white Steam (water vapor) is given off Droplets of water may condense on cooler parts of the tube The mass decreases (water is lost) The solid becomes powdery The Reverse Reaction (Rehydration) When water is added back to anhydrous copper(II) sulfate, it becomes hydrated again: CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s) white powder + water → blue crystals Observations: White powder turns blue A lot of heat is released (very exothermic) May produce a hissing sound Test for Water Anhydrous copper(II) sulfate is used as a chemical test for water .
If water is present, the white powder turns blue. Comparing Hydrated and Anhydrous Salts Property Hydrated Salt Anhydrous Salt Water content Contains water of crystallization No water of crystallization Formula Includes ·nH₂O (e.g., CuSO₄·5H₂O) No water in formula (e.g., CuSO₄) Appearance Usually well-formed crystals Usually powder Color (CuSO₄) Blue White Color (CoCl₂) Pink Blue Relative mass Higher (includes water) Lower (no water) Stability Stable at room temperature May absorb water from air Formation Crystallization from solution Heating hydrated salt Naming Hydrated Salts The number of water molecules is indicated by a Greek prefix: Number of H₂O Prefix Example 1 Mono- CuSO₄·H₂O = Copper sulfate monohydrate 2 Di- CaSO₄·2H₂O = Calcium sulfate dihydrate 3 Tri- - 4 Tetra- - 5 Penta- CuSO₄·5H₂O = Copper sulfate pentahydrate 6 Hexa- CoCl₂·6H₂O = Cobalt chloride hexahydrate 7 Hepta- MgSO₄·7H₂O = Magnesium sulfate heptahydrate 10 Deca- Na₂CO₃·10H₂O = Sodium carbonate decahydrate Calculating Water of Crystallization We can use mass measurements to calculate the number of water molecules in a hydrated salt.
Method Weigh the hydrated salt Heat it to drive off all the water Weigh the anhydrous salt Calculate the mass of water lost Use mole calculations to find the ratio Example Calculation Q: 12.5 g of hydrated copper(II) sulfate (CuSO₄·xH₂O) is heated.
After heating, 8.0 g of anhydrous copper sulfate remains. Calculate the value of x. A: Step 1: Find mass of water lost Mass of water = 12.5 - 8.0 = 4.5 g Step 2: Calculate moles of CuSO₄ Mr of CuSO₄ = 64 + 32 + 4(16) = 160 Moles of CuSO₄ = 8.0 ÷ 160 = 0.05 mol Step 3: Calculate moles of H₂O Mr of H₂O = 2 + 16 = 18 Moles of H₂O = 4.5 ÷ 18 = 0.25 mol Step 4: Find the ratio Ratio = moles of H₂O : moles of CuSO₄ = 0.25 : 0.05 = 5 : 1 Answer: x = 5, so the formula is CuSO₄·5H₂O Key Point The ratio tells us how many molecules of water are associated with each formula unit of the salt.
Testing for Water…
Interactive revision notes, videos and practice questions load below.