Water of Crystallization
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flagWhat you'll discover
- arrow_forwardDefine water of crystallization and hydrated versus anhydrous salts
- arrow_forwardCarry out the heat-cool-weigh cycle to constant mass and explain why it matters
- arrow_forwardRecord masses correctly and compute the mass of water lost
- arrow_forwardConvert masses to moles and determine x in CuSO₄·xH₂O
- arrow_forwardList the main sources of error in a gravimetric experiment
Water hidden inside a crystal
Many salts crystallise with a fixed number of water molecules chemically bonded into their crystal lattice — the water of crystallization. Blue vitriol is the textbook case: CuSO₄·5H₂O, with exactly five water molecules per formula unit. This water is not dampness; it is part of the crystal's structure, present in an exact stoichiometric ratio, and the deep blue colour depends on it.
Gentle heating drives the water off: CuSO₄·5H₂O → CuSO₄ + 5H₂O, and the blue crystals collapse into a white powder of anhydrous copper sulphate. The change is reversible — add a drop of water to the white powder and it flashes blue again (and warms up!), which is why anhydrous CuSO₄ serves as a chemical test for the presence of water.
Heating to constant mass
The whole experiment hangs on one technique: heating to constant mass. Weigh the empty crucible, add about 2-3 g of the hydrated salt, and weigh again. Heat gently (strong heating decomposes CuSO₄ itself into black CuO — ruining the result), cool, and weigh. Then heat again, cool, and weigh again. Only when two consecutive weighings agree within about 0.02 g can you be sure all the water has gone.
Why cool before weighing? A hot crucible sets up convection currents around the balance pan that make the reading drift light, and heat damages the balance. Real labs cool the crucible in a desiccator — a sealed vessel with drying agent — so the anhydrous salt cannot reabsorb moisture from the air while cooling, because anhydrous CuSO₄ is hygroscopic and would silently regain mass.
From masses to the formula
The arithmetic is two mole calculations. Mass of water lost = (initial crucible + sample) − (final constant mass). Mass of anhydrous CuSO₄ = final mass − empty crucible. Then moles of water = mass lost / 18, moles of CuSO₄ = anhydrous mass / 159.6, and x = moles of water / moles of CuSO₄. With careful work x comes out close to 5.
Error analysis is where marks are won. Insufficient heating leaves water behind → x too small. Overheating to black CuO changes the residue's identity → calculation invalid. Weighing hot → mass reads low. Letting the residue stand in humid air → it reabsorbs water and x drifts low for the wrong reason. Every error should be stated with its direction — that is the difference between a pass and a distinction in the practical report.