Hydrated salts and the tests for water sim

What the sim shows

Blue copper(II) sulfate crystals contain water of crystallisation, CuSO₄·xH₂O. Heat them gently and stir, and the water leaves as steam, leaving white (greyish-white) anhydrous copper(II) sulfate: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Cool in a desiccator before weighing (a hot crucible reads 0.02–0.04 g low, and powder cooled in air takes back 0.01–0.03 g of water), then reheat, cool and reweigh until two masses agree within 0.01 g: constant mass means all the water has gone. With Mᵣ CuSO₄ = 160 and H₂O = 18, 18.43 / 20.93 / 20.03 g gives 1.60 g of CuSO₄ (n = m ÷ Mᵣ = 0.0100 mol) and 0.90 g of water (0.0500 mol), a ratio of 1 : 5.00, so x = 5. A roaring flame heats the bottom of the dry powder to about 600–700 °C, where copper(II) sulfate itself breaks down: CuSO₄ → CuO + SO₃. Black copper(II) oxide appears, the mass keeps falling and x comes out too big; spitting while it is still wet loses solid too. Stopping before constant mass, or cooling in air, makes x too small. Water dropped back on the white powder turns it blue and makes it hot (ΔH = −79.3 kJ per mol of CuSO₄): the reaction is reversible. On bench 2, water turns white anhydrous copper(II) sulfate blue and blue cobalt(II) chloride paper pink. These tests show that water is present, not that it is pure: only pure water boils at 100.0 °C and melts at 0.0 °C (at 1 atm). Salt solution passes both tests but boils at about 101.0 °C and melts at about −3.5 °C (ΔT = i K m). Model limits: the drying rates (two first-order pools, the first four H₂O at 0.55 per min and the last at 0.25 per min on a gentle flame), the temperatures in the crucible, the uptake in air, the spitting and the balance scatter (sd 0.003 g, read to 0.01 g) are the desk's estimates. Each heating click is one minute of heating, shown in about 3 s; cooling takes about 10 minutes and does not count as heating time. Really the water leaves in stages (four below about 150 °C, the last near 250 °C). Wear eye protection; cobalt(II) chloride paper is toxic, so handle it with forceps.

• Balance display, read to 0.01 g
• Copper(II) sulfate: blue CuSO₄·5H₂O, white CuSO₄, black specks of CuO if overheated
• Porcelain crucible: moved by tongs from the bench to the balance, triangle and desiccator

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