States of Matter

Aim

• Determine the solubility (g per 100 g water) of a soluble salt, typically copper(II) sulfate or potassium nitrate, at a stated temperature

Apparatus

• Evaporating basin
• Boiling tube
• Water bath (a heated beaker of water will substitute)
• Thermometer
• Balance reading to 0.01 g
• Bunsen burner, tripod, gauze
• Stirring rod
• The chosen soluble solid

Method

1. Weigh the empty evaporating basin; record its mass
2. Half-fill a boiling tube with water and warm it in the water bath to just above the target temperature (say, just above 30 °C)
3. Add the chosen solid in small portions, stirring after each addition, until further additions stop dissolving and a small amount of solid residue settles on the base of the tube
4. Cool the boiling tube to exactly the target temperature (30 °C), stirring gently; let the residue settle
5. Decant the clear saturated solution into the pre-weighed basin (leaving the residue behind in the tube); reweigh the basin
6. Heat the basin gently to evaporate the water; reduce the heat before the basin runs dry, to prevent spitting
7. Heat to constant mass: cool, weigh, reheat, cool, reweigh, repeating until two consecutive masses agree
8. Calculate the solubility from the recorded masses

Sample calculation

• Mass of solution: 90.00 − 30.00 = 60.00 g
• Mass of solute: 50.00 − 30.00 = 20.00 g
• Mass of water: 60.00 − 20.00 = 40.00 g
• Solubility at 30 °C = (20.00 / 40.00) × 100 = 50.0 g per 100 g of water
• Mass % of solute in the solution = (20.00 / 60.00) × 100 = 33.3 %

Sources of error

• Transferring undissolved residue into the basin → overestimates solubility; decant carefully, or filter at the target temperature
• Loss of solid by spitting during evaporation → heat gently near dryness; finish on residual heat
• Solution cooling below the target temperature before decanting → solubility falls; less solid in the decanted liquid; decant promptly
• Incomplete drying → trapped water inflates the apparent solute mass; always heat to constant mass