Investigate the cooling curve of a substance passing through a change of state from liquid to solid, observe the isothermal solidification arrest plateau, and determine the latent heat of fusion of salol (phenyl salicylate) from the duration of the solidification arrest under a known steady rate of heat loss. Explore a virtual apparatus, collect readings and analyse a graph.
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Adjust the apparatus, then take a reading.
Readings are generated by an ideal model. The diagram is schematic and is not to scale.
The teaching simulation models the phase change of pure phenyl salicylate (salol) with an ideal, flat solidification plateau at T_m = 41.50 °C under a calibrated steady heat loss rate R_loss = 270.0 J min⁻¹ (4.50 W) into an isothermal ambient air jacket. In physical laboratory practice, chemical impurities can depress the melting point and cause a slight slope on the plateau, supercooling may delay crystallization onset by several degrees if unseeded, radial temperature gradients across the viscous solid salol cause the termination of the plateau to round off gradually, and ambient room temperature drafts can induce small variations in the effective heat loss rate. The simulation evaluates the solidification plateau duration using the exact thermodynamic conservation relation Δt = (m · L_f) / R_loss, where m is the sample mass in kilograms, L_f = 79,500 J kg⁻¹ (79.50 kJ kg⁻¹) is the reference latent heat of fusion of salol, and R_loss = 270.0 J min⁻¹ (4.50 W) is the calibrated heat loss rate. Plateau durations are rounded to 0.01 min to model standard laboratory stopclock precision, and ordinary least-squares linear regression of Q_latent = R_loss · Δt against mass m provides the reported latent heat of fusion.
A thin-walled borosilicate glass test tube (25 mm diameter, 150 mm length) held coaxially inside a wider glass cylinder by a split cork ring, providing a stagnant annular air buffer that ensures uniform, draft-free convective and radiative cooling to ambient surroundings.
A short-range mercury-in-glass thermometer graduated from 20°C to 70°C in 0.1°C subdivisions, positioned centrally within the molten salol to record the temperature descent, supercooling dip, and the steady horizontal solidification plateau.
A fine nichrome wire formed into a loop encircling the thermometer stem, used to agitate the liquid gently prior to crystallization and to introduce a microscopic seed crystal to arrest supercooling promptly at 41.5 °C.
A precision digital timer recording elapsed time to 0.01 min, paired with a thermostatically controlled warm water bath used to melt the salol sample completely to approximately 60 °C before cooling runs.
Investigate the cooling curve of a substance passing through a change of state from liquid to solid, observe the isothermal solidification arrest plateau, and determine the latent heat of fusion of salol (phenyl salicylate) from the duration of the solidification arrest under a known steady rate of heat loss. Suggested independent settings: 15.0, 20.0, 25.0, 30.0, 35.0 g.
Weigh the clean, dry boiling tube together with its wire stirrer to ±0.01 g using an analytical balance. Introduce a known mass m of pure crystalline salol (e.g., 15.0 g) into the tube, reweigh the assembly to confirm the exact specimen charge, and secure the tube in a clamp ready for melting.
Accurate gravimetric determination of the sample mass is critical because the total latent heat liberated during phase transition scales directly with mass (Q = m · L_f).
Immerse the lower half of the boiling tube in a beaker of water maintained between 60 °C and 65 °C. Allow the salol crystals to melt entirely until an optical inspection confirms complete liquefaction with no residual solid crust adhering to the inner walls, reaching a uniform liquid temperature of approximately 55 °C to 60 °C.
Complete melting dissolves all pre-existing crystal nuclei, ensuring that subsequent cooling commences from an entirely homogeneous liquid state and that thermal history does not introduce erratic crystallization artifacts.
Remove the tube from the warm water bath, wipe its outer surface thoroughly dry, and seat it firmly inside the wider outer air-jacket cylinder using the split cork collar. Insert the precision thermometer through the cork so its mercury bulb is centered vertically and horizontally within the molten salol volume.
Drying the outer surface prevents evaporative cooling from lingering water droplets, while the concentric air jacket eliminates ambient air drafts and establishes a reproducible, uniform heat loss rate R_loss.
Start the digital stopclock and record the temperature of the salol every 30 seconds as it cools. As the temperature nears 41.5 °C, monitor for supercooling (where the liquid drops 1°C to 2°C below T_m without solidifying). Touch a microscopic speck of solid salol on the tip of the stirrer wire to seed nucleation, observing the immediate thermal rebound to the steady arrest temperature T_m = 41.5 °C.
Salol readily supercools in clean glassware. Prompt seeding initiates crystallization at the true thermodynamic transition point, preventing prolonged metastable liquid states from corrupting the plateau duration measurement.
Observe the thermometer reading during the crystallization process. Note the precise time t_start when the temperature stabilizes at 41.5 °C and the time t_end when all liquid has transformed into crystalline solid and the temperature resumes a rapid downward decline. Evaluate the plateau duration Δt = t_end − t_start in minutes.
The duration of the horizontal temperature plateau represents the exact interval during which the rate of latent heat liberation matches the steady rate of heat loss to the jacket (Q_latent = R_loss · Δt).
Repeat the measurement for five distinct masses of salol (15.0 g, 20.0 g, 25.0 g, 30.0 g, and 35.0 g). For each run, compute the total heat liberated Q_latent = R_loss · Δt in joules. Plot Q_latent against mass m (kg), perform an ordinary least-squares fit through the origin, and determine the latent heat of fusion L_f from the gradient.
Testing multiple sample masses distributes instrumental timing and mass reading uncertainties across the regression line, verifying the direct proportionality Q = m · L_f and suppressing random errors.
Points are your observations. The fit uses SI x and y values before display scaling.
| # | Mass of salol m (g) | Solidification arrest duration Δt (min) | Mass of substance m (kg) | Total latent heat liberated Q_latent (J) | Remove |
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