90 86 84 pure liquid 80 first crystals appear 76- 74 72 70 freezing pure solid t
ID: 1026698 • Letter: 9
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90 86 84 pure liquid 80 first crystals appear 76- 74 72 70 freezing pure solid time, min Figure 1 Cooling curve for pure naphthalene 80 78 76 74 72 70 68 temperature only liquid is present plus solid true freezing first crystals 246 8 10 12 14 16 18 time, min Figure 2 Cooling curve for a solution of para-dichlorobenzene in naphthalene properties are freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. We can relate the freezing point depression of a pure solvent, At, to the molality of the solution (m), as shown in Equation 1: Atf-tsolvent-tsolution = Kern where fsolvent is the freezing point of the pure solvent; tsolution is the freezing point of the solvent from the solution: and K is the molal freezing pointExplanation / Answer
In the first curve, pure naphthalene freezes at a constant rate as it has formation of intermolecular forces occuring from liquid to solid phase change.
In the second curve, when para-dichlorobenzene is added, it disrupts the intermolecular bonds present between the naphthlene molecules and thus causes a lowering of freezing point of the resulting solution. Therefore, we do not have a constant freezing curve in this case. The amount of disruption of intermolcular forces is different at different locations in solution causing a not so smooth cooling curve.
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