When I freeze water with some impurities (1% NaCl by mass), the mixture seems to have a much lower enthalpy of fusion that pure water. The impurities cause the freezing point to depress to 0.6°C. I only have a simple calorimeter available but it looks like the enthalpy of fusion is dropping by 25%+ compared to pure water (tested by melting the ice into water and measuring the temperature drop).

Additionally, the ice does not have a nice crystal structure. It is "mushy" and easily deformed.

This effect is surprising to me since colligative theory tells us that most properties changes in a pretty small linear way at low concentrations (like the freezing point).

Am I observing a real phenomenon or is there some problem with my testing?

If its a real phenomenon, is there a way to calculate/estimate the effect?

  • $\begingroup$ are you mesuring change in freezing point, $\Delta T_f$ or change in enthalpy of fusion $\Delta H_f$ ? $\endgroup$ – RE60K Apr 23 '15 at 16:48
  • $\begingroup$ @ADG Both. The change in freeze point matches theory nicely- $\Delta T_f = i*K_b*m = 0.6°C$. I back-calculated the enthalpy of fusion from $m_{water}*c_p*(T_f-T_i) = m_{ice}*h_f$ (where the final temp is 0C so the ice has no sensible energy change.) $\endgroup$ – ericksonla Apr 23 '15 at 17:43
  • $\begingroup$ ok the change in freezing point is matching but not the heat of fusion? $\endgroup$ – RE60K Apr 23 '15 at 17:44
  • $\begingroup$ Thats the question. I don't know of a way to calculate the enthalpy of fusion of a salt/water mixture to compare the experimental value against. Is there a formula for this? $\endgroup$ – ericksonla Apr 23 '15 at 17:45
  • $\begingroup$ Maybe 1% isn't "low" concentration $\endgroup$ – Babounet Apr 24 '15 at 8:07

According to the article An Empirical Correlation between the Enthalpy of Solution of Aqueous Salts and Their Ability to Form Hydrates (Pandelov et al. 2010), your observation of a lower enthalpy is correct:

We have found a correlation between the enthalpy of solution of the salt and its ability to form a hydrate, namely, that the salt’s enthalpy of solution is lower than the standard enthalpy of fusion of ice (6 kJ/mol)

However, the drop in enthalpy you observed of 25%+ seems excessive, as can be seen in the graph below:

enter image description here

Latent heat of fusion comparison between water, $\ce{NaCl}$ solution and ethanol solution.

Image source: Colligative properties of water cited from the original article Formulation of the latent heat of fusion of ice in aqueous solution (Kumano et al. 2009)

KUmano et al. 2009's methodology centred around the use of

Differential scanning calorimetry analysis was performed and the measurements and calculated values compared with respect to the specific enthalpy of the ice/solution mixture.

As for the 'mushy ice' that you observed, it is likely to be a mixture of ice and brine, as per the diagram below:

enter image description here

Image source: Wikipedia

  • 1
    $\begingroup$ Thanks for the references, these are very helpful. Update for anyone else who might ever be reading this: I put the mushy stuff in a chest freezer for a week and it is a nice solid block. The issue must have been in the way I was making the ice. $\endgroup$ – ericksonla May 4 '15 at 22:24

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