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?