Q: You have 10 g NaCl (s) and 90 g Lactose (s) (both easily soluble in water.) You want to achieve a 100 g mixture of these two compounds in solid form where they are completely evenly mixed. (A 1 mg sample of this mixture will always have 0.1 mg NaCl and 0.9 mg Lactose no matter from where in the mixture the sample is taken.)

Attempted A: Boil a litre of distilled water in a clean container. In a sterile container put the pulverized NaCl and Lactose. Wait until boiled water cools to at least 50 C because high temperature increases risk of hydrolyzing lactose. While stirring with a sterile glass stir rod add the hot water step-wise until just enough water has been added to completely dissolve all the powder. Leave the solution on a warm plate (temperature shouldn't exceed 40-50 Celsius) and use a fan or similar device for airflow to speed up evaporation. Once most of water has evaporated put in a dessicator chamber to finish drying.

What I'm not sure about: Would this actually create a completely even mixture because lactose and NaCl have slightly different solubilities so as the amount of water drops wouldn't one start precipitating earlier? If this is true how could you prevent that and are there any other issues?

  • $\begingroup$ Well you could freeze dry it rather than evaporate of the water $\endgroup$ – Waylander Feb 7 '18 at 15:36

The final solid mass would be a powder of sort, right? If you grind it / mix it will be homogeneous irregardless of the selectivity of their precipitation in the prior solution.


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