In general impurities lower the melting(freezing) point and raise the boiling point of liquids. This is why in 70's movies you would see heroin being tested using a thermometer and heater. The images in the film often made it appear that the melting point could be dramatically altered and it could be extremely high for pure stuff. In reality, the delta is only a few degrees for most substances, although I don't really know what they were cutting heroin with.
Another example is the freezing point of water being lowered by the addition of ethylene glycol. I don't recall the exact ratio, but if you use more ethylene glycol than that, the freezing point goes back up some. In essence, the water also acts as antifreeze for the ethylene glycol.
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