Could someone help me with these problems?

1. Why is it advantageous to choose a solvent with a large value for k(fp)?

I don't really know

2. Why is it not necessary to wait for an entire sample of water to freeze in order to determine its freezing point?

I think it has something to do with supercooling but I don't know.

3. What difference would be expected in the solutions freezing point if an ionic compound, such as NaCl, was added to 10mL of water instead of urea?

I think the freezing point would lower, but could someone explain why?