[joshuacalcu]

what can i add to ice that will make it last longer?  I'm trying baking soda, but i am unsure of what would really be a good substance to add that will cause it not to melt as quickly.

[enahs]

Add less energy in the form of heat!

[AlexKun]

I'm not sure. If you added salt, for example, to the water before freezing it, the temperature at which the water froze would be less than zero. So it would actually start melting at a lower point. Adding something to the water before freezing it actually would make it not last as long. If anything, purifying the water would raise the freezing point, which is what you want.

[madscientist]

Not sure if you have to use ordinary water (H₂O) but what about freezing heavy water, i.e. Deuterium oxide (²H₂O or D₂O).  The melting point is 4 degrees C.

[Ψ×Ψ]

Not sure if you have to use ordinary water (H₂O) but what about freezing heavy water, i.e. Deuterium oxide (²H₂O or D₂O). The melting point is 4 degrees C.

Yeah, but don't put it in a drink... 

[madscientist]

only if you want to increase your mutant powers..