h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Why does salt melt ice? and does the slipping effect increase or decrease when used

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Salt lowers the freezing point of the water because it's an impurity, so the ice melts. If the temperature is too cold, however, the water still freezes because it is still below the new freezing point of water.

Ice has unique properties. Hydrogen bonding means that it expands when it freezes so the solid (ice) floats on the liquid (water). No other substance does this (I think).
If you put pressure on ice the molecules get broken and a liquid forms (it is not because the ice has warmed up). As soon as you take the pressure off the liquid goes back to ice.

Putting salt on ice breaks the bonds and forms a liquid. An ice/salt mixture is colder than 0C (zero Celsius). The ice does not melt because it warms up. Years ago scientists used ice/salt mixtures to obtain low temperatures.
Further topics to look up; thermometry, Fahrenheit.