Pure water freezes at 32°F (0°C). Water with salt (or any other substance in it) will freeze at some lower temperature. Just how low this temperature will be depends on the de-icing agent. If you put salt on ice in a situation where the temperature will never get up to the new freezing point of the salt-water solution, you won't see any benefit. For example, tossing table salt (sodium chloride) onto ice when it's 0°F won't do anything more than coat the ice with a layer of salt. On the other hand, if you put the same salt on ice at 15°F, the salt will be able to prevent melting ice from re-freezing. Magnesium chloride works down to 5°F while calcium chloride works down to -20°F.
Use Salt to Melt Ice - Activities
You can demonstrate the effect of freezing point depression yourself, even if you don't have an icy sidewalk handy. One way is to make your own ice cream in a baggie, where adding salt to water produces a mixture so cold it can freeze your treat. If you just want to see an example of how cold ice plus salt can get, mix 33 ounces of salt with 100 ounces of crushed ice or snow. Be careful! The mixture will be about -6°F (-21°C), which is cold enough to give you frostbite if you hold it too long.