Salt can be used to melt ice, or lower the freezing point of water, by taking advantage of the fact that it is a hygroscopic substance. This means that it absorbs water molecules from the surrounding environment, which can cause the water to freeze at a lower temperature.

The exact temperature at which salt will no longer melt ice depends on the type of salt and the concentration of salt in the water. However, as a general rule, salt is most effective at melting ice at temperatures below 0°C (32°F). At temperatures above 0°C, the ability of salt to melt ice decreases significantly.

For example, common table salt (sodium chloride) is typically effective at melting ice down to temperatures of about -18°C (0°F). Calcium chloride, on the other hand, is a more effective ice melter and can lower the freezing point of water to as low as -52°C (-62°F).

It's important to note that salt will not completely prevent ice from forming, but it can make it more difficult for ice to form and can help to melt any ice that does form. The effectiveness of salt as an ice melter is also influenced by factors such as humidity and air temperature.