Briefly explain why adding salt to icy roads melt the ice and prevent it from freezing again.

See explanation below

Explanation:

The melting point (the temperature that it changes from solid to liquid) of a pure substance depends on its mass, the forces between its molecules, and the pressure of the system.

The melting point, or the freezing point, of a mixture, is a temperature between the melting point of its components. Salt has a huge negative melting point, so when it is added to the water, the melting point decreases from 0ºC to -21ºC approximately.

So, the water will only freeze again if the temperature becomes below -21ºC which is very difficult to happen.