Many substances that dissolve in water will also reduce its freezing point, allowing water to stay liquid at lower temperatures, or melting ice if it's frozen. Substances that do this include salt, sugar and alcohol. The amount of the change depends on the substance you use. The effect, which scientists call freezing point depression, helps keep roads and sidewalks free of ice and snow in winter months.
Salt as De-Icer
When roads freeze in the winter, the highway department is quick to spread salt on the roads to melt the ice. The salt lowers the freezing point. As long as the temperature is higher than the new freezing point, the ice will melt away. This is the same principle used when rock salt is used to cool an ice bath to below 0 degrees Celsius (32 degrees Fahrenheit) to create ice cream.
This works because the presence of the salt replaces some of the water molecules, meaning the ice and water can’t exist in equilibrium at the freezing temperature. The ice does not come into contact with as many pure water molecules and therefore is unable to maintain the free exchange of molecules between the water and the ice. The result is melting ice, according to the Frostburg State University General Chemistry website.
Alcohol and the Freezing Point
If you’ve ever poured hard liquor over ice, you’ve probably noticed that the ice melts unusually fast. That’s because alcohol brings the freezing temperature of water down significantly. Though most alcoholic beverages contain some water, high alcohol content beverages like bourbon or vodka will not freeze in your home freezer -- nor will rubbing alcohol.
So as the required freezing temperature drops when alcohol is added to ice, the temperature of the glass or elsewhere surrounding the ice is now higher than the new freezing point. Since it's no longer cold enough to stay frozen, the ice melts.
Sugar's Effect on Ice
Sugar water will react with ice in a similar way as alcohol, although at a much less noticeable degree. A sugary beverage such as sweet tea or Kool Aid will allow the ice to slowly melt and actually cool the liquid to a point slightly lower than the 0 degrees Celsius (32 degrees Fahrenheit) without freezing the entire beverage.