Ice melts back into a liquid form when the external temperature around the ice goes above the freezing point. But when some substances come in contact with ice it seems as if the rate of melting increases. It actually doesn’t, but there is a good reason why it seems that way. Some substances change the physical properties of the water or ice, making the melting process speed up. The change depends on the substance you use.
When roads freeze in the winter, the highway department is quick to spread salt on the roads to melt the ice. But the salt itself does not affect the rate of melting. No foreign substance will do that to ice. But salt can affect the freezing point. Adding salt water to ice brings the temperature at which water freezes down considerably. So as long as the external 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 less then freezing 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.
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. Water is an ingredient in most alcoholic beverages, but high alcohol content beverages like bourbon or vodka will not freeze in your home freezer—nor will rubbing alcohol. This lowered rate of freezing is the reason why this happens.
So as the required freezing temperature drops when alcohol is added to ice, the temperature in the glass or elsewhere surrounding the ice becomes higher than the new freezing point. This is why the ice melts more quickly. It’s all because of the heat, not the alcohol itself, according to the University of Illinois Physics website.
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 32 degrees Fahrenheit without freezing the entire beverage.