A trivia game I was playing made the claim that a vinegar and water mixture can be used to melt ice. Google lead me to Snopes.com, which claims that the mixture will prevent ice from forming instead of melting ice (when used on a car windshield). Is this true? If so, how does it work?
When a solute (like salt, or alcohol) is added to water a freezing-point temperature depression occurs. It usually depends only on the molarity of the solution not on the type of solute. This phenomenon is often used in winter to clear roads from ice by salt spreading. The freezing (or melting) temperature lowering should also happen when vinegar is added to water. Thus when you pour a water-vinegar mixture on water ice this should melt the ice because the water-vinegar mixture has a lower melting point.
The freezing point of "average" household vinegar (5% w/w) is around -3°C, and diluted further with water raises the freezing point to -2°C or -1°C. To put it another way, even a small amount of vinegar lowers the freezing point of water to below 0°C. But if you pour cold water on ice, it will melt the ice anyway. Why? Because the water is above freezing and will warm the ice and cause it to melt.
Adding vinegar to the water will lower the freezing point of the liquid water and help prevent re-freezing, but this benefit is probably quite limited in practice and isn't going to work in environments where the ice is forming at temperatures below a few degrees, and where the temperature of the glass stays below the freezing point.
And Snopes says, "In general, we’ve found no consensus about how effective the use of a vinegar-water mixture to remove or prevent windshield ice might be"
Also, putting hot water on cold glass is probably a bad idea too.