Adding a substance such as sugar or salt to water or ice lowers the freezing point and increases melting of existing ice. This is why salt is spread on icy roads in wintertime.
Effect of Sugar
Sugar lowers the freezing point of water. This is because if sugar molecules are added to water, the resultant solute isn't water anymore. The sugar-water mixture has different freezing properties to pure water.
How Does This Work
When sugar (or another solid such as salt, for example) is sprinkled on ice it mixes with the top, warmest layer of the ice that is melting. This creates a layer of melted sugar water on top of the ice. The layer underneath the sugar layer is freezing, but the icy molecules cannot move though the sugary water from the bottom layer to the top and extend the freezing zone. Movement of pure water is slowed through sugar water, which has less water molecules and a different density. Freezing necessitates molecule movement through solutions.
Altering Freezing Point
For each mole of a substance (about 270 million molecules, a scientific measurement) added to a kilogram of water, the freezing point is lowered by about 1.8 degrees Celsius.