Water molecules are made up of two hydrogen atoms and one oxygen atom (H2O). At freezing temperatures, the atoms that make up the molecules bond, causing the water molecules to hold together in a static form. Ice melts as its temperature rises above 32 degrees Farenheit. Ice cubes melt by convection, or the transfer of heat from one substance to another. For ice cubes, the heat transferring substance will either be liquid or air.
As soon as you take an ice cube out of the freezer, it begins to absorb surrounding energy (heat), causing the frozen water molecules to begin vibrating. The molecules then bump each other, causing the overall vibrations to increase. The hydrogen and oxygen atoms become unbound, water molecules break away from each other and the ice cube begins to turn from a frozen to a liquid state.
How fast ice cubes melt depends on size, how much surface area is exposed and whether they are in liquid or surrounded by air. Larger ice cubes will take more time to melt than smaller ones because it takes more external energy to get all of the molecules to begin vibrating. Ice cubes with more surface area (like a square ice cube vs. a round one) will also melt faster because more of the ice is exposed to the external, heat-transferring environment.
Ice cubes in liquid melt faster than ice cubes in air because liquid has a greater concentration of molecules than air does. This means that when ice cubes are surrounded by liquid, a large concentration of molecular activity is interacting with the frozen water molecules, causing a faster chain reaction of molecular vibration. Continuing to heat the water until it boils will cause the molecules to vibrate even more, and eventually turn into steam.
When ice cubes are surrounded by air, there is a much less concentrated amount of molecular activity surrounding the ice cubes, resulting in a slower chain reaction of molecular vibration. In certain cases air, when hot and fast moving (like from a blow dryer), will melt an ice cube faster than liquid.
