What causes ice to melt and water to boil

Ice melts due to the chemical properties of water. There are more hydrogen bonds between the molecules of ice than in water.

Water boils when the thermal energy in the water, which is a type of kinetic energy which causes the water molecules to move around, exceeds the strength of the hydrogen bonds between the other molecules.

When water is boiled, the heat energy is transferred to the molecules of the water, which begin to move more quickly. Eventually, the molecules have too much energy to stay connected as a liquid. When this occurs, they form gaseous molecules of water vapor, which floats to the surface as bubbles and travels into the air.