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how we explain the observation of water boiling at steady temperature while being heated continuously?

Heat is being added to ice as it melts, but that heat is going into turning the water from a solid into a liquid (breaking intermolecular bonds), rather than increasing the temperature. This is also true when it boils, and that is why a boiling pot of water won't reach a higher temperature than 100 °C.

When something changes phase from solid to liquid, or from liquid to gas, it takes energy to break the intermolecular interactions. These interactions between the water molecules are what make it solid. When you have ice, these interactions are strongest, which is why ice is hard. Then when you have water, the interactions are not as strong, and although the water still "stays together" it is now a liquid and moves and flows freely. Then when all the interactions are broken, it become a gas, or steam, and now none of the water molecules are attached to any other molecules. Whenever it goes through a phase change like this, the energy goes into breaking up these interactions, and so the temperature stays constant until all the interactions are broken. Once all the ice is melted, or all the water has turned to steam, then any added heat will act to raise them temperature again.


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