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Phase change is a fundamental scientific concept where matter changes from one state to another. The states, or phases, typically include solid, liquid, and gas. For example, ice turning into water and water turning into steam represent phase changes.

During a phase change, energy is either absorbed or released, but it doesn't result in any change in mass within a closed system. This is because a phase change only alters the state of the substance, not the amount of substance itself.

It’s fascinating to note how this simple process of altering states involves impactful processes at the molecular level. Ice melting into water involves molecules gaining enough energy to break out of their rigid structure. Conversely, water turning into steam involves molecules gaining enough kinetic energy to spread out and move freely.

Water is a unique substance with special properties that make it vital for life on Earth. One of the most interesting aspects of water is its ability to exist in all three states (ice, liquid, and steam) within relatively short temperature ranges found on Earth.

With its high specific heat, water can absorb or release substantial amounts of heat without significant changes in temperature. This property plays a crucial role during phase changes:

• When ice melts, it absorbs heat energy but stays at 0°C until entirely transformed into water.
• Similarly, when water boils, it requires heat to turn into steam, with temperature holding constant at 100°C until the process is complete.

This persistence at specific temperatures during phase changes is known as latent heat, indicating water’s ability to consume or release energy to facilitate the phase transition, without altering its mass.

Mass calculation in the context of phase changes involves understanding the law of conservation of mass. According to this principle, the mass in a closed system remains constant over time. Therefore, during a phase change, even though the state may change, the total mass does not.

In the given exercise, we are dealing with water going through phase changes:

• Ice at 0°C turning into water at 0°C.
• Liquid water at 100°C converting into steam at 100°C.

In both scenarios, even though water transforms between solid, liquid, and gas phases, the mass remains at 2 kg.

This principle highlights an important concept in physics: whilst energy changes during phase transitions can be significant, mass itself remains unchanged, reiterating the stability and predictability of physical systems.