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These changes represent a change in the physical state of matter, namely water. It means that enough heat as been transferred into or out of the water to cause it to change to another physical state. When you heat up liquid water in a pan on the stove, the heat from the stove is being absorbed by the water, causing its molecular bond rotations, vibrations, and collisions to increase in speed and frequency. Once a certain threshold of heat is crossed, the liquid water can absorb no more heat and the liquid molecules change into gas molecules. Enough energy has been put into the system to cause the molecules to change physical states.
Similarly, when frozen water (ice) is exposed to enough heat, the molecules get enough energy to break free of the solid matrix and exist in the higher energy liquid state. Of course these processes are also completely reversible. So the change in the physical states of water really has to do with the amount of energy in the form of heat that the system has gained or lost.
A phase chart is very helpful when dealing with phase changes (I have provided a link of one below).
Phase changes from solid to liquid or liquid to solid occur when the pressure reaches a certain point in relation to the temperature or when the temperature reaches a certain point in relation to the pressure. For example, water boils at 100 degrees Celsius at 1 atm. At a lower pressure, the boiling point would also be lower.
The subject is changing state of form or states of matter
If the water boils it is liquid to gas
If it melts it is solid to liquid
The water is changing states of matter.
water boils: liquid to gas
ice cube melts: solid to liquid
These are both examples of physical chanes. water boiling is the change from water to water vapor. It can be reversed just like ice melting is the change from a solid to a liquid. All items still have the composition of water. Energy (heat) is being applied to the water and released into the enVironment (water vapor).
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