The three characteristics of physical change

A physical change is always reversible.

A physical change only affects the physical properties of the substance like volume and size as well as its state (solid, liquid, gas).

A physical change does not change the chemical composition of the substance like the number of atoms and protons in a molecule or element.

Explanation:

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E.g. An ice melting does not change the chemical composition of the water, but it only changes its ability to take the shape of its beholder (the cup). To reverse it, simply lowering the temperature allows the water to turn back into ice.

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

c. establishing proper sleep hygiene