GED Science Practice Test: State / Phases of Matter

Density is a useful property in terms of describing another property of matter:  its phase, or state. There are four states of matter observable in everyday life: solid, liquid, gas, and plasma. You should already have an intuitive understanding of the first three.  For example, water can exist as a solid (ice), a liquid (water), and a gas (water vapor or steam).

Solids: In a solid the particles are closely packed together, and thus have a high density. Due to this tight packing, particles in a solid cannot move freely but can only vibrate. As a result, a solid has a stable, definite shape, and a definite volume. Solids can only change their shape by force, as when broken or cut.  There are two categories of solids:  crystalline and amorphous. In crystalline solids, the particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern. Sugar and diamond are examples of crystalline solids. In amorphous solids, the particles do not have a regular structure.  Styrofoam is an example of an amorphous solid.

Liquids: In a liquid, the particles are not as closely packed together as a solid, and thus have a lower density than solids.  Due to this lower density, the particles in liquid are free to move past each other.  As a result, a liquid has no definite shape, and can take the shape of the container it is placed in.  The density of a liquid, while lower than a solid, is still high enough for attractive forces to hold the particles of a liquid together.  Thus, liquids also have a definite volume.

Gases: In a gas, the particles are spread out very far, and thus, have a very low density.  Due to this very low density, the particles in a liquid are free to move past each other and the gas then has no definite shape.  Additionally, because the particles are so far apart, the force of attraction between is very weak.  Thus, gases do not have a definite volume.

The following table summarizes some of the properties of solids, liquids, and gases:


It was suggested that you have an intuitive understanding of solids, liquids, and gases through your understanding of water.  It is important to note that water is actually an exception to one characteristic of solids, liquids, and gases.  Water’s solid form is actually slightly less dense than its liquid form.  For this reason, solid ice can float in liquid water.  The reasons for this are due to hydrogen bonding, which will be discussed in the next lesson.

Plasma: One state or phase of matter with which you are likely not familiar is a plasma. Plasma is actually quite common on Earth, and the majority of people observe it on a regular basis without even realizing it. Fire, lightning, electric sparks, fluorescent lights, neon lights, plasma televisions, and the stars are all examples of illuminated matter in the plasma state. Like a gas, plasma does not have definite shape or volume. However, plasmas are different in structure at the atomic level, which you will learn more about in the next lesson. Due to these unique structural characteristics, plasmas are electrically conductive, produce magnetic fields and electric currents, and respond strongly to electromagnetic forces.


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