Properties of Gases, Solids, and Liquids

Study Notes

Solids have a definite volume that does not change, and they cannot change their shape easily.
Liquids have a definite volume that does not change, but they can flow and change their shape.
Gases take up all the available space, take the shape of their container, can compress into a small space, and can expand.

Liquids and solids compress in very small proportion because the molecules are close, and repulsive forces appear when they try to approach.
Solids do not flow and keep their shape, while liquids can flow and do not keep their shape.
Solids and liquids practically do not expand, and when they do, it is in very small proportion.

Types of solid include amorphous and crystalline solids.
Amorphous solids have molecules arranged in a random manner, tend to flow when subjected to sufficient pressure, and do not have definite melting points.
Crystalline solids have particles arranged in a specific manner, cannot flow, and have sharp melting points.
Examples of crystalline solids include sodium chloride crystal and diamonds.

Methods of characterization include X-ray diffraction, melting point and heat of fusion, and double refraction of light.

The liquid crystalline state can result from heating of the solid (thermotropic liquid crystals) or the action of certain solvents on solids (lyotropic liquid crystals).
Liquid crystals are used to detect areas of elevated temperature under the skin and in developing display systems (e.g., LCD).

The kinetic-molecular theory explains the behavior of matter based on the following assumptions:
- Any material thing of the universe has a very large number of very tiny particles called “molecules” that are in continuous movement.
- Molecules are not all equal, but they can have different sizes depending on the type of material.
- Among molecules, there are empty spaces.
The kinetic-molecular theory also explains the behavior of molecules in different states:
- Solid molecules are close, tidy, and can only vibrate.
- Liquid molecules are close, untidy, and move relative to each other.
- Gas molecules are very distant, in continuous movement, and crash into each other and into every object that surrounds them.

Gases expand when heated, and their speed of molecules depends on their temperature, increasing with temperature and decreasing with temperature.

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