Talha's Physics Academy

Forces and Kinetic Theory

According to the kinetic molecular model, the state of a substance is governed by particle energy and intermolecular forces:

Gases: Molecules have large kinetic energy, resulting in negligible forces of attraction. They move freely, go farther apart, occupy any available space, and compress easily. Their melting and boiling points are very low.
Liquids: Molecules possess less kinetic energy than gases, bringing intermolecular forces into play. Molecules stay close yet remain free to move past one another, giving liquids a fixed volume but no fixed shape.
Solids: Molecules possess extremely low energies and experience strong attractive forces, restricting them to small vibrations about their mean positions. This yields high densities, fixed shapes, and rigid volumes.

The state of a substance can be altered through thermal energy transfer (heating or cooling):

Heating a Solid (Fig. 7.3): Molecules vibrate more strongly and violently, weakening intermolecular forces until the substance melts into a liquid. Further heating provides enough energy to overcome attractive forces entirely, turning the liquid into a gas (boiling).

Figure: Fig. 7.3

Cooling a Gas (Fig. 7.4): Molecules slow down, collide, and stick together as attractive forces increase. Continued cooling condenses the gas into a liquid, and further cooling freezes the liquid into a solid.

Figure: Fig. 7.4

Helium: Possesses the lowest boiling and melting points among all substances; it solidifies only under combined cooling and compression.
Mercury: Notable as the only metal that remains in a liquid state at room temperature.

Class 9 > Unit # 07:Properties of Matter > Forces & Kinetic Theory