Water can exist in three states (or three phases):

• Solid phase:  The particles in a solid are strongly bonded to one another. Ice cubes maintain their form regardless of the container that holds them.

• Liquid phase: The particles are no longer in an ordered state. The bonds between molecules are broken, and the liquid water takes the shape of its container.  The particles are very close to one another, and so a liquid is incompressible.

• Gaseous phase: Agitation and disorder are at the maximum level. Water vapor occupies all of the space in a container. The distances between molecules are large. A gas in compressible.

Note that water vapor is invisible.

Gases, unlike solids and liquids, occupy all of the space inside their container.

The kinetic molecular theory of Maxwell and Boltzmann proposes that the macroscopic properties of an ideal  gas  derive  from the fact that it is  composed of a large number of colliding point particles that  are  in rapid motion.  The particles do not interact with one another except in elastic collisions. The piston receives from each such collision a certain amount of momentum. The macroscopic effect of these collisions is pressure.

Symbolic illustration of the ideal gas law, PV=nRT. The molecular model of an ideal gas involves  a large number of point molecules moving independently and experiencing elastic collisions.

This sliding array shows the great differences in the resistance values of conductors and insulators. The superconductors included in the animation are not new elements, but represent a physical phenomenon observable under certain conditions.

The half-life of  a given isotope is the amount of time it takes for half of the atoms in a sample to decay. This animation  allows you to address, using three different isotopes,  notions like radioactive decay, carbon dating, half life constant…

Hexagonal compact lattice.