It's not entirely accurate to say that particles of gas change while those of solids don't. The particles themselves (atoms or molecules) remain the same. What changes is the arrangement and movement of these particles. Here's a breakdown:

Gases:

* Arrangement: Particles are far apart and randomly arranged.

* Movement: Particles move very quickly and freely in all directions, colliding with each other and the container walls.

* Changes: Because of their high kinetic energy, gas particles can easily spread out to fill any container they're in. They are also compressible, meaning their volume can change significantly with pressure.

Solids:

* Arrangement: Particles are tightly packed in a regular, repeating pattern (like a crystal lattice).

* Movement: Particles vibrate in fixed positions, with limited movement.

* Changes: While individual particles vibrate, their overall arrangement remains largely fixed. This gives solids a definite shape and volume. They are not easily compressed.

The key difference:

The strength of the forces of attraction between particles determines their state.

* In gases, these forces are very weak.

* In solids, these forces are very strong.

This is why gas particles can move freely, while solid particles are held in a fixed structure.

Important Note: It's worth mentioning that even solids have some degree of particle movement. This is why solids expand when heated. The increased energy allows particles to vibrate with greater amplitude, leading to a slight expansion. However, the overall structure remains intact.