Key Points

All matter, whether solid, liquid, or gas, is composed of a vast number of extremely small particles. These particles are the fundamental building blocks of every substance.

The particles of matter have empty spaces between them, known as interparticle spaces. These spaces are smallest in solids, larger in liquids, and largest in gases.

Particles of matter attract each other with forces called interparticle attractions. The strength of this force determines the state of matter: strongest in solids, weaker in liquids, and negligible in gases.

Solids have a definite shape and a fixed volume. This is because their particles are held in fixed positions by very strong interparticle forces and can only vibrate.

Liquids have a definite volume but no fixed shape, taking the shape of their container. Their particles have weaker interparticle forces than solids, allowing them to move and slide past one another.

Gases have neither a fixed shape nor a fixed volume, expanding to fill any available space. Their particles are far apart with negligible interparticle forces, allowing for free and random movement.

The constituent particles of matter are in continuous, random motion. This motion is most restricted in solids (vibration), more free in liquids (sliding), and completely free in gases.

Heating a substance increases the energy of its particles, causing them to move or vibrate more vigorously. This increased motion can lead to a change in the state of matter.

The melting point is the specific temperature at which a solid substance melts to become a liquid at atmospheric pressure. At this point, particles gain enough energy to overcome the strong forces holding them in fixed positions.

The boiling point is the temperature at which a liquid rapidly turns into a gas (vapor) at atmospheric pressure. Particles gain enough energy to overcome nearly all interparticle forces and escape the liquid.

When a soluble substance dissolves in a liquid, its particles break away and spread out, occupying the interparticle spaces of the liquid. This is why the total volume may not increase significantly.

Gases are highly compressible because of the large interparticle spaces, which can be reduced by applying pressure. Liquids and solids are almost incompressible because their particles are already closely packed.

The smell of perfume or an incense stick spreads across a room because gas particles move freely and mix with air particles. This intermixing of particles is called diffusion.

In solids, particles are orderly and tightly packed. In liquids, particles are randomly arranged but still close. In gases, particles are randomly arranged and very far apart.