Rigidity
This is the resistance of a substance to changing its shape. Solids are highly rigid, maintaining a fixed shape. Liquids and gases are not rigid.
Compressibility
This refers to the ability of a substance to be compressed into a smaller volume. Gases are highly compressible because their particles have large spaces between them. Liquids and solids are considered virtually incompressible.
Fluidity
This is the ability of a substance to flow. Both liquids and gases are classified as fluids because their particles can move freely past one another. Solids do not exhibit fluidity.
Filling a gas container
Gases completely fill the container they are in, regardless of its size or shape. This is due to the high kinetic energy of their particles, which move randomly and spread out to occupy the entire volume.
Shape
Solids have a definite, fixed shape. Liquids do not have a fixed shape and take the shape of their container. Gases also have no fixed shape and conform to the shape of their container.
Kinetic EnergyThis is the energy of motion of particles. Particles in solids have the lowest kinetic energy, vibrating in fixed positions. Liquids have more kinetic energy, allowing particles to slide past each other. Gases have the highest kinetic energy, with particles moving randomly and rapidly.
Density
This is the mass per unit volume. Solids generally have the highest density due to their tightly packed particles. Liquids are less dense than solids, and gases have the lowest density because their particles are very far apart.