A substance can be found in liquid, solid or gaseous physical state. These different aspects are called aggregation phases and depend on temperature and pressure.
For each substance, there is a temperature and pressure range in which it maintains its characteristics as a species, changing only the phase of aggregation.
For example, the substance water: at or below 0 ° C, subjected to the pressure of 1 atm, is in the solid phase; between 0 ° C and 100 ° C, under the same pressure, it is in the liquid phase and at 100 ° C also under the same pressure, it will change to water vapor, that is, gas phase.
At this stage, the particles of the substance are with the highest kinetic energy. They are very far apart. They move very fast and collide with each other.
Any gas placed inside a 1 liter bottle takes the shape of the bottle and its volume will be 1 liter. We can say that a substance in the gas phase has variable shape and volume.
Why are gases compressible? Knowing that gases (unlike liquids and solids) have no fixed volume, with increased pressure we can compress them or reduce their volume. Gases are compressible because there is so much space between the particles that compose them.
In the liquid phase, the particles are a little closer together than the gas phase particles, but not fully together. There is no definite arrangement.
Kinetic energy is intermediate between the gas phase and the solid phase. The particles in the liquids "slide" over each other and move. This is what provides the fluidity in the liquid. All liquids can flow, and some more than others. Water, for example, flows more easily than honey.
So we say that water has low viscosity and honey has high viscosity. Low viscosity liquids offer less resistance to flow.
In the solid phase, the particles that form the substance have the lowest kinetic energy; they remain practically immobile, united by mutual forces of attraction and generally arranged according to a definite geometric arrangement.
In the case of water molecules, this arrangement is ring-shaped, in which there is always one hydrogen atom between two oxygen. The arrangement of water molecules in the solid phase is responsible for increasing their volume. Then, when freezing water expands, forming ice, which is less dense than water in the liquid phase.
Does a marble block on a table change shape and volume over time? We can conclude that a solid substance has its own shape and volume.