All physical matter has mass, weight, volume and density. Matter also has certain characteristics restricted only to specific physical states. Physical matter is made up of atoms, minuscule particles that form physical structures via their electrically charged attraction to one another.
Atoms by themselves possess weight, mass, volume and density. All atoms constantly move. The more energy that is applied to a group of atoms, the more they move. Atoms in a less energetic state do not move very much at all; they stay in place and vibrate. Atoms in such a state form solid matter.
Solids have a fixed mass, weight, volume and density. Mass is the amount of matter in an object while weight is the pull of gravity on the mass of that object. Volume is the amount of space the object takes up, while density is a description of how the mass takes up that space. Objects with high density have a lot of mass in a small amount of space.
Atoms that are more energetic and, hence, move more freely are in a liquid state. Liquids also have a fixed mass, weight, volume and density. Particles in a liquid can flow past one another rather than being stuck in a single position. While solids do not change shape easily, liquids can conform instantly to the shape of their container.
Atoms in an even more energetic state form gases. Gases can change not only their shape, but also their volume because the particles have enough energy to widely separate from one another while flowing.