Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, water, soil, mountains, even our bodies are all examples of matter. Matter is divided into three states — solid, liquid, and gas — that are commonly found on earth. The fourth state of matter, plasma, occurs naturally in the interiors of stars. 

Solids are characterized by a definite shape and volume. In a solid, the particles (atoms or molecules) pack very closely and are held together tightly by strong forces, with very little freedom of motion. The particles vibrate, only slightly, in their otherwise fixed positions. This makes solids incompressible. Ice, aluminum, and gold are examples of solids.

Liquids have a definite volume but no definite shape; they take the shape of their container. In a liquid, although the particles are closely spaced, the interparticle forces are weaker than solids. The particles vibrate and move past one another. Thus, a liquid, though incompressible, flows and can be poured easily. Water, alcohol, and oil are all liquids.

Gases have no fixed volume or shape. They conform to the volume and shape of their container. In a gas, the particles are widely separated with much weaker (or almost negligible) interparticle forces. The particles are free to move relative to one another, making gases highly compressible. Thus, unlike solids or liquids, a gas can be compressed to occupy a smaller volume, or it can expand to occupy a larger space. Substances that are gases at room temperature include helium, oxygen, nitrogen, and carbon dioxide.

Plasma 

The fourth state of matter, plasma, has evolved from the gaseous state and contains appreciable numbers of electrically charged particles.  Like gases, plasmas have no fixed shape or volume, and they are less dense than solids or liquids. The interiors of the stars, lightning strikes, auroras that surround the poles, the tail of a comet, plasma TV screens, fluorescent light bulbs, and neon signs are all examples of plasmas.  

Physical Changes: Transitioning From One State of Matter Into Another

Changes in temperature and/or pressure can lead to a conversion from one state of matter to another. This is illustrated by familiar processes such as the melting of ice or evaporation of water. For example, upon heating, solid ice melts to liquid water. Further heating will convert the water into gaseous vapors. Cooling reverses these processes.

This text is adapted from Openstax, Chemistry 2e, Section 1.2: Phases and Classification of Matter.