1.3:
Classifying Matter by State
Matter is anything that has mass and occupies space. Everything around us is matter, including our bodies, the air we breathe, and the ground we walk on.
Matter exists in three main forms — solid, liquid, and gas. Scientists have also identified a fourth fundamental state of matter — plasma.
Solids have a definite shape and volume. For example, take ice cubes in a glass. The ice cubes have a rigid shape, do not flow, and cannot be compressed. This rigidity is maintained because the particles in the ice are arranged close to each other, with strong interparticle forces holding them in place. These particles vibrate only slightly at their otherwise fixed positions, giving them fixed shape and volume.
Liquids, on the other hand, have a fixed volume, but no definite shape. If the same amount of water is placed into another container, its shape will vary according to the container, unlike the ice cube. Like solids, liquids too cannot be compressed. Their particles are still close together but the interparticle forces in a liquid are weaker than in solids, meaning the particles are freer to move and slide past over each other. This gives liquids their fluidity.
Gases have no fixed shape or volume. Gases take the shape and volume of their container. For example, imagine air from a cylinder inflating a balloon. In a gas, the particles are very loosely held together, with little to almost no interparticle forces. As a result, they have freedom of shape and volume, can move rapidly, and are less dense than both solids and liquids.
Gases are highly compressible, due to the large empty spaces between their particles, meaning they can be confined inside a tiny balloon, or expanded to occupy a large room.
Plasma is the fourth state of matter — produced when gas gets heated to very high temperatures. It is comprised of electrically charged particles. Plasmas make up the interior of stars, auroras surrounding the poles, and fluorescent light bulbs.
The different types of matter are also capable of transitioning from one state to another, typically triggered by changes in temperature or pressure or both. For example, upon heating, solid ice melts to become liquid water. With further heating, water changes into a vapor. With cooling, these state changes can be reversed.
1.3:
Classifying Matter by State
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.