Pure substances refer to a class of substances composed of only the same kind of chemical substances (or molecules). As opposed to mixtures, they can be divided into two categories: simple substances and compounds. The basic phases of pure substances mainly include solid phase, liquid phase and gas phase. Under one basic phase, there can be several phases, and each phase has a different molecular structure. For homogeneous pure substances, when two strength properties are given (usually any two of p, V, and T, there are exceptions), other thermodynamic properties can be calculated. The model used is mainly the equation of state. The principle of correspondence state can be used.
Substances with a fixed chemical composition as a whole are called pure substances. For example, water, oxygen, hydrogen, nitrogen, and carbon dioxide are all pure substances. In chemistry, pure substances have fixed chemical and physical properties. Ice-water mixtures, crystal hydrates, etc. are also pure substances. There is no absolute ideal pure substance in the world.
A pure substance is not necessarily a single chemical element, nor is it necessarily a single compound. A homogeneous mixture of chemical elements or compounds can also be pure substances, as long as the mixture is homogeneous. For example, gaseous air is a mixture of several gases. It has the same chemical composition, so it is a pure substance. The mixture of oil and water is not a pure substance, because the oil is insoluble in water. After mixing, the oil is on the water, and the two form two areas with different chemical compositions. In other words, this mixture is not uniform.
In addition, as long as the chemical composition of each phase is consistent, a mixture composed of two-phase or multi-phase pure substances is still a pure substance. For example, a mixture of ice and liquid water is a pure substance because the two phases have the same chemical composition. However, the mixture of liquid air and gaseous air is not a pure substance, because the chemical composition of the two is different and it is an inhomogeneous mixture.
PHASE OF PURE MATTER
Three basic phases
There are several phases of matter. At room temperature and pressure, copper and iron are solids, mercury and water are liquids, and oxygen and nitrogen are gases. In other words, they present a solid phase, a liquid phase, and a gas phase, respectively. These three are the basic phases of matter. Under different conditions, matter will transform into different phases.
Under one basic phase, there can be several phases, and each phase has a different molecular structure. For example, carbon may exist in the form of graphite, or it may exist in the form of diamond and activated carbon, although their chemical composition is the same. For another example, helium has two liquid phases, iron has three solid phases, and ice can have seven different phases at high temperatures. Matter exists in different phases, adding more forms to matter in nature.
The complex phase system is composed of two or more phases, and these phases are separated from each other by the phase boundary, that is, the phase step surface. There are many practical examples of the balanced coexistence of two phases of pure matter. In the field of thermal engineering, in the condenser of a thermal power plant, the water transitions in the form of a mixture of liquid and steam, and gradually turns into liquid. In the boiler, water also transitions in the form of a mixture of liquid and steam, gradually turning into steam. In the evaporator of the refrigerator, the refrigerant transitions in the form of a mixture of liquid and vapor, from liquid to vapor; in the condenser, the refrigerant transitions in the form of a mixture of liquid and vapor, from vapor to liquid, etc.
You can also check out the real element acrylic periodic table for some inspiration.