Liquid
Liquid is one of the three fundamental states of matter, existing as an intermediary between solids and gases. In a liquid state, particles possess enough kinetic energy to move freely while still maintaining some attraction to one another, preventing them from dispersing completely. This unique balance grants liquids a fixed volume and the ability to flow, allowing them to conform to the shape of their containers without being compressible. The molecular arrangement in liquids is often described as an "imperfect crystal," as their particles are organized, yet not as rigidly as in solids.
Key properties of liquids include cohesion, which is the attraction between like particles, resulting in surface tension, and adhesion, the attraction between different types of particles, which causes meniscus formation in containers. Viscosity, a measure of a liquid's resistance to flow, and volatility, which relates to a liquid’s tendency to evaporate, are also important characteristics. Temperature significantly affects these properties; for instance, as temperature increases, the viscosity of a liquid typically decreases. Understanding these properties and behaviors has been pivotal in the advancement of science and has historical roots tracing back to early philosophical inquiries into the nature of matter.
Subject Terms
Liquid
Liquid is one of the three states of matter. It is an intermediate phase between solids and gases. Matter exists in a liquid state when its particles gain enough kinetic energy in the form of heat to begin moving freely, but not so much energy as to be capable of fully overcoming their molecular attraction to one another. When a liquid loses or gains a sufficient amount of kinetic energy, it either reverts to a solid state or evaporates into a gaseous state.
Historical Background
The historical study of liquid matter was instrumental in the development of modern science. This is due in no small part to the fact that such studies helped scientists connect the behavior of matter at the atomic level (microscopic behavior) with its more easily observable properties (macroscopic properties), like pressure, temperature, density, and viscosity. This breakthrough, which took centuries to achieve, forever changed humankind's understanding of chemistry and physics.
Scientists first began formally studying the states of matter during ancient times. Naturally, given its abundance and obvious importance in everyday life, water was the most commonly studied liquid. Around 600 B.C.E., a Greek philosopher named Thales of Miletus put forth the first theory of matter–a theory in which water was the essential element of life. Specifically, Thales suggested that all mater originated as water and would inevitably return to water at some point. Although his theory was inaccurate, Thales's efforts set off a wave of interest in chemistry and the states of matter, which eventually led scientists to discover the relationship between matter's microscopic behaviors and macroscopic properties during the Scientific Revolution of the early modern era.
Properties of Liquid Matter
As with solids and gases, the primary defining characteristic of a liquid is in the movement of its particles. The particles of a solid have only a small amount of kinetic energy and remain mostly locked in a rigid geometric arrangement, but the particles of a liquid have a greater amount of kinetic energy and can slide past one another. This circumstance is directly responsible for two of a liquid's most notable properties: a definite volume and the ability to flow.
The kinetic energy of a liquid's particles is delicately balanced between that of a solid and that of a gas. Because a liquid's kinetic energy is greater than a solid's, the molecular bond between its particles is weaker. This allows the particles a greater degree of movement and allows the liquid to alter its shape to fit that of any container in which it is held. This means that liquid has the ability to flow. On the other hand, the kinetic energy of a liquid's particles is also low enough that the molecular bond holding them together is not completely broken, which means that the liquid's volume does not change based on the size of the container. This also means that a liquid cannot be compressed like a gas.
In addition to defining its key properties in terms of volume and flow, the arrangement of a liquid's particles also underlies its frequent characterization as an imperfect crystal. In a solid state, the particles of a sample of matter exist in a rigid crystalline structure in which they all follow the same pattern of organization. When that matter reaches its melting point and changes from a solid to a liquid, the crystalline structure is retained over volumes comparable to the distance between its particles but not any farther. Therefore, when matter is in liquid form, any given particle of that matter is constantly surrounded by other particles in positions almost identical to those they would be in if the matter were in its solid state. This makes liquid an imperfect crystal.
Again, the underlying key to a liquid's characteristics is the bond between its particles. These bonds also make liquid cohesive and adhesive. Cohesion is the tendency of like particles to be attracted to one another. This is why liquids typically have surface tension. The outermost layer of a liquid's particles are more attracted to themselves than they are to the particles of the surrounding air. As a result, the surface of a liquid can support a certain amount of weight before allowing an object to penetrate its surface. Adhesion, on the other hand, occurs when there is an attraction between different types of particles. Liquid particles are attracted not only to one another but also to the particles that make up the container in which the liquid is being held. As a result of this attraction, the particles at the surface of a liquid are drawn up slightly above the surface where they make contact with the container. This is why most liquids held in containers have a meniscus, or concave curve, at the surface.
Two other properties that are of particular importance in regards to liquids are viscosityand volatility. Viscosity is a measure of the speed at which a liquid flows. In short, a liquid that flows slowly is more viscous, while a liquid that flows quickly is less viscous. Highly viscous liquids, like syrup or molasses, are generally thicker and flow very slowly. Low viscosity liquids, like water, are much thinner and flow much more quickly. In most cases, viscosity is tied to temperature. As the temperature of a thick viscous liquid increases, it gradually becomes thinner and less viscous.
The volatility of a liquid is tied directly to the concept of evaporation. When a liquid evaporates, it changes into a gas. If a liquid is forced to evaporate inside a closed container, the particles are unable to escape. They then exert pressure within the container. This pressure is referred to as vapor pressure. The higher a liquid's vapor pressure, the more likely that liquid is to vaporize. In other words, higher vapor pressure means greater volatility. Particularly volatile liquids, like gasoline, can be ignited very easily and may result in fires or explosions.
Bibliography
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Bagley, Mary. "Properties of Matter: Liquids." LiveScience. Purch. Web. 22 Dec. 2014. http://www.livescience.com/46972-liquids.html
Considine, Glenn D., ed. "Liquid State." Van Nostrand's Encyclopedia of Chemistry. 5th ed. Hoboken, NJ: Wiley-Interscience, 2005, 937–941. Print.
McLaughlin, Charles William, Marilyn Thompson, and Dinah Zike. "States of Matter." Physical Science. Columbus, OH: Glencoe/McGraw-Hill, 2008, 476–480. Print.