Chemistry (from Egyptian kēme (chem), meaning “earth”) is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions. It is a physical science for studies of various atoms, molecules, crystals and other aggregates of matter whether in isolation or combination, which incorporates the concepts of energy and entropy in relation to the spontaneity of chemical processes. Modern chemistry evolved out of alchemy following the chemical revolution (1773).
Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include inorganic chemistry, the study of inorganic matter; organic chemistry, the study of organic matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the energy related studies of chemical systems at macro, molecular and submolecular scales; analytical chemistry, the analysis of material samples to gain an understanding of their chemical composition and structure. Many more specialized disciplines have emerged in recent years, e.g. neurochemistry the chemical study of the nervous system (see subdisciplines).
Science (from the Latin scientia, meaning “knowledge”) refers in its broadest sense to any systematic knowledge-base or prescriptive practice that is capable of resulting in a prediction or predictable type of outcome. In this sense, science may refer to a highly skilled technique or practice.
In its more restricted contemporary sense, science refers to a system of acquiring knowledge based on scientific method, and to the organized body of knowledge gained through such research. This article focuses on the more restricted use of the word. Science as discussed in this article is sometimes called experimental science to differentiate it from applied science, which is the application of scientific research to specific human needs—although the two are commonly interconnected.
Science is a continuing effort to discover and increase human knowledge and understanding through disciplined research. Using controlled methods, scientists collect observable evidence of natural or social phenomena, record measurable data relating to the observations, and analyze this information to construct theoretical explanations of how things work. The methods of scientific research include the generation of hypotheses about how phenomena work, and experimentation that tests these hypotheses under controlled conditions. Scientists are also expected to publish their information so other scientists can do similar experiments to double-check their conclusions. The results of this process enable better understanding of past events, and better ability to predict future events of the same kind as those that have been tested.
The term matter traditionally refers to the substance that all objects are made of. One common way to identify this “substance” is through its physical properties; a common definition of matter is anything that has mass and occupies a volume. However, this definition has to be revised in light of quantum mechanics, where the concept of “having mass”, and “occupying space” are not as well-defined as in everyday life. A more general view is that bodies are made of several substances, and the properties of matter (among them, mass and volume) are determined not only by the substances themselves, but by how they interact. In other words, matter is made up of interacting “building blocks”, the so-called particulate theory of matter.
The concept of matter has been refined many times in history, in light of the improvement in knowledge of just what the basic building blocks are, and in how they interact. For example, in the early 18th century, Isaac Newton viewed matter as “solid, massy, hard, impenetrable, movable particles”, which were “even so very hard as never to wear or break in pieces” The “primary” properties of matter were amenable to mathematical description, unlike “secondary” qualities such as color or taste. In the 19th century, following the development of the periodic table, and of atomic theory, atoms were seen as the being the fundamental constituents of matter; atoms formed molecules and compounds.
In the late 19th century with the discovery of the electron, and in the early 20th century, with the discovery of the atomic nucleus, and the birth of particle physics, matter was seen as made up of electrons, protons and neutrons interacting to form atoms. Today, we know that even protons and neutrons are not indivisible, they can be divided into quarks, while electrons are part of a particle family called leptons. Both quarks and leptons are elementary particles, and are currently seen as being more fundamental constituents of matter.
These quarks and leptons interact through four fundamental forces: gravity, electromagnetism, weak interactions, and strong interactions. The Standard Model of particle physics is currently the best explanation for all of physics, but despite decades of efforts, gravity cannot yet be accounted for at the quantum-level; it is only described by classical physics (see quantum gravity and graviton). Interactions between quarks and leptons are the result of an exchange of force-carrying particles (such as photons) between quarks and leptons. The force-carrying particles are not themselves building blocks. As one consequence, mass and energy cannot always be related to matter. For example, the carriers of the electric force (photons) possess energy (see Planck relation) and the carriers of the weak force (W bosons) are massive, but neither are considered matter either. However, while these particles are not considered matter, they do contribute to the total mass of atoms or subatomic particles.
Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental technique have realized other phases, previously only theoretical constructs, such as Bose–Einstein condensates and Fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark-gluon plasma.
In physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave-particle duality.
In the realm of cosmology, extensions of the term matter are invoked to include dark matter and dark energy, concepts introduced to explain some odd phenomena of the observable universe, such as the galactic rotation curve. These exotic forms of “matter” do not refer to matter as “building blocks”, but rather to currently poorly-understood forms of mass and energy.
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