Information: - An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are very small; typical sizes are around 100picometers (a ten-billionth of a meter, in the short scale). - Chemical species are atoms, molecules, molecular fragments, ions, etc., subjected to a chemical process or to a measurement. Generally, a chemical species is an ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. The term may be applied equally to a set of chemically identical atomic or molecular structural units. - An ion is an atom or a molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. Ions can be created, by either chemical or physical means, via ionization. - In a scientific sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by an outside force, and involves a chemical reaction of some sort. In an "engineering" sense, a chemical process is a method intended to be used in manufacturing or on an industrial scale (see Industrial process) to change the composition of chemical(s) or material(s), usually using technology similar or related to that used in chemical plants or the chemical industry. - Measurement is the assignment of a number to a characteristic of an object or event, which can be compared with other objects or events. The scope and application of a measurement is dependent on the context and discipline. In the natural sciences and engineering, measurements do not apply to nominal properties of objects or events, which is consistent with the guidelines of the "International vocabulary of metrology" published by the International Bureau of Weights and Measures. However, in other fields such as statistics as well as the social and behavioral sciences, measurements can have multiple levels, which would include nominal, ordinal, interval, and ratio scales. - A salt metathesis reaction ( from the Greek , '' transposition '' ) , sometimes called a double replacement reaction or double displacement reaction , is a chemical process involving the exchange of bonds between two reacting chemical species , which results in the creation of products with similar or identical bonding affiliations . This reaction is represented by the general scheme : A-B + C - D A-D + C - B The bond between the reacting species can either be ionic or covalent . Classically , these reactions result in the precipitation of one product . In older literature , the term double decomposition is frequently encountered . The term double decomposition is more specifically used when at least one of the substances does not dissolve in the solvent , as the ligand or ion exchange takes place in the solid state of the reactant , for example : AX ( aq ) + BY ( s ) AY ( aq ) + BX ( s ) . - A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, and biochemistry, the term "molecule" is often used less strictly, also being applied to polyatomic ions. Given the information above, choose from the list below the object entity that exhibits the relation 'subclass of' with the subject 'salt metathesis reaction'. Choices: - application - atom - being - charge - chemical element - chemical process - chemical reaction - chemistry - composition - constituent - element - event - interval - ion - liquid - manufacturing - matter - measurement - measurements - molecule - natural - number - physics - process - scale - sense - solid - species - statistics - technology - vocabulary
The answer to this question is:
chemical reaction