In a double-replacement reaction: a. the reactants are usually a metal and a nonmetal. b. the reactants are generally two ionic compounds in aqueous solution. c. one of the reactants is often water. d. energy in the form of heat or light is always produced.

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the reactants are generally two ionic compounds in aqueous solution.

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In a double-replacement reaction the reactants are generally two ionic compounds in aqueous solution.

Explanation:

Double Substitution, Double Displacement or Metastasis Reactions are those in which two elements found in different compounds exchange their positions forming two new compounds. These chemical reactions do not present changes in the number of oxidation or relative load of the elements. So they are not considered redox reactions.

The solvent of the double displacement reactions usually is water and the reagents and products are usually ionic compounds (cations or anions are exchanged), although they can also be acids or bases.

In general, this type of reaction can be expressed as:

AB + CD ⇒ AD + CD

An example is:

BaO2 + H2SO4 ⇒ BaSO4 + H2O2

In the reaction their ions combine with each other very easily, to form substances that remain stable in the reactant medium (solvent)