The output from a double displacement reaction is either a gas, precipitate, or water. However, one point to be noted here is that if all the reactants and products happen to be in the aqueous form, then it is not considered as a double displacement reaction.

Each reaction has reactants that react with each other to form compounds called products. In most cases, the reactants are compounds. Each of the reacting compound lends one molecule/compound for exchange with the molecule/compound offered by the other compound. Anions and cations of compounds change their placement to react and form altogether different compounds. This type of reaction is also called metasynthesis or double replacement. The standard representation of this type of equation is as given below: This equation can be represented in terms of the ionic charges as follows: Let us understand the features of this type of reaction with a chemical equation in which silver nitrate reacts with sodium chloride to form the resultant products―sodium nitrate and silver chloride. ▶ In the standard representation, U, V, X, and Y represent a compound or an element. In the given example, AgNO₃ comprises silver and nitrate. Nitrate, in turn, is a compound of nitrogen and oxygen. NaCl contains only two elements, sodium and chlorine. ▶ The reactants or the compounds reacting with each other are ionic in nature. In the given example, Cl^- and NO₃^- are negative ions. ▶ They interchange elements with each other. Silver nitrate gives away NO₃^- and sodium chloride gives away Cl^-. These ions are interchanged by the two compounds, thus forming sodium nitrate and silver chloride. ▶ Here, we can observe the fact that silver nitrate is a compound containing three elements. The entire NO₃^- ion gets attached to the sodium atom. In most of the cases, the reactants of these equations are in the aqueous form. However, the product may be a precipitate, gas, or a liquid. If the product obtained is in the gaseous or solid form, it becomes easier to separate it.

The reaction in which a salt reacts with an acid, the resulting product is in the gaseous form. #1: Na₂CO₃ (aq) + 2 HCl (aq) ➜ 2NaCl (aq) + H₂O (l) + CO₂ (g) In this case, sodium carbonate reacts with hydrochloric acid. The resulting compounds produced are sodium chloride, water, and carbon dioxide (which is a gas). #2: NH₄OH (aq) ➜ NH₃ (g) + H₂O (l) In this reaction, two hydrogen atoms react with an oxygen atom to form a water molecule. The remaining molecules of ammonium hydroxide are given out as ammonia, which is a gas. #3: Na₂S (aq) + 2 HCl (aq) ➜ H₂S (g) + 2NaCl (aq) In the above reaction, sodium sulfide reacts with hydrochloric acid. The resulting products are hydrogen sulfide gas and sodium chloride.