[Mikez]

How do you determine/predict whether a double displacement has occurred. For example (from textbook)  NaCl + KNO3 -->  NO REACTION but there was no explanation why N.R/

thanks

[kankerfist]

Does it have in parentheses what state the reactants are in?

[Bakegaku]

If the reaction happens between two solutions, a double-replacement reaction means a precipitate has formed.  If both products are soluble, no reaction will happen.

[UnintentionalChaos]

Like bakegaku said, in solution a double replacement will only occur if there is a precipitate to drive the reaction toward the products. Otherwise, you have ion soup and no individual product or reactant really exists in it. All reactions are attempting to reach equilibrium. If you mix silver nitrate and sodium chloride, as the ions try to achieve a mix (if you will) of reactants and products, silver chloride will precipitate. The ions then push toward balance again in a continous process until all the silver remaining in solution is held there by the small solubility of silver chloride.

Example 2NaCl + CuSO4 when placed in solution form Na⁺, Cl^-, Cu²⁺, and SO₄^2-. So far we have ion soup. If the mix is chilled, Na₂SO₄*10H₂O will crystallize out to some extent. This is a double replacement since there is now an increased ratio of ions favoring product formation in solution and the solid is a product which has been removed from the system.