The formation of a white precipitate and the increase in temperature suggest a chemical reaction has taken place. This indicates that the components of the two liquids underwent a transformation resulting in new substances. Therefore, the best explanation is that the chemical identity of the substances involved changed.
When two liquids are mixed and a solid precipitate forms, it typically means that an insoluble compound has been produced as a result of the reaction. Additionally, the rise in temperature indicates that the reaction is exothermic, meaning that heat is released during the process. This release of energy further supports the idea that a new chemical identity has formed, as the original reactants have transformed into different products.
In summary, the changes observed in the experiment—specifically the formation of a precipitate and the increase in temperature—are strong indicators that new substances have formed, contradicting the statement that the chemical identity of the substances involved did not change.