HCl(aq) + NaOH(aq) —> NaCl(aq) + H2O(l)
When you mixed HCl and NaOH in a beaker, it produces NaCl and H2O. HCl is a strong acid and NaOH is a strong base. Acid-base reactions are exothermic. Heat was released by the reaction of the acid and the base to water.
Exothermic reactions mean that the energy of the products is less than the energy of the reactants, and so the change in enthalpy is negative.
