A chemical reaction was producing a new substance.
Explanation:
Equation of the reaction
NaOH + HCl --> NaCl + H2O
Heat of neutralization is the amount of heat evolved when one mole of acid reacts with one mole of an alkaline to form one mole of water.
When a neutralisation reaction is carried out, there is an exothermic change; energy released into the surroundings therefore since heat is given out which is what is felt.