Because more energy is released than is used on adding NaOH. The temperature now drops because the acid now being added is at a lower temperature than the reaction mixture.
As a byproduct of chlorine synthesis, sodium hydroxide is created. It is crystalline solid and colourless in its purest form. This substance, which is made up of sodium cations and hydroxide anions, is very water soluble. NaOH draws moisture out of the atmosphere. It is quite corrosive, and it can burn your skin badly and irritate your eyes and other parts of your body.
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