Question

How do the two differ $\left( i \right)$ adding water to an acid $\left( {ii} \right)$ adding acid to water?

Answer 1

Chemical reactions are of two types depending on the evolution of energy in the reaction. Reaction may be endothermic or exothermic which include either absorption of energy or release of energy.

Complete answer:
The basic characteristic of an endothermic reaction is that these reactions require an input of heat in the form of energy or as radiation to initiate the chemical reaction. This energy facilitates the conversion of reactant to product. At the end of the reaction, it is noted that the energy of the product is always larger than the reactant.
On the other hand, an exothermic reaction is characterized by the release of energy as the reaction proceeds. It is observed that the energy of reactants is lower than energy of products in case of exothermic reaction.
For example: addition of water and acid together is a type of exothermic reaction which is associated with release of heat.
When we add acid into water with proper stirring of the solution, the temperature rises slowly because water has a better heat capacity than acid. Heat capacity is a physical property of any compound which is defined as the total amount of heat that must be supplied to the substance to increase its temperature by one degree.
As the heat capacity is larger than acid, the temperature of water does not rise suddenly as a result of which reaction does not become violent.
But when we add water in acid, due to poor heat capacity of an acid, the temperature of acid rises suddenly and the temperature of the mixture becomes too high and reactions become violent in nature.
Therefore, it is always suggested to add acid in water.

Note:
The final compound formed by endothermic reaction is less stable than compounds which are formed by exothermic reaction because the energy in the endothermic product is high. Another example of exothermic reaction is like burning a candle etc.