While preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring. Explain why?

Hint: For answering this type of question we must remember the difference between exothermic and endothermic processes.

Complete step-by-step answer:
Exothermic reactions are chemical reactions which release energy by light or heat. It's the opposite of an endothermic reaction. Expressed in an equation of chemicals: $reac\tan ts \to products + energy$. In an exothermic reaction ‘exo’ means releases and ‘thermic’ means heat.
An endothermic process is any process that requires or consumes thermal energy, usually in the form of heat from its surroundings. It can be a chemical process, such as ammonium nitrate dissolving in water or a physical process, such as ice cube melting
When Dilute Sulphuric Acid is being prepared in a laboratory, acid is gradually applied to water with regular stirring because it will not cause any harm to the person or any equipment present in the laboratory. But water is never applied to the acid because it will be an exothermic reaction and it will be an exothermic phase and can cause the person to be fatal also when water is applied directly to the acid then the temperature will unexpectedly increase and the acids in bulk will begin to spur out with serious consequences.
Hence when we prepare dilute sulphuric acid in a laboratory, acid is gradually applied to water with regular stirring because it does not cause any harm to the person or equipment in the laboratory.

Note: In an exothermic process the heat generated during the reaction if heat produced is very much then it may cause an accident resulting in damage of property and hurting the person performing the reaction. So, we must remember the definition and exothermic and endothermic reactions.