Question

when concentrated sulphuric acid is added to water what chsnge

Answer 1

If you add water to concentrated sulfuric acid, it can boil and spit and you may get a nasty acid burn. If you're wondering about thetemperature change, mixing 100 ml of concentrated sulfuric acid and 100 ml of water initially at 19°C reaches a
temperature over 131°C within a minute.

Answer 2

The reaction of water (H2O) and sulphuric acid (H2SO4) is very exothermic and gives out heat.(however, this answer is in accordance with school textbooks. in real cases the reaction of even 98% pure H2SO4 with water isn't to much exothermic)

If water is added to the concentrated sulphuric acid, it can boil and spit dangerously. One should always add the acid to the water rather than the water to the acid. The necessity for this safety precaution is due to the relative densities of these two liquids. Water is less dense than sulphuric acid, meaning water will tend to float on top of this acid.

When H2SO4 is dissolved in water it dissociates to produce ions. The H+ ions react with the water molecules to form the hydrogen ions.  

The dissociation takes place in two steps, as in:

H2SO4 + H2O ↔ H3O+ HSO4-

The HSO4- ion also dissociates to a small degree:  

HSO4- + H2O → H3O+ + SO42-