If water is added to an acid, what will happen?

in the water you have both OH negative ions (@ hydroxide ion) and H positive ions (actually hydronium to be exact H30+) if you add more water to the acid you'll get an explosion basically due to high levels exothermic reaction.

So add acid to water. As the OH negative ions (@ hydroxide ion) are the sources of alkali, it neutralises the hydronium ions present in acid as acid ionises (ionisation ->> formation of ions from atom or molecule) in water to form hyrdonium ion.

Definition of weak acid -> acid that ionises and dissociates PARTIALLY in water to form H+ @ hyrdonium ion (H30+)

e.g. HCOOH + H20 -> (HCOO-) + (H30+)
see, here it forms methanoate ion and hydrogen positive. If you look closer you'd actually see that there is still hydrogen in the methanoate ion, so it ionises partially only; not all hydrogen atoms are ionised into hydronium

Definition of strong acid -> acid that ionises and dissociates COMPELTELY in water to form H+ @ hyrdonium ion (H30+)

e.g. H2SO4 + 2H20 -> (SO4 ^2- ) + 2(H30+)
here, it forms sulphate ion (the sign '^' is to show the charge) and hydronium ion and none of the hydrogen atoms in the hydrochloric acid initially remains. All are completely reacted into hydronium

So, at the same concentration you'll never get the same pH value.