* Photo: The left vial contains 1 ml of water of 18.6° C,
   the vial in the middle 1 ml concentrated sulfuric acid (It is coloured because it is not clean).
* Transfer the acid into the vial with water  (not the other direction!).
* Stirr with the thermometer.
* Observation: During mixing 1 ml conc. sulfuric acid and 1 ml water the temperature
   increases by 70°C.
Explanation:
An acid-base reaction  between molecules of sulfuric acid and of water took place:
H₂SO₄ + 2 H₂O ----> SO₄^2-   + 2 H₃O⁺
The resulting ions connect more water molecule releasing heat energy.