Electronic thermometer measuring the change of temperature during an exothermic reaction. In the beaker at right, a coil of magnesium metal is added to hydrochloric acid. This reaction generates heat, and the temperature is seen to rise from 27.2 degrees Celsius to 36.3, in just less than two minutes. The reaction between the metal and acid produces soluble magnesium chloride, and hydrogen gas, which is seen bubbling off.