Electronic thermometer in a beaker showing the temperature change during an exothermic reaction. Initially, the beaker contains hydrochloric acid solution at a temperature of 27.9 degrees Celsius. A strip of magnesium ribbon is then added, which reacts vigorously with the ribbon, producing hydrogen gas bubbles and soluble magnesium chloride. This reaction is exothermic, it gives out heat, and the temperature is seen to rise to more than 45 degrees Celsius.