Electronic thermometer measuring the change of temperature during an endothermic reaction. In the beaker at centre, an electronic probe measures the temperature of a beaker of water, as potassium chloride crystals are added. Dissolving potassium chloride in water requires energy, as it takes more energy to break bonds between the potassium and chloride ions than is recovered by forming a solution of them with the water molecules. The temperature is seen to drop 24.7 degrees Celsius to 18.2, in less than 30 seconds. The energy change associated with the dissolving of a substance is called the enthalpy change of solution.