Thermal changes accompanying physical and chemical changes Thermal changes accompanying physical changes - melting temperature It is the temperature change when one mole of solute is dissolved in a given amount of solvent (The amount of heat energy absorbed or released when one mole of solute is dissolved in a given amount of solvent). During the dissolution process: two processes occur: A- Dissociation: the breaking of bonds (endothermic reaction) Sodium and chloride ions dissociate from the crystal lattice as a result of the ionic bond breaking. The energy absorbed to break the bond is called the heat of the crystal lattice (bond energy). - Crystal lattice temperature: It is the energy needed to break the bond between ions in the crystal lattice. B- Hydration: The dissociated ions bind with water molecules to form hydration ions (exothermic process). Rehydration temperature: It is the amount of heat released when hydrating ions bind to water molecules. If the crystal lattice temperature > heat of hydration Dilution temperature: It is the heat change per mole of solute when diluted from a concentrated solution to a dilute solution. (It is the heat change of one mole of solute when diluted in a large amount of water (about 1000 moles of solvent)). Sedimentation temperature: The amount of heat released when one mole of an insoluble substance is completely precipitated. - heat of combustion It is the amount of heat released when one mole of a substance is completely burned in an abundance of oxygen. (The reaction is always exothermic). Formation temperature: It is the amount of heat released or absorbed when one mole of a compound is formed from its elemental elements so that the elements are in their standard state. (at 25 °C, a pressure of 76 x g). Note: Heat of formation of the elements = zero