Explain why some salts absorb heat when dissolved and others release heat. Define the term "enthalpy of solution".

Step 1: Address Post-lab Question 1.
Some salts absorb heat (endothermic) during dissolution when the energy required to break solute-solute and solvent-solvent bonds is greater than the energy released by forming new solute-solvent bonds. Other salts release heat (exothermic) when the energy released from forming new solute-solvent bonds exceeds the energy absorbed to break existing bonds. This net energy change is known as the enthalpy of solution.

The answer is:
Some salts absorb heat because the energy needed to break their bonds and separate solvent molecules is greater than the energy released when new solute-solvent bonds form. Others release heat when the energy released by forming new solute-solvent bonds is greater than the energy absorbed to break the initial bonds. This difference determines if the dissolution is endothermic or exothermic.