Heat of Solution
When sodium hydroxide, NaOH, dissolves in water, the solution becomes warmer. The solution releases energy to the surroundings. In contrast, when ammonium nitrate, NH4NO3, dissolves in water, the solution becomes colder. The solution absorbs energy from the surroundings. 
During the formation of a solution, energy is either released or absorbed.
Like chemical reactions, the solution process can be described as exothermic or endothermic. Dissolving sodium hydroxide in water is exothermic, as it releases heat. Dissolving ammonium nitrate in water is endothermic, as it absorbs heat. The How It Works box on page 233 describes how dissolving ammonium nitrate is used in cold packs.
In order for a solution to form, both the attractions among solute particles and the attractions among solvent particles must be broken. Breaking attractions requires energy. As the solute dissolves, new attractions form between solute and solvent particles. The formation of attractions releases energy. The difference between these energies is known as the heat of solution. For example, dissolving one mole of sodium hydroxide in water releases 44.5 kilojoules of heat. In this exothermic change, energy is released as NaOH and H2O form new attractions. It is 44.5 kilojoules greater than the energy required to break the attractions among NaOH crystals and among H2O molecules.
Does the breaking of attractions among solvent particles release energy or absorb energy?
Quick Lab
Comparing Heats of Solution
Materials
2 large test tubes, 10-mL graduated cylinder, distilled water, thermometer, 1 g potassium chloride, 5 mL 95% isopropyl alcohol solution, stirring rod
Procedure 
Add 5 mL of distilled water to each test tube. Measure and record the temperature of the water in each test tube to the nearest 0.2°C.
Remove the thermometer. Add the potassium chloride to one of the test tubes. Stir until the potassium chloride dissolves. CAUTION: Use the stirring rod, not the thermometer, to stir the solution.
Measure and record the final temperature of the solution. Rinse the thermometer.
Add 5 mL of the alcohol solution to the second test tube. Stir the mixture. Measure and record the final temperature.
Analyze and Conclude
Observing What happened to the temperature of each solution?
Classifying Which process was exothermic? Which was endothermic?
Inferring If there were no change in temperature during the formation of a solution, how would you explain this observation?
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