Heat of Solution
Dissolving can absorb or release heat.
About this topic
The heat of solution is the energy change when a solute dissolves. Dissolving involves breaking solute-solute attractions (endothermic), breaking solvent-solvent attractions (endothermic) and forming solute-solvent attractions (exothermic). If the energy released exceeds what is absorbed, the process is exothermic and the solution warms up; otherwise it is endothermic and cools.
Calcium chloride dissolves exothermically and is used in some instant heat packs, while ammonium nitrate dissolves endothermically and is used in instant cold packs. For ionic solids, the heat of solution is lattice energy (positive) plus hydration energy (negative). If the sum is small, the temperature effect is small, as for table salt. The temperature change can be measured with a coffee-cup calorimeter using q = m c ΔT.
Worked example: dissolving 5.0 g of NH4NO3 in 50 g of water drops the temperature by about 6 degrees Celsius, so energy was absorbed from the surroundings. Le Chatelier's principle also predicts that solubility of endothermic dissolvers rises with temperature, matching most solid solubility curves.
A misconception is that dissolving is always cooling or always heating. It depends on the substance. Another is that a temperature drop means no energy is involved; it means energy flowed from the water into the dissolving process. Also, do not confuse heat of solution with the rate of dissolving.
Key ideas
- Endothermic: cold packs
- Exothermic: hand warmers
- Lattice and hydration energies compete
Quick check
Use cases: where Heat of Solution helps
- Homework: review the key idea behind Heat of Solution before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Heat of Solution without notes.
- Visual learning: play with the interactive lab to see how Heat of Solution changes when you adjust its inputs.
- Teaching a friend: explain Heat of Solution in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Heat of Solution?
Dissolving can absorb or release heat.
What will I learn in Heat of Solution?
Endothermic: cold packs. Exothermic: hand warmers. Lattice and hydration energies compete.
Is Heat of Solution part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Heat of Solution lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Heat of Solution?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.