System and surroundings
To study heat, first mark off the system体系: the part you care about, such as substances reacting or a salt dissolving. Everything else is the surroundings环境, including the solvent water, the beaker, the air, and your hand.
Heat always flows between the system and the surroundings, and signs are taken from the system’s point of view:
- In an exothermic process放热过程, heat flows from the system to the surroundings, so q < 0 and ΔH < 0. The surroundings get warmer.
- In an endothermic process吸热过程, heat flows from the surroundings to the system, so q > 0 and ΔH > 0. The surroundings get colder.
Using temperature changes
In an experiment, the thermometer usually measures the surroundings, most often the water in a solution. So:
- if the solution or container warms up, the system is releasing heat;
- if the solution or container cools down, the system is absorbing heat.
When NH₄NO₃ dissolves in a cold pack, for example, the solution gets cold: the dissolving process absorbs heat from the water, so it is endothermic. Don’t describe it as “releasing cold.”
Physical changes have heat effects too
Endothermic and exothermic aren’t only for chemical reactions (2.10.2). The AP exam also expects you to judge physical changes:
| Process | Endothermic or exothermic | Why |
|---|---|---|
| Melting, vaporization, sublimation | Endothermic | Intermolecular forces must be overcome to pull particles apart |
| Freezing, condensation, deposition | Exothermic | Attractions form as particles come together, releasing energy |
| Dissolving | Either | NaOH dissolving is exothermic; NH₄NO₃ and KNO₃ dissolving are endothermic (U6.2 explains why) |
So a temperature change alone can’t tell you whether a change is chemical or physical. For that, look for new substances (U4.4).
Judge each process below: endothermic or exothermic? Chemical or physical?
Ammonium nitrate dissolving in water (a cold pack)
NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq)
Observation: Squeezing a cold pack makes it feel cold within seconds.
ΔH = +25.69 kJ/mol (CRC Handbook data)
The solution’s temperature falls, so the system absorbs heat from its surroundings (including the water): endothermic. Dissolving only spreads ions through the water and makes no new substance, so it is a physical change.
Sodium hydroxide dissolving in water
NaOH(s) → Na⁺(aq) + OH⁻(aq)
Observation: As the solid dissolves, the beaker feels hot.
ΔH = −44.51 kJ/mol (CRC Handbook data)
The system releases heat, warming its surroundings: exothermic. Dissolving is a physical change.
Ice melting
H₂O(s) → H₂O(l)
Observation: Holding an ice cube makes your hand feel cold.
ΔH = +6.01 kJ/mol (CRC Handbook data)
The ice absorbs heat from your hand (the surroundings) to overcome intermolecular forces: endothermic. Only the state changes, so it is a physical change.
Steam condensing
H₂O(g) → H₂O(l)
Observation: A burn from 100 °C steam is worse than one from 100 °C water.
ΔH = −40.7 kJ/mol (CRC Handbook data)
Steam releases a lot of heat as it condenses to liquid: exothermic. That heat goes into the skin, making the burn worse. Condensing is a physical change.
Water freezing
H₂O(l) → H₂O(s)
Observation: Tubs of water placed in a root cellar on a freezing night help keep stored vegetables from freezing.
ΔH = −6.01 kJ/mol (CRC Handbook data)
Water releases heat as it freezes, which keeps the cellar from dropping quickly below 0 °C: exothermic. A physical change.
Methane burning
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Observation: A gas burner’s flame heats a pot of water.
ΔH = −890.3 kJ/mol (calculated from the NBS enthalpy-of-formation table)
Combustion turns chemical energy into heat, so the system releases heat to the surroundings: exothermic. CO₂ and H₂O form, so it is a chemical change.
Heating limestone
CaCO₃(s) → CaO(s) + CO₂(g)
Observation: The solid must be heated strongly and continuously; when the heating stops, so does the reaction.
ΔH = +178.3 kJ/mol (calculated from the NBS enthalpy-of-formation table)
The decomposition keeps absorbing heat from outside: endothermic. CaO and CO₂ form, so it is a chemical change.
Quicklime reacting with water (a self-heating meal)
CaO(s) + H₂O(l) → Ca(OH)₂(s)
Observation: After water is added, the bottom of the container gets very hot.
ΔH = −65.2 kJ/mol (calculated from the NBS enthalpy-of-formation table)
The system releases heat to its surroundings: exothermic. Ca(OH)₂ forms, so it is a chemical change.
Barium hydroxide crystals with ammonium chloride
Ba(OH)₂·8H₂O(s) + 2NH₄Cl(s) → BaCl₂(aq) + 2NH₃(g) + 10H₂O(l)
Observation: Two solids are stirred in a beaker resting on a wet board, and the water on the board freezes, sticking the beaker to it.
This reaction is described qualitatively only.
The beaker and the water on the board both get colder as heat flows into the reacting system: endothermic. New substances form (you can smell ammonia), so it is a chemical change.
Potassium nitrate dissolving in water
KNO₃(s) → K⁺(aq) + NO₃⁻(aq)
Observation: Stirring potassium nitrate into water makes the thermometer reading drop.
ΔH = +34.89 kJ/mol (CRC Handbook data)
The solution’s temperature falls, so the system absorbs heat: endothermic. Dissolving is a physical change.
Example 1: Drawing conclusions from dissolving data
5.0 g of a salt is stirred into 100 mL of water, and the solution’s temperature falls from 22.0 °C to 18.5 °C. Is dissolving endothermic or exothermic? Which way does heat flow? Is this a chemical or physical change?
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Identify the system and surroundings. The system is the dissolving salt (the dissolving process); the surroundings are mainly the water in the solution, and the thermometer reads the surroundings’ temperature.
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Look at the surroundings’ temperature change.
The water’s temperature falls, so the water lost heat.
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Direction of heat flow. The heat the water lost went into the dissolving process: heat flows from the surroundings into the system, so the system absorbs heat. Dissolving is endothermic, with ΔH > 0.
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Kind of change. The salt only dissolves into ions spread through the water; no new substance forms, so it is a physical change.
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Note. A falling thermometer doesn’t mean the system “released cold”: the thermometer reads the surroundings, and surroundings that cool mean the system absorbed heat.
Example 2: Evaluating a claim
Anhydrous calcium chloride (CaCl₂) is stirred into water, and the solution gets very hot (some instant heat packs use this). A student says, “A lot of heat was released, so a chemical reaction happened.” Evaluate the claim.
- Heat effect. The solution gets hot, so the surroundings (the water) gain heat; the process is exothermic, ΔH < 0. That part is right.
- Identify the student’s reasoning. The student reasons “releases heat → chemical reaction”.
- Test that reasoning. Releasing heat doesn’t show a chemical change: physical changes such as dissolving, condensing, and freezing release heat too. A chemical change is decided by whether a new substance forms.
- Identify the process. dissolves into and ions spread through the water; ion–water attractions release energy, but no new substance forms: a physical change.
- Conclusion. The claim doesn’t hold: releasing heat only shows that ΔH < 0, not that a new substance formed.
- Ice melting to water
Intermolecular forces are overcome.
- Water freezing to ice
The reverse of melting.
- Sweat evaporating
Vaporization absorbs heat, which is why sweating cools you.
- Water droplets forming on the inside of a window in winter
Condensing water vapor releases heat.
- Dry ice subliming
The solid turns directly into gas, which means pulling molecules apart.
- Frost forming on grass on a cold night (vapor turning directly to ice)
Deposition releases heat.
- Wood burning
All combustion reactions are exothermic.
- Photosynthesis in a plant
Light energy is absorbed and stored as chemical energy.
- Ammonium nitrate dissolving as the solution cools
The surroundings cool, so the system absorbs heat.
- HCl and NaOH solutions neutralizing as the solution warms
The surroundings warm, so the system releases heat.
Common mistakes
Mistake 1: “The solution got cold, so the process released cold.” There’s no such thing as “cold” to release. The solution cools because heat flows from it (the surroundings) into the system: the process is endothermic.
Mistake 2: “If there’s a heat change, there must have been a chemical reaction.” Physical changes such as dissolving and phase changes have heat effects too. A chemical change is identified by new substances forming.
Mistake 3: “An endothermic process needs continuous heating to happen.” Some endothermic processes happen on their own: ammonium nitrate dissolves at room temperature while absorbing heat from its surroundings. Whether a process needs heating is a separate question from whether it is endothermic.
Quick check
A solid is added to water, and the solution’s temperature drops from 25.0 °C to 21.0 °C. Which statement is correct?
The surroundings (water) cool, so heat flows from the surroundings into the system, which is endothermic. Physical changes can be judged as endothermic or exothermic too.
The iron powder in a hand warmer is oxidized by air, and the hand warmer gets hot. What kind of process is this?
Iron is oxidized to a new substance (an iron oxide), so it is a chemical change; the hand warmer (surroundings) gets hot, so the system releases heat.
Which of these processes is exothermic?
Condensation, the reverse of vaporization, releases heat; A, B, and D all pull particles apart, which absorbs heat.
Which is the best explanation for why sweating helps cool the body?
Vaporization is endothermic: heat flows from the skin (surroundings) into the evaporating sweat (system), so the skin cools.
Two liquids are mixed and the mixture warms up, but no new substance forms. A student says, “Heat was released, so a chemical reaction happened.” Which evaluation is best?
Heat is also released when stronger attractions form between molecules. A heat effect can’t distinguish a physical change from a chemical one.
A reaction releases 50 kJ of heat. In terms of system and surroundings, which statement is correct?
The heat the system releases is absorbed by the surroundings: equal in size and opposite in sign.
AP-style free response
A cold pack holds solid NH₄NO₃ and a small pouch of water. When the pouch is broken, the NH₄NO₃ dissolves: . In an experiment, 8.00 g of NH₄NO₃ is dissolved in 100.0 g of water, and the solution’s temperature falls from 22.0 °C to 16.3 °C.
(a) Is this dissolving process endothermic or exothermic? Describe how heat flows between the system and the surroundings.
(b) Is this a chemical or physical change? Justify your answer.
(c) Taking the solution’s mass as 108 g and its specific heat as 4.18 J/(g·°C), calculate the heat lost by the solution.
(d) Use your answer to (a) to explain why a cold pack cools a sprained ankle.
(e) A student says, “Dissolving is endothermic, so NH₄NO₃ must be heated before it will dissolve.” Evaluate the claim.
Model answer and scoring
(a) Endothermic. The solution (surroundings) cools, so heat flows from the solution into the dissolving process (system), and ΔH > 0.
(b) A physical change. NH₄NO₃ only dissolves into NH₄⁺ and NO₃⁻ ions spread through the water; no new substance forms.
(c) q = m·c·ΔT = 108 g × 4.18 J/(g·°C) × (16.3 − 22.0) °C = −2.6 × 10³ J, so the solution loses about 2.6 kJ of heat (absorbed by the dissolving process).
(d) When the cold pack sits on the ankle, the dissolving process absorbs heat from its surroundings, including the ankle, so the ankle’s temperature drops.
(e) Incorrect. NH₄NO₃ dissolves on its own at room temperature, taking the heat it needs directly from the surrounding water, which is why the solution gets cold. Being endothermic is a separate question from needing outside heating.
Scoring: (a) endothermic, with the direction of heat flow; (b) no new substance; (c) the calculation and sign of q; (d) linked to endothermic; (e) notes that the process happens on its own.
Summary
- The system is what you study, and everything else is the surroundings. Exothermic: heat flows from the system to the surroundings, which warm up, and ΔH < 0. Endothermic: heat flows from the surroundings to the system, which cool down, and ΔH > 0.
- A thermometer usually measures the surroundings (the water in a solution): a warming solution means an exothermic process, and a cooling one means endothermic.
- Physical changes have heat effects too: melting, vaporization, and sublimation are endothermic; freezing, condensation, and deposition are exothermic; dissolving can be either.
- Heat effects can’t distinguish chemical changes from physical ones.
AP tip: In your justification, say which part’s temperature rises (or falls) and which way heat flows. When asked whether a change is chemical or physical, answer in terms of whether new substances form, not the temperature change.