Turn up the stove all you want: boiling water will never get hotter than 100 °C.
You turn a burner from low to high and pour five times more heat into a pot. The thermometer in the rolling boil does not rise past 100 °C. Where does the energy go? This page separates two kinds of energy: faster particles (thermal energy) and particles pulling apart (phase energy). You use that split to read heating curves and to add up the heat for every stage.
Core Claims
- Slopes and plateaus. On a slope, added energy speeds up the particles (thermal energy, Eth) and the temperature rises: q = mcΔT. On a plateau, added energy pulls particles apart (phase energy, Eph) and the temperature stays constant.
- One temperature during a change. While a phase change runs, two states of matter coexist. All the added heat (latent heat) separates particles. The average kinetic energy does not change.
- Boiling takes more than melting. Vaporization (ΔHv = 2,260 J/g) takes nearly 7 times the energy of fusion (ΔHf = 334 J/g). Melting loosens the lattice but the molecules stay in contact. Boiling pulls them apart completely.
- Bonds stay whole. A phase change disrupts the attractions between molecules, such as hydrogen bonds. It never breaks the covalent bonds inside a molecule. Steam is still H2O.
- Add the stages. To find the heat across several stages, calculate each slope and each plateau separately. Then add them: qtotal = ∑ qi.
Heating curve layout
Retrieval Checklist
- Explain why the temperature stays constant during melting and boiling.
- Contrast thermal energy (Eth) with phase energy (Eph).
- Explain why ΔHv is much larger than ΔHf for water.
- Calculate the total heat for a process that crosses phase boundaries.
Why the thermometer stops rising.
Heat ice at −20 °C with constant power. The temperature does not climb steadily. The thermometer traces a staircase called a heating curve.
A heating curve has two kinds of region.
Slopes: thermal energy (Eth)
In regions I (ice), III (liquid water), and V (steam), the substance stays in one state. Every joule you add speeds up the particles, so the temperature rises. The heat follows q = m × c × ΔT.
Plateaus: phase energy (Eph)
In regions II (melting at 0 °C) and IV (boiling at 100 °C), heat keeps entering but the temperature stays fixed. The added energy is latent heat. It pulls particles apart against the intermolecular forces. The particles do not speed up, so the energy is stored as potential phase energy. The heat follows q = m × ΔHf for melting and q = m × ΔHv for boiling.
Thinking boiling tears H2O molecules into hydrogen and oxygen. It does not. The covalent bonds inside a molecule (intramolecular forces) need about 460 kJ/mol to break. A phase change only overcomes the weaker attractions between molecules (intermolecular forces), such as hydrogen bonds at about 20 kJ/mol. Steam from a kettle is still H2O. The molecules are just far apart.
Heating curve and particle explorer.
Add heat to a 50.0 g sample of water. Drag the heat slider or press a preset. Watch the particle picture change as the sample moves along the curve.
Why does the temperature stop rising while ice melts?
The explanation appears after you predict.
- One water molecule (24 stand for 50 g)
- Hydrogen bond between molecules in ice
- The heating curve so far, with a dot at the sample
- Add 10 kJ, then press Record. Look for: the temperature sits at 0.0 °C while the lattice loosens.
- Add 30 kJ, then press Record. Look for: all the ice has melted and the water has warmed to about 54 °C.
- Add 100 kJ, then press Record. Look for: the temperature sits at 100.0 °C while molecules leave the liquid.
- Add 154 kJ, then press Record. Look for: every molecule is a gas and the temperature rises again.
Use your evidence
Why boiling takes 7 times more energy than melting.
Look at the curve for 50 g of water. Melting takes 16.7 kJ. Boiling takes 113.0 kJ. The heat of vaporization (ΔHv = 2,260 J/g) is 6.8 times the heat of fusion (ΔHf = 334 J/g). Why does turning liquid into gas take so much more energy than turning solid into liquid?
The reason is the distance between molecules.
- Melting (ΔHf = 334 J/g). The rigid ice lattice collapses into a fluid. The molecules stay packed together. Liquid water is even denser than ice. The molecules keep about 85% of the hydrogen bonds that ice has. The energy only disrupts the ordered arrangement. It does not separate the molecules.
- Vaporization (ΔHv = 2,260 J/g). Liquid molecules must separate completely. The average distance between molecules grows about tenfold. The volume grows over 1,600 times. Overcoming all the intermolecular attractions takes nearly 7 times the energy of melting.
One gram of boiling water cools from 100 °C to skin temperature (37 °C) and releases 264 J. Here q = 1 g × 4.184 J/(g·°C) × (37 − 100 °C) = −264 J. One gram of steam at 100 °C must condense first. It dumps 2,260 J into the skin before the liquid starts to cool. In total the steam releases 2,260 + 264 = 2,524 J, nearly 10 times as much as boiling water.
Expecting boiling to cost about the same as melting because both are “just a phase change.” Boiling takes nearly 7 times more energy per gram. Melting only loosens the molecules. Boiling pulls them completely apart.
Constants for water (H2O)
| Quantity | Symbol | Value | Where it is used on the curve |
|---|---|---|---|
| Specific heat of ice | cice | 2.09 J/(g·°C) | Region I (ice warming, below 0 °C) |
| Heat of fusion | ΔHf | 334 J/g (6.01 kJ/mol) | Region II (melting plateau at 0 °C) |
| Specific heat of liquid water | cliquid | 4.184 J/(g·°C) | Region III (liquid warming, 0 to 100 °C) |
| Heat of vaporization | ΔHv | 2,260 J/g (40.7 kJ/mol) | Region IV (boiling plateau at 100 °C) |
| Specific heat of steam | csteam | 2.01 J/(g·°C) | Region V (steam warming, above 100 °C) |
Multi-stage phase change calculator.
A sample that changes temperature and phase needs more than one equation. Build a roadmap: one equation for each stage, then add the results.
- Slope stage: q = m × c × ΔT
- Plateau stage: q = m × ΔH
Using q = mcΔT across a plateau. The temperature does not change on a plateau, so ΔT = 0 and the formula gives zero. A phase change needs q = mΔH.
How much heat does a multi-stage change need?
- A slope stage: the heat changes the temperature, q = mcΔT
- A plateau stage: the heat changes the phase, q = mΔH
- Load the melting ice case. Look for: two stages, a warming slope and a melting plateau.
- Load the five-stage case. Look for: five stages, and the boiling plateau is the largest.
- Load the ethanol case. Look for: one stage only, because 78 °C is below the boiling point of 78.4 °C.
Using latent heat: sweat, coolers, and smart materials.
Water's high heat of vaporization (ΔHv = 2,260 J/g) does real work. It helps your body shed heat, and it can cool a building without electricity.
Evaporative cooling in your body
Sprint on a 35 °C day and your muscles make extra thermal energy. Your skin releases sweat. The fastest surface molecules escape as vapor. Each gram of sweat that evaporates takes 2,260 J of thermal energy from your body. The slower molecules stay behind, so your skin cools and your organs stay safe.
Phase change materials (PCMs)
Some builders put microencapsulated paraffin wax into walls and ceilings. When a room warms to about 23 °C, the wax begins to melt. It absorbs hundreds of kilojoules of latent heat while the room stays at the same temperature. At night the wax freezes and gives the energy back, so the building needs less air conditioning.
Thinking sweat cools you because sweat is cold. Sweat cools you only when it evaporates. Sweat that drips off takes no energy with it.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. On a heating curve, the flat plateaus show a change in while the temperature stays constant.
2. The energy that melts 1 g of a solid at its melting point is the heat of .
3. For water, the heat of vaporization (ΔHv) is about times the heat of fusion (ΔHf).
4. Boiling water does not break bonds. It only overcomes attractions between molecules.
5. Sweating cools you because each gram of evaporating water absorbs J from your skin.
A student watches a pot of water boil and says two things:
“First, if I turn the heat to maximum, the water will reach 110 °C and cook the pasta faster. Second, the bubbles at the bottom are hydrogen and oxygen gas that the heat has split apart.”
Critique both parts. Use what you know about temperature during a phase change and about particles.
Say what happens to the temperature during a phase change, where the burner's energy goes, and what the gas in the bubbles is. Write your answer first, then compare it with the model answer.
Both claims are wrong. First, liquid water cannot get hotter than 100 °C at normal pressure. A higher flame raises the heat flow, not the temperature. The extra energy goes into phase energy (Eph), so the water boils faster at the same 100 °C. The pasta cooks at the same temperature.
Second, boiling is a physical change. The energy supplied is far too small to break the O–H bonds inside water molecules (about 460 kJ/mol). The bubbles hold water vapor, made of H2O molecules. They do not hold H2 or O2 gas.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
A student heats a 30.0 g sample of ice that starts at −15.0 °C.
Data for H2O:
• cice = 2.09 J/(g·°C)
• cliquid = 4.184 J/(g·°C)
• ΔHf = 334 J/g
• ΔHv = 2,260 J/g
(a) Calculate the total energy in joules to warm the ice to 0.0 °C and melt it. Show each step. [2 marks]
(b) The student then boils the 30.0 g of liquid water at 100.0 °C. Calculate the heat needed to boil it (q = mΔHv). Then explain with particles why boiling takes over six times more energy than melting. [2 marks]
- Ice warming (1 mark): You calculate q1 = 30.0 × 2.09 × 15.0 = 940.5 J.
- Melting and total (1 mark): You find q2 = 10,020 J and a total of 10,960.5 J.
- Boiling heat (1 mark): You calculate q = 30.0 × 2,260 = 67,800 J.
- Particle reason (1 mark): You say melting keeps molecules in contact, and boiling separates them completely.
Hurricanes run on the phase change of water. Tropical oceans at 27 °C or warmer evaporate huge amounts of water. The moist air rises, cools, and condenses into clouds. Each kilogram of water that condenses releases 2.26 million joules into the air. That energy drives the storm's updrafts and winds.
Adaptive Competency Practice: HS-PS3-1
Test your mastery of phase transitions and heating curves. The adaptive engine models plateau regions, latent heat of fusion and vaporization (q = mΔH), multi-stage energy summations (Σq), and slope variations from specific heat differences.