IMAT Chemistry · States of Matter and Gas Laws
Changes of State, Boiling and Phase Diagrams
A change of state happens at constant temperature while energy breaks or makes the attractions between particles; boiling happens when the vapour pressure reaches the outside pressure, and dissolved solutes move the freezing and boiling points.
Why this matters
The 2013 paper asked for the correct names of the changes of state. The 2019 and 2021 papers asked why a solute makes ice melt below 0 °C, and how the outside pressure changes the boiling point of water.
Concept 1 of 4: The six changes of state and their energy
To turn a solid into a liquid, or a liquid into a gas, the particles must be pulled further apart against their attractions, and that takes energy. Going the other way, the attractions pull the particles together and energy is given out. While the change is happening the energy goes into the attractions, so the temperature stays the same.
Definition
- Changes that absorb energy (endothermic): melting (fusion), evaporation or boiling (vaporisation), sublimation.
- Changes that release energy (exothermic): freezing (solidification), condensation, deposition.
- Sublimation goes straight from solid to gas (iodine, dry ice). Deposition goes straight from gas to solid (frost on a cold window).
- During a change of state of a pure substance at constant pressure, the temperature stays constant. The energy, called latent heat, changes the potential energy of the particles, not their kinetic energy.
| Change | Name | Energy |
|---|---|---|
| Solid to liquid | Melting (fusion) | Absorbed |
| Liquid to solid | Freezing (solidification) | Released |
| Liquid to gas | Evaporation or boiling (vaporisation) | Absorbed |
| Gas to liquid | Condensation | Released |
| Solid to gas | Sublimation | Absorbed |
| Gas to solid | Deposition | Released |
Practice this conceptself-check · 3 quick reps
Freezing means liquid to solid
Freezing is the change from liquid to solid. A gas turning directly into a solid is deposition, and a solid turning directly into a gas is sublimation. Options that call gas to solid freezing are wrong.
The temperature does not rise during melting or boiling
While a pure substance melts or boils, the heat supplied breaks attractions between particles and the temperature stays constant. Only when the change is complete does the temperature start rising again.
Concept 2 of 4: Vapour pressure, evaporation and the boiling point
At the surface of a liquid, the fastest molecules escape into the air, and in a closed container they build up a pressure of vapour. The hotter the liquid, the more molecules escape and the higher this vapour pressure. When it becomes as large as the pressure pushing down on the liquid, bubbles of vapour can form inside the liquid itself: that is boiling.
Definition
- Vapour pressure: the pressure of the vapour in equilibrium with its liquid in a closed container. It rises steeply with temperature and depends on the liquid: weaker attractions between molecules give a higher vapour pressure (a more volatile liquid).
- A liquid boils when its vapour pressure equals the external pressure. The normal boiling point is the boiling point at 1 atm (100 °C for water).
- Higher external pressure raises the boiling point (a pressure cooker at about 2 atm boils water near 120 °C). Lower pressure lowers it (high in the mountains water boils below 100 °C).
- Evaporation happens at the surface at any temperature. It cools the liquid left behind, because the fastest molecules leave.
| Feature | Evaporation | Boiling |
|---|---|---|
| Where | At the surface only | Throughout the liquid, with bubbles |
| Temperature | Any temperature | Only where vapour pressure equals the external pressure |
| Speeded up by | Higher temperature, larger surface, wind | Supplying heat faster (the temperature stays the same) |
| Effect on the liquid | Cools the liquid left behind | Temperature stays at the boiling point |
Practice this conceptself-check · 4 quick reps
More pressure on a liquid means a higher boiling point
Raising the pressure above water from 100 kPa to more than that raises its boiling point; it does not lower it. That is how a pressure cooker cooks faster. Heating more strongly only makes the water boil faster, at the same temperature.
Concept 3 of 4: Phase diagrams in outline: triple point and critical point
A phase diagram is a map with pressure up the side and temperature along the bottom. Each region shows which state is stable at that pressure and temperature, and the lines between regions are the conditions where two states can exist together. Reading it is just finding a point and seeing which region it is in.
Definition
- Three regions: solid (low temperature), liquid, gas (high temperature, low pressure).
- The lines show where two states coexist in equilibrium. The liquid to gas line is the vapour pressure curve: it shows the boiling point at each pressure.
- The triple point is the one pressure and temperature where solid, liquid and gas all coexist.
- Above the critical point there is no difference between liquid and gas, and the gas cannot be liquefied by pressure alone.
- For most substances the solid to liquid line leans to the right. For water it leans to the left, because ice is less dense: pressure favours the denser liquid and slightly lowers the melting point.
| Feature | Meaning | Example values |
|---|---|---|
| Triple point | Solid, liquid and gas coexist | Water: 0.01 °C and about 0.006 atm |
| Critical point | End of the liquid to gas line | Water: about 374 °C and 218 atm |
| Normal melting and boiling points | Where the 1 atm line crosses the two lines | Water: 0 °C and 100 °C |
| Triple point above 1 atm | No liquid can exist at 1 atm; the solid sublimes | Carbon dioxide: triple point 5.1 atm, so dry ice sublimes at about at 1 atm |
Practice this conceptself-check · 3 quick reps
The triple point of water is not 0 °C at 1 atm
0 °C at 1 atm is the normal melting point of ice. The triple point of water is at 0.01 °C and a very low pressure, about 0.006 atm.
Concept 4 of 4: Solutes lower the freezing point and raise the boiling point
Dissolved particles get in the way of the solvent molecules. They make it harder for water molecules to lock into ice, so the solution must be colder to freeze, and harder for water molecules to escape, so it must be hotter to boil. Only the number of dissolved particles matters, not what they are.
Definition
- A dissolved, non-volatile solute lowers the freezing point and raises the boiling point of a solvent, and lowers its vapour pressure. These are colligative properties: they depend on the number of solute particles, not their identity.
- Molality is moles of solute per kilogram of solvent.
- Ionic solutes split into ions: NaCl gives 2 particles per formula unit (), gives 3. Sugar and urea stay as molecules ().
- Ice mixed with salt or sugar below 0 °C can melt if the freezing point of the mixture is below the current temperature. The energy to melt it comes from the surroundings, so the mixture gets colder: the solute supplies no energy.
Freezing point depression
- fall in freezing point, rise in boiling point, in K
- constants for the solvent; for water 1.86 and 0.512 K kg/mol
- molality, in mol of solute per kg of solvent
- number of particles each formula unit gives in solution
Worked example
0.50 mol of glucose is dissolved in 1.0 kg of water. Find the freezing point of the solution (). What would it be with 0.50 mol of sodium chloride instead?
Practice this conceptself-check · 4 quick reps
A solute lowers the freezing point; it never raises it
Adding salt or sugar to water lowers the temperature at which it freezes. Options saying a solute raises the freezing point, or raises the melting point of ice, are wrong.
The solute does not supply the energy to melt ice
When salt or sugar makes ice melt below 0 °C, the energy for melting comes from the surroundings and the mixture gets colder. The solute only changes the melting point; it gives the water molecules no energy.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Solutes lower the freezing point and raise the boiling point
Freezing point depression
Reference tables (3)
The six changes of state and their energy6 rows
| Change | Name | Energy |
|---|---|---|
| Solid to liquid | Melting (fusion) | Absorbed |
| Liquid to solid | Freezing (solidification) | Released |
| Liquid to gas | Evaporation or boiling (vaporisation) | Absorbed |
| Gas to liquid | Condensation | Released |
| Solid to gas | Sublimation | Absorbed |
| Gas to solid | Deposition | Released |
Vapour pressure, evaporation and the boiling point4 rows
| Feature | Evaporation | Boiling |
|---|---|---|
| Where | At the surface only | Throughout the liquid, with bubbles |
| Temperature | Any temperature | Only where vapour pressure equals the external pressure |
| Speeded up by | Higher temperature, larger surface, wind | Supplying heat faster (the temperature stays the same) |
| Effect on the liquid | Cools the liquid left behind | Temperature stays at the boiling point |
Phase diagrams in outline: triple point and critical point4 rows
| Feature | Meaning | Example values |
|---|---|---|
| Triple point | Solid, liquid and gas coexist | Water: 0.01 °C and about 0.006 atm |
| Critical point | End of the liquid to gas line | Water: about 374 °C and 218 atm |
| Normal melting and boiling points | Where the 1 atm line crosses the two lines | Water: 0 °C and 100 °C |
| Triple point above 1 atm | No liquid can exist at 1 atm; the solid sublimes | Carbon dioxide: triple point 5.1 atm, so dry ice sublimes at about at 1 atm |
Watch out for (6)
- Freezing means liquid to solid→ The six changes of state and their energy
- The temperature does not rise during melting or boiling→ The six changes of state and their energy
- More pressure on a liquid means a higher boiling point→ Vapour pressure, evaporation and the boiling point
- The triple point of water is not 0 °C at 1 atm→ Phase diagrams in outline: triple point and critical point
- A solute lowers the freezing point; it never raises it→ Solutes lower the freezing point and raise the boiling point
- The solute does not supply the energy to melt ice→ Solutes lower the freezing point and raise the boiling point
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