Interconversion of states: melting, boiling and condensation

Heat moves particles, then frees them

Melting: heat makes the particles of a solid vibrate faster. At the melting point the vibration overcomes the cohesive forces. The particles lose their mean positions and arrangement, and the solid collapses into a liquid.

Once melting starts, further heating does not raise the temperature. All the heat goes into turning the solid into liquid.

Molecules escape from the surface of a liquid at all temperatures; this is evaporation, and heating speeds it up. With more heating, bubbles form rapidly. When the vapour pressure equals the external pressure, the liquid boils. That temperature is its boiling point.

At the boiling point, the temperature stays constant: the heat only breaks the attractions between particles. Condensation is the reverse. A cooled gas loses kinetic energy, attractions pull its molecules close, and it turns liquid at constant temperature.

Below a change of state, heat raises the temperature. During melting, boiling or condensing, the temperature stays constant while particles are freed from, or captured by, their attractions.

Key terms

Melting point

The temperature at which vibration overcomes the cohesive forces.

Evaporation

Molecules leaving a liquid's surface at any temperature.

Boiling point

The temperature at which a liquid boils.

Condensation

A cooled gas turning into liquid at constant temperature.

Short questions with model answers

1. Q1. Ice at its melting point is heated for two minutes. Where does the heat go?

   • At the melting point.
   • Heat use.

   Into melting more ice. The temperature stays at 0 °C until all the ice has melted.

2. Q2. A liquid is warm and bubbles are rising, but it is not yet boiling. What is still missing?

   • Condition for boiling.
   • More heat.

   Its vapour pressure must rise to equal the external pressure; then it boils at a constant temperature.

Common mistakes

• ✗ While ice melts, its temperature keeps rising.

  ✓ It stays at the melting point until all the solid has melted.

• ✗ A liquid only turns into vapour at its boiling point.

  ✓ Evaporation happens at all temperatures, from the surface.

• ✗ A liquid boils as soon as bubbles appear.

  ✓ It boils when its vapour pressure equals the external pressure.

MCQs

1. 1. Upon heating, the rate of evaporation:

   1. (a) Decreases
   2. (b) Increases
   3. (c) Remains the same
   4. (d) First decreases, then increases
   Show answer

   (b) Exercise MCQ 2 (p. 13): heating raises kinetic energy, so more molecules escape (p. 3).

2. 2. The phase changes A and B in gas →(A) liquid →(B) solid are:

   1. (a) Melting, evaporating
   2. (b) Condensation, melting
   3. (c) Condensation, freezing
   4. (d) Boiling, freezing
   Show answer

   (c) Exercise MCQ 8 (p. 14): gas to liquid is condensation; liquid to solid is freezing.

3. 3. The boiling point of a liquid is the temperature at which:

   1. (a) Its vapour pressure equals the external pressure
   2. (b) The first bubble appears
   3. (c) Molecules start leaving the surface
   4. (d) It reaches 100 °C
   Show answer

   (a) p. 4: boiling point is where vapour pressure equals the atmospheric or external pressure.

4. 4. Why does the temperature remain constant during a phase change?

   1. (a) The heat supply stops
   2. (b) Particles stop moving
   3. (c) The substance changes chemically
   4. (d) The heat is used to break the attractions between particles
   Show answer

   (d) Short Answer 14.4 (p. 14); p. 4: at the boiling point the heat only breaks the attractions.

5. 5. When a gas is cooled, its molecules:

   1. (a) Gain kinetic energy and move apart
   2. (b) Lose kinetic energy and come closer
   3. (c) Change into different molecules
   4. (d) Stop attracting each other
   Show answer

   (b) p. 4: cooling lowers kinetic energy; attractions bring the molecules close enough to condense.

Quick revision

• At the melting point, vibration overcomes cohesive forces and the solid collapses into a liquid.
• Evaporation happens at every temperature; boiling needs vapour pressure equal to external pressure.
• During melting, boiling and condensation the temperature stays constant.