Change in state

Energy in, energy out

Absorbing or losing energy changes the kinetic energy of a substance's particles, and that can change its state.

Energy in: particles move faster and need more space. A solid can turn to liquid (melting) and a liquid to gas (boiling).

Energy out: particles slow down and come closer. A gas can turn to liquid (condensation) and a liquid to solid (solidification).

Intermolecular forces are strongest in solids, where particles are closest, and weakest in gases. So gases show the greatest relative thermal expansion and solids the least.

Order of expansion for the same heating: gases > liquids > solids. The weaker the hold between particles, the more they spread.

Key terms

Melting

A change of state when a solid absorbs energy.

Boiling

A change of state when a liquid absorbs energy.

Condensation

A change of state when a gas loses energy.

Solidification

A change of state when a liquid loses energy.

Short questions with model answers

1. Q1. Molten metal is poured into a mould and sets. Name the change and say whether energy is absorbed or lost.

   • Liquid → solid.
   • Particles.

   Solidification; the metal loses energy.

2. Q2. Equal volumes of air, water and steel are heated by the same amount. Rank them by expansion.

   • Forces.

   Air most, then water, then steel least.

Common mistakes

• ✗ Condensation needs energy to be supplied.

  ✓ Condensation happens when a gas loses energy.

• ✗ Solids expand most because they are dense.

  ✓ Solids expand least: their particles are held most strongly.

• ✗ Melting makes a new substance.

  ✓ Only the spacing and motion of the same particles change.

MCQs

1. 1. A liquid turning into a solid is called:

   1. (a) Melting
   2. (b) Condensation
   3. (c) Boiling
   4. (d) Solidification
   Show answer

   (d) p. 10.

2. 2. Intermolecular forces are strongest in:

   1. (a) Solids
   2. (b) Liquids
   3. (c) Gases
   4. (d) All equally
   Show answer

   (a) p. 10: least separation in solids.

3. 3. Which shows the greatest relative thermal expansion?

   1. (a) Solids
   2. (b) Liquids
   3. (c) Gases
   4. (d) They are equal
   Show answer

   (c) p. 10.

4. 4. When a substance absorbs energy, its particles:

   1. (a) Slow down
   2. (b) Move faster and need more space
   3. (c) Stop moving
   4. (d) Get smaller
   Show answer

   (b) p. 10.

5. 5. Gas → liquid is:

   1. (a) Melting
   2. (b) Condensation
   3. (c) Solidification
   4. (d) Boiling
   Show answer

   (b) p. 10: by losing energy, a gas turns to liquid.

Quick revision

• Energy in: melting and boiling. Energy out: condensation and solidification.
• Forces between particles are strongest in solids and weakest in gases.
• So gases expand most and solids least.