Sublimation, deposition and their uses

Skipping the liquid

Sublimation is the direct change of a solid into vapour without melting. Dry ice, which is solid carbon dioxide, turns straight into gas at room temperature. Naphthalene balls kept with woollen clothes slowly disappear the same way.

As in evaporation, the energy first comes from within the substance, which then absorbs energy from its surroundings. It is enough to overcome the attractions, and the molecules escape as vapour.

Deposition is the reverse: a gas changes directly into a solid without becoming liquid. Frost forming on a winter night is deposition.

Sublimation goes solid → gas. Deposition goes gas → solid. Neither passes through the liquid state.

Key terms

Sublimation

A solid turning directly into vapour without melting.

Deposition

A gas turning directly into a solid without becoming liquid.

Dry ice

Solid carbon dioxide; it sublimes at room temperature.

Sublimation printing

Printing a design into fabric with ink, heat and pressure.

Short questions with model answers

1. Q1. Why do naphthalene balls disappear after some time? (Short Answer 14.3, p. 14)

   • No liquid is seen.
   • Solid → vapour.

   They sublime: the solid turns directly into vapour, which spreads into the air.

2. Q2. Frost covers the ground at 6:30 am and is gone by 8:30 am, with no puddles. Which phase changes made and removed it? (Exercise MCQ 9, p. 14)

   • Night.
   • Morning.

   Deposition formed the frost; sublimation removed it.

Common mistakes

• ✗ Naphthalene melts first and then evaporates.

  ✓ It sublimes: solid straight to vapour, no liquid.

• ✗ Frost is formed by condensation.

  ✓ Condensation makes a liquid. Frost forms by deposition, gas to solid.

• ✗ Sublimation printing sticks ink on top of the fabric.

  ✓ The ink vapour goes into the fabric and turns solid there, so it does not fade.

MCQs

1. 1. Cooling the vapours of some gases changes them directly into a solid. This is called:

   1. (a) Evaporation
   2. (b) Condensation
   3. (c) Sublimation
   4. (d) Deposition
   Show answer

   (d) Exercise MCQ 4 (p. 14): gas to solid without the liquid is deposition (p. 8).

2. 2. Frost on the ground at 6:30 am has gone by 8:30 am. Which change turned it into water vapour?

   1. (a) Melting
   2. (b) Evaporation
   3. (c) Sublimation
   4. (d) Deposition
   Show answer

   (c) Exercise MCQ 9 (p. 14): solid frost to vapour directly is sublimation.

3. 3. Sublimation is:

   1. (a) A solid changing to vapour without melting
   2. (b) A liquid changing to vapour at its surface
   3. (c) A gas changing to a liquid
   4. (d) A solid melting at its melting point
   Show answer

   (a) p. 7: the direct conversion of a solid to vapours without melting.

4. 4. Where does the energy for sublimation come from, according to your book?

   1. (a) Only from a flame
   2. (b) From within the substance, then from the surroundings
   3. (c) From the light only
   4. (d) No energy is needed
   Show answer

   (b) p. 7: just like evaporation, from within the substance, which then absorbs energy from the surroundings.

5. 5. Sublimation printing does not fade because the ink:

   1. (a) Stays on top of the fabric
   2. (b) Is glued on with heat
   3. (c) Is embedded in the fabric
   4. (d) Changes the fabric chemically
   Show answer

   (c) p. 8: the ink is embedded in the fabric rather than attached to its top.

Quick revision

• Sublimation is solid → vapour without melting: dry ice, naphthalene balls.
• Deposition is gas → solid without becoming liquid: frost.
• Air fresheners and sublimation printing put sublimation to work.