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STATES OF MATTER - properties of gases and liquids (fluids) and solids

12. The special case of sublimation (solid <=> gas)

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.


12. What is sublimation?

Examples of sublimation explained using the kinetic particle theory of gases and solids

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  • Sublimation:

    • This is when a solid, on heating, directly changes into a gas without melting, AND the gas on cooling re–forms a solid directly without condensing to a liquid. Sublimation usually just involves a physical change BUT its not always that simple (see ammonium chloride!).

    • The opposite of sublimation is sometimes referred to as deposition or 'reverse sublimation'.

  • Theory in terms of particles:

    • When the solid is heated the particles vibrate with increasing force from the added thermal energy.

      • If the particles have enough kinetic energy of vibration to partially overcome the particle–particle attractive forces you would expect the solid to melt.

      • HOWEVER, if the particles at this point have enough energy at this point that would have led to boiling, the liquid will NOT form and the solid turns directly into a gas.

        • Overall endothermic change, thermal energy absorbed and 'taken in' to the system.

    • On cooling, the particles move slower and have less kinetic energy.

      • Eventually, when the particle kinetic energy is low enough, it will allow the particle–particle attractive forces to produce a liquid.

      • BUT the energy may be low enough to permit direct formation of the solid, i.e. the particles do NOT have enough kinetic energy to maintain a liquid state!

        • Overall exothermic change, energy released and 'given out' to the surroundings.

  • Examples of sublimation:

    1. Even at room temperature bottles of solid iodine show crystals forming at the top of the bottle above the solid. The warmer the laboratory, the more crystals form when it cools down at night!

      • I2 (s) reversible I2 (g)   (physical change only)

      • If you gently heat iodine in a test tube you see the iodine readily sublime and recrystallise on the cooler surface near the top of the test tube.

    2. The formation of a particular form of frost involves the direct freezing of water vapour (gas).  Frost can also evaporate directly back to water vapour (gas) and this happens in the 'dry' and extremely cold winters of the Gobi Desert on a sunny day.

      • H2O (s) reversible H2O (g)   (physical change only)

      • See pictures of 'hoar frost' and the process of freeze drying food in sections below.

    3. Solid carbon dioxide (dry ice) is formed on cooling the gas down to less than –78oC. On warming it changes directly to a very cold gas!, condensing any water vapour in the air to a 'mist', hence its use in stage effects.
      • CO2 (s) reversible CO2 (g)   (physical change only)
      • Dry ice is added to water where the sublimation causes evaporation of water which is cooled to fine droplets to give a 'smoke effect' in stage effects.
    4. On heating strongly in a test tube, white solid ammonium chloride, decomposes into a mixture of two colourless gases ammonia and hydrogen chloride. On cooling the reaction is reversed and solid ammonium chloride reforms at the cooler top surface of the test tube.
      • Ammonium chloride + heat energy reversible ammonia + hydrogen chloride

      • NH4Cl(s) reversible NH3(g) + HCl(g)     

      • This involves both chemical and physical changes and is so is more complicated than examples 1. to 3, but in all cases total mass is conserved.

      • In fact the ionic ammonium chloride crystals change into covalent ammonia and hydrogen chloride molecule gases which are naturally, and normally, far more volatile than ionic solids like salt crystals (covalent substances generally have much lower melting and boiling points than ionic substances).

    The liquid particle picture does not figure here, but the other models fully apply apart from state changes involving liquid formation. GAS particle model and SOLID particle model links.

    You can also use the reverse reaction to illustrate diffusion and the fact that the rate of diffusion depends on the molecular mass See section 4. Examples of demonstrating diffusion in gases

    PLEASE NOTE, At a higher level of study, you need to study the g–l–s phase diagram for water and the vapour pressure curve of ice at particular temperatures. For example, if the ambient vapour pressure is less than the equilibrium vapour pressure at the temperature of the ice, sublimation can readily take place. The snow and ice in the colder regions of the Gobi Desert do not melt in the Sun, they just slowly 'sublimely' disappear!

gcse chemistry reverse sublimation deposition hoar frost gas/vapour to solid state change

The formation of hoar frost - the reverse of sublimation

Frost is a thin layer of ice on a solid surface.

Hoar frost forms directly from water vapour in air above 0oC, coming in contact with a solid surface whose temperature is below freezing (<0oC).

The water vapor changes directly from gas (vapour) to solid (ice) as it comes into contact with the solid surface.


How freeze drying food works

Freeze-drying is a process in which water in the form of ice under low pressure is removed from a material by sublimation.

The food material is cooled to below 0oC so that any liquid water is frozen to ice.

The ice is vapourised directly by sublimation (solid ==> gas) under low pressure to dry the food and avoid the presence of liquid water.

This process has found many applications for the production of high quality food and pharmaceuticals.

Freeze drying is widely used for the stabilization of high-quality food, biological materials, and pharmaceuticals, such as proteins, vaccines, bacteria, and mammal cells. In the freeze drying process, the quality of the dried product is preserved because freezing water in the material inhibits chemical, biochemical, and microbiological degradation processes. This improves retention of the taste, smell, and content of various nutrients inhibiting change.


Learning objectives for sublimation reverse sublimation (deposition)

Know what we mean by sublimation, the interchange of vapour/gas and solid without an intermediate liquid state of matter.

Be able to draw particle pictures to illustrate and explain sublimation and its reverse i.e. deposition..

Be able to use the kinetic particle theory of matter to explain the state of matter change of sublimation,

Know that thermal energy must be absorbed by a solid before it can be sublimed.

Know that iodine and ammonium chloride are good examples of solids to heat and observe the phenomenon of sublimation.


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