SITEMAP   Physics Notes: Thermal energy 4.3 Energy transfer and change of state

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Thermal energy & particle theory: 4.3 Thermal energy transfer in state changes, conservation of mass with no chemical change of the substance

Doc Brown's Physics exam study revision notes

4.3 Energy transfer in state changes and conservation of mass

Read in conjunction with Part 4.4

Introduction to latent heat and physical changes of state

 FREEZING MELTING SUBLIMING BOILING or EVAPORATING SUMMARY of the CHANGES of STATE between a gas, liquid and solid All mass conserved in these PHYSICAL CHANGES CONDENSING These are NOT chemical changes !
• As well as the transfer of heat energy by conduction, convection and radiation, state changes like evaporation and condensation also involves heat energy transfers and the particle model can be used to explain them.

• ON HEATING - adding thermal kinetic energy increases the internal energy of a substance

• When you heat a solid, the vibrational kinetic energy of the particles is increased until they have enough KE to weaken the interparticle bonds to allow melting and the particles are free to move around in the liquid state.

• With further heating above the melting point, the particles gain more kinetic energy and the inter-particle bonds are further weakened so that the particles at the surface with the highest KE can escape the surface (evaporate) or vapourise to the gaseous state in the bulk liquid (bubbles!) at the boiling point.

• The graph below shows how the distribution of kinetic energy and speed of particles changes with changes in temperature - with increase in temperature, the average speed and kinetic energy of the particles increases.

• Note that the random movement and collisions of the particles creates a wide range of speeds/kinetic energies.

• When the temperature is increased, more particles have a greater kinetic energy and greater speed, but only the highest speed/kinetic energy particles can escape from the surface (only the very right-hand section of the graph curves)

• Below is a particle model of evaporation.

• ON COOLING - removing thermal kinetic energy - decreasing internal energy

• If you cool the substance, the reverse happens e.g. cool a gas so the interparticle bonds bring the particles together to condense and form a liquid.

• Further cooling reduces the KE of the liquid particles so that when the temperature is reduced to the freezing point, the interparticle forces are sufficient to 'club' the particles together to form a solid.

• All these physical state changes are reversible by adding or removing thermal energy, no new substances are formed (NOT a chemical change) and all mass is conserved. What you start with is what you finish with and all the original properties are retained.

• The only difference between the states of a substance is how the particles are arranged (as described in section 1. above).

• Note that in a closed system, mass is conserved in a system undergoing a change in state.

• If you melt 100 g of ice, you get 100 g of water!

• However, even with mass conservation, you can get a volume change, except for water, for the same mass, liquids occupy a slightly larger volume and gases occupy a massively greater volume than the liquid or solid form.

• Ice is unusual that the solid ice crystals are less dense than water - which is why ice floats!

Read in conjunction with Part 4.4

Introduction to latent heat and physical changes of state

Keywords, phrases and learning objectives for particle models and thermal energy transfers in state changes

Be able to describe and explain the thermal energy transfers that happen in state changes.

Know that in state changes there is complete conservation of mass with no chemical change of the substance.

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