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4.5 A heating Curve - steadily increasing the internal energy of a system

• When a solid is heated from the solid state to the gaseous state and the temperature of the system measured continuously, there are two horizontal sections on the graph where the temperature does not rise, despite the constant input of thermal energy (continuous heating). Typical results are shown in the heating curve graph below.

• This is called a HEATING CURVE

  • You need to be able to accurately label and sketch a heating curve graph AND explain it!

• As you heat the substance you are increasing the internal energy. BUT the temperature stays constant during the state changes of melting at temperature Tm and boiling at temperature Tb (see diagram above).

• This is because all the extra ('hidden') energy absorbed in heating at these two temperatures (called the latent heat of state change), goes into weakening the inter–particle forces (intermolecular bonds) .

  • This induces the state change without temperature rise, to cause melting and then boiling to take place.

• The thermal energy gain at this point equals the heat energy absorbed needed to reduce the interparticle forces in melting or boiling - the latent heat.

• During the state change the temperature stays constant until all the latent heat is absorbed and the state change completed, so no temperature rise can occur.

• In between the 'horizontal' state change sections of the graph, you can see the energy input increases the kinetic energy of the particles and raising the temperature of the substance as you expect as the internal energy increases.

• For these state changes you have the addition of the latent heat of melting at temperature Tm and the addition of the latent heat of boiling at temperature Tb.

• The diagram involving the brown half-arrows illustrates what is happening to the energy stores in a heating curve.

• A simple experiment to illustrate a 'heating curve'

  • You start with a beaker of crushed ice into which you place a thermometer (-10 to 100^oC thermometer).

  • Place on a tripod and gauze and record the temperature at the start.

  • To speed things up, heat the beaker of ice steadily with a bunsen flame.

  • Continue to record the temperature every minute until all the ice has melted and eventually the water will boil.

  • Finish taking temperature readings after 5 minutes of boiling.

  • Plot a graph of temperature versus time.

  • It should look like the graph above, apart from the initial rise of temperature of solid ice.

  • You should get two horizontal sections on the graph where the latent heat of fusion (melting at 0^oC) or the latent heat of boiling (vapourising at 100^oC) are being absorbed to weaken the intermolecular forces between the water molecules, without rise in temperature.

    • There is one rising section on your graph as the liquid water from the melted ice rises in temperature until the water boils - see the graph below.

INDEX of notes on Particle model theory state changes and latent heat

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Keywords, phrases and learning objectives for particle models and a heating curve

Be able to describe, draw and explain a graph of a heating curve.

Know that heating a substance increases the internal energy of system, eventually causing the state changes of melting and boiling when the latent heat is absorbed.

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INDEX of notes on Particle model theory state changes and latent heat