Sep 2020

Thermal Properties Of Matter: Melting, Boiling & Evaporation

September 15, 2020

Have you ever wondered why you feel cold the moment you step out of the pool? Or, why all the liquid “disappears” when you leave it on the heated stove for too long? Then you’ve come to the right place. In this article, we’ll be covering the thermal properties of matter to help you understand the concepts of melting, boiling, evaporation, and beyond.

Internal energy

In any object, the internal energy encapsulates its total molecular potential energy (PE) and kinetic energy (KE):

  • PE is caused by the intermolecular bonds, related to the distance between the molecules. The higher the PE, the further apart the molecules, which could result in a change in state.
  • KE is the vibrations of the molecules that is influenced by the temperature of the substance. The higher the KE of the molecules, the higher the temperature.

Hence, internal energy can be defined as the total sum of the KE and PE of its molecules. So, when you heat a substance, it’ll absorb thermal energy, which could lead to either of these two scenarios: an increase in KE and hence, temperature, or an increase in PE which could result in a change of state, such as during melting or boiling.

Heat capacity

The heat capacity (C) of an object refers to the amount of thermal energy (Q), needed to increase the temperature by 1ºC or 1K.

  • SI Unit of heat capacity: Joules per Kelvin, JK−1
  • Another common unit: Joules per degree Celsius (JºC−1)

Specific heat capacity

The specific heat capacity (c) of an object refers to the amount of thermal energy (Q), needed to increase the temperature of a unit mass (1kg) of an object by 1ºC or 1K.

  • SI Unit: Joules per Kilogram per Kelvin (Jkg−1K−1)
  • Another common unit: Joules per Kilogram per degree Celsius (Jkg−1ºC−1)
  • As such, the relationship can be seen by this equation: c = Q/m(Δθ)

The relationship between heat capacity and specific heat capacity is C = mc. If you substitute and rearrange the terms, you can form an equation for thermal energy: Q = mc(Δθ).

Latent heat

Latent heat of fusion refers to the amount of thermal energy needed to change a physical state from solid to liquid, or vice versa, without any change in temperature. The latent heat of vaporisation follows a similar process, but it changes the liquid to gas, or vice versa.

When a substance is undergoing a change in state, latent heat energy is either absorbed or given out. However, the average KE of the molecules remains the same. As KE is related to temperature, the temperature remains constant throughout the process.

In a nutshell, the terms for the change in a physical property without any change in temperature is as follows:

  • Melting: Solid to liquid
  • Solidification: Liquid to solid
  • Boiling: Liquid to gas
  • Condensation: Gas to liquid

Aside from boiling, a change in physical property from liquid to gas can also be the case of evaporation. Unlike boiling, evaporation occurs at any temperature below a liquid’s boiling point. For the differences between boiling and evaporation, refer to the table below:

BoilingEvaporation
Occurs at a fixed temperatureOccurs at any temperature
It happens quicklyIt happens slowly
Takes place throughout the liquidTakes place only at the surface of the liquid
Bubbles formedNo bubbles formed
Constant temperatureChange in temperature

Specific latent heat

Specific latent heat of fusion refers to the amount of thermal energy needed to change the physical property of a unit mass (1kg), from solid to liquid, vice versa, without any change in temperature. For a change in liquid to gas, vice versa, the process is called ‘specific heat vaporisation’.

For both specific latent heat and fusion, the SI unit is Joules per Kilogram (Jkg−1). One other common unit is Joules per gram (Jg−1).

Conclusion

And that’s it for our brief overview of the thermal properties of matter! Be sure to take note of the various units and SI units when applying them into a formula. Anytime you require help to comprehend the concepts better, it’s never too late to enrol yourself in a Physics tuition class. With relevant learning materials and succinct delivery, we aim to enhance students’ understanding of Physics.

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