Thermal Properties of Matter

Internal Energy Definition

The sum of the kinetic energy due to random motion of particles and potential energy due to forces of attraction between the particles

Heat capacity definition

Heat capacity, C, is the amount of heat required to raise the temperature of a substance by 1K or 1^C

Specific heat capacity definition

The amount of heat needed to raise the temperature of 1g (or unit mass) of substance by 1K or 1C

What is melting?

Melting is change from the solid state the to liquid state when energy is transferred to the substance

Temperature of pure substance remains constant

What is solidification?

The change from liquid state to solid state when energy is transferred out of the substance

What is boiling?

Boiling is the change from the liquid state the to gaseous state when energy is transferred to the substance

What is condensation?

The change from gaseous state to liquid state when energy is transferred out of the substance

What is latent heat?

Latent heat is the energy absored or released to change the state of a substance at constant temperature

Why is temperature constant during changes in state?

Melting

During melting, the energy supplied is used to do work against the forces of attraction between the particles. Energy released to the surroundings causes the forces of attraction to reform around the particles. The particles now have a wider range of movement and the average distance of the particles increases. This increases the internal potential energy of the substance whereas the internal kinetic energy remains constant

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Boiling

During boiling, the energy supplied is used to do work against the forces of attraction between the particles so that they can move much further apart, as well as to do work against atmospheric pressure in order to escape to the surroundings. This increases the internal potential energy of the substance whereas the internal kinetic energy remains constant.

Latent heat of fusion definition

The amount of heat energy required to change a substance between the solid and liquid states, at constant temperature.

Specific latent heat of fusion definition

The amount of heat energy required to change a unit mass of the substance between the solid and liquid states, at constant temperature.

Latent heat of vapourisation definition

The amount of heat energy required to change a substance between the liquid and gaseous states, at constant temperature.

Specific latent heat of vapourisation definition

The amount of heat energy required to change a unit mass of the substance between the liquid and gaseous states, at constant temperature.

Define evaporation

The process where a liquid changes into a gas without boiling

Difference between boiling and evaporation

Boiling	Evaporation
Occurs at a specific temperature (for a fixed pressure)	Occurs at any temperature
Relatively fast	Relatively slow
Occurs throughout the liquid	Takes place only at the liquid surface
Bubbles are formed in the liquid	No bubbles are formed in the liquid
Temperature remains constant	Temperature may change
External heat source required	External heat source not required

How does evaporation occur

• The particles in a liquid are always moving randomly at different speeds.

• Particles at the surface of the liquid can escape into the atmosphere, provided they have sufficient kinetic energy to:

  • overcome attractive forces of other liquid particles

  • do work against atmospheric pressure

• The less energetic particles are left behind and the average kinetic energy of the particles in a liquid decrease. Therefore, the average temperature decreases. This results in a cooling effect of evaporation.

factors affecting rate of evaporation

temperature, surface area, humidity, windspeed, pressure, nature of the liquid