• A?phase change?happens whenever matter changes its state
• During a phase change, thermal energy is transferred to or from a substance
• This energy transfer does?not change the temperature?of the substance undergoing the phase change
• This means:
  • The thermal energy provided (or removed) does not affect the?kinetic energy?of the molecules within the substance
  • Only the?potential energy?(i.e. the spacing between the atoms or molecules) is affected

• The four main phase changes are:
  • Melting?- i.e. when a substance changes from solid to liquid as it absorbs thermal energy
  • Freezing?- i.e. when a substance changes from liquid to solid as it releases thermal energy
  • Vaporisation?(or?boiling) - i.e. when a substance changes from liquid to gas as it absorbs thermal energy
  • Condensation?- i.e. when a substance changes from gas to liquid as it releases thermal energy

Water

• Each substance has its own melting (or freezing) and boiling points
  • For example, the freezing point of water is 0°C and its boiling point is 100°C

• Possible phase changes of water include:
  • Solid ice?melting?into liquid water at 0°C
  • Liquid water?boiling?and changing into gaseous water vapour at 100°C

• Both these changes happen when?thermal energy?is absorbed
  • If thermal energy is released from water vapour at 100°C, it condenses back into water
  • If water continues to release thermal energy, it cools down until it reaches 0°C and freezes into ice

Phase changes for water

• Melting and freezing happen at the?melting?/?freezing?point?of a substance
• Vaporisation and condensation happen at the?boiling point?of a substance

• A?heating?or?cooling curve?shows how the temperature of a substance changes with time
• The 'flat' sections of the graph indicate that there is?no change in temperature?over time, hence the substance is undergoing a?phase change
  • The thermal energy supplied to or removed from the substance only affects the?potential energy?of the particles??

• The regions of the graph that are not flat indicate that the substance is being heated or cooled down
  • The thermal energy supplied to or removed from the substance changes the?average kinetic energy of the particles, hence resulting in an overall change in temperature of the substance

Heating Curves

• As energy is being supplied to a solid substance, its temperature increases until it reaches its melting point
• The temperature remains constant until the substance has melted completely
• If energy continues to be supplied, the liquid substance warms up until the boiling point is reached, and the substance vaporises
• Then, the temperature of the gas increases

Cooling Curves

• As energy is being removed from a gaseous substance, its temperature decreases until the boiling point is reached
• The temperature remains constant until the substance has condensed completely
• If energy continues to be removed, the liquid substance cools down until its freezing point and changes into a solid
• Then, the temperature of the solid decreases

• Heating or cooling curves can also display how the temperature of a substance changes with energy
  • In the following worked example,?energy?(in J) is plotted on the?x-axis?instead of time

Step 1: Write down the mass?m?of ice in kilograms (kg)

m?= 25 g = 0.025 kg

Step 2:? Read from the graph the amount of energy?E₁?being supplied to the water in its liquid phase as it warms up

• • The first flat section of the graph indicates the change of phase of ice into water
  • The non-flat region that follows is the one relating to water in its liquid phase being heated up

E₁?= 20 kJ – 10 kJ = 10 kJ

Step 3: Convert this energy from kilojoules into joules?

E₁?= 10 kJ = 10000 J

Step 4:? Read from the graph the change in temperature?ΔT₁?of the water as it warms up

ΔT₁?= 100°C – 0°C = 100°C

Step 5: Write down the equation linking thermal energy?E₁?to mass?m, specific heat capacity?c?and change in temperature?ΔT₁

E₁?=?mcΔT₁

Step 6: Solve for the specific heat capacity?c

c?= 4000 J kg^–1?°C^–1

Step 7: Read from the graph the amount of energy?E₂?being supplied to the water as it changes into a gas at 100°C

• • The second flat section of the graph indicates the change of phase of water into water vapour
  • This energy must be converted from kilojoules (kJ) into joules (J)

E₂= 56 kJ = 56000 J

Step 8: Write down the equation linking thermal energy?E₂?to mass?m?and specific latent heat of vaporisation?L_v

E₂?=?mL_v

Step 9: Solve for the specific latent heat of fusion?L_v

L_v?= 2.2 × 10⁶?J kg^–1