As we all know, heat is stored inside substances and the amount of heat depends on its temperature and its state.
Heat energy is needed to convert a solid into a liquid (melting) and a liquid into a gas (evaporation). Heat is released when a gas turns into a liquid (condensation) and when a liquid turns into a solid (freezing).
The Latent heat of fusion/melting (\( L_f \)) = the amount of heat needed to convert 1 kg of a solid into a 1kg of liquid under standard conditions.
= Energy released when 1kg of liquid turns into a solid \( L_f \)
Similarly, \( L_v \) (latent heat of vaporisation) = Energy needed to evaporate 1 kg of liquid into a gas.
Q = mL
One important thing to notice is that
Latent heat of sublimation = Latent heat of fusion + Latent heat of vaporisation
How much energy is needed to convert 0.5kg of water into water vapour?
(Given that \( L_v \) of water = 2.26 × 10⁶ J/kg)
\( Q = m \times L_v \)
Substitute \( m = 0.5 \text{ kg} \) and \( L_v = 2.26 \times 10^6 \text{ J/kg} \)
\( Q = 0.5 \times 2.26 \times 10^6 = 1.13 \times 10^6 \text{ J} \)
A 2 kg block of ice at 0°C is placed in a container. If 670 kJ of heat is supplied to the ice, determine how much of it melts. (Latent heat of fusion of ice = 334 kJ/kg). If any ice remains, what will be its final mass? If all ice melts, how much energy will be left?
To melt all the ice, it takes energy:
\( Q = mL_f = 2 \times 334 = 668 \text{ kJ} \)
Since 670kJ is supplied:
\( 670 - 668 = 2 \text{ kJ} \) is left. This remaining energy will be used to heat the water.
Written by Thenura Dilruk