LATENT HEAT Changes in State

E NERGY AND S TATES OF M ATTER  The energy and organization of the particles in a sample of matter determine the physical state, or phase, of the matter: solid, liquid, or gas.

E NERGY AND S TATES OF M ATTER  When the temperature of a substance is at the substance’s melting or boiling point, a change of state, or phase transition, begins.

E NERGY AND S TATES OF M ATTER  When a solid is heated, the kinetic energy of the vibrating particles increases and the temperature of the substance rises.  When the melting point is reached, continued heating causes the bonds between particles to begin to break,  The potential energy of the particles increases.

E NERGY AND S TATES OF M ATTER

C HANGE OF S TATE At the boiling point, the water molecules in the steam and liquid water have the same kinetic energy. But the molecules in the steam have a lot more potential energy than those in the liquid water, So the steam has more thermal energy than the liquid water.

L ATENT H EAT  Latent heat is the thermal energy that is needed for a phase change.  Latent means “hidden.”  The increase in thermal energy of the object or system is hidden, because no corresponding change in temperature is observed.  The quantity of thermal energy transferred in a phase change from solid to liquid is called the latent heat of fusion, Q F.

S PECIFIC L ATENT H EAT OF F USION  Fusion is another term for melting  The specific latent heat of fusion, L F, of a substance is the quantity of thermal energy needed to melt 1 kg of the substance at its melting point.  It is also the quantity of thermal energy that must be removed from 1 kg of the substance to make it freeze or solidify without temperature change.

S PECIFIC L ATENT H EAT OF F USION  The specific latent heat of fusion, L F, of a substance is the quantity of thermal energy needed to melt 1 kg of the substance at its melting point.  A phase change requires a huge amount of energy.  For example, the specific latent heat of fusion of water is 333 kJ/kg. It takes 333 kJ of energy to melt 1 kg of ice at 0°C.

S PECIFIC L ATENT H EAT OF V APORIZATION  The specific latent heat of fusion, L V, of a substance is the quantity of thermal energy needed to vaporize 1 kg of the substance at its vaporization point.

L ATENT H EAT Gold melts at 1063°C. It takes 3250 J of energy to melt 50.0 g of gold at this temperature. Determine the specific latent heat of fusion of gold.

L ATENT H EAT The boiling point of ethanol is 78.4°C. Calculate the quantity of energy needed to completely vaporize 75.0 g of ethanol if its initial temperature is (a) 78.4°C and (b) 22.4°C. The specific latent heat of vaporization of ethanol is 854 kJ/kg and its specific heat capacity is 2.72 kJ/(kgK).