CHANGES IN TEMPERATURE AND PHASE Holt Chapter 10, Section 3

10.3 Changes in Temperature and Phase Melting glacier as a result of increasing global temperatures.

Specific Heat Capacity  Substances vary in their capacity to absorb heat and change temperature.  Specific heat capacity, C p (or, specific heat) – energy needed to change 1 kg of a substance by 1 o C at constant pressure.

 Subscript p?  When is Q positive?  When is Q negative?

 Determining specific heat capacity  Mass and change in temperature can be easily measured, but not heat.  Heat capacity of water is well known: kJ/kg  o C  If a hot substance is placed in an insulated container of cool water, energy conservation requires that the energy the substance gives up must equal the energy absorbed by the water. Q substance = Q water C x m x  T x = C w m w  T w

 The approach is called calorimetry  an experimental procedure used to measure the energy transferred from one substance to another as heat.  Calorimeter

Pre-sample Problem 10C Calculate the mass of a sample of silver (Ag) if it increase in temperature by 5.0 o C after absorbing 5.0 kJ of heat.

Latent Heat

PPhase change – the physical change of a substance from one state (solid, liquid, or gas) to another at constant temperature and pressure. RRequires energy to be added or removed  Changes U of substance w/o changing its T. PPhase changes require an expansion of definition for heat. HHeat is the energy that is exchanged between two objects at different temperatures OR between two objects at the same temperature when one of them is undergoing a phase change.

 Phase changes involve potential energy between particles.

 Energy required to melt a substance goes into rearranging the molecules.

 Energy required to vaporize a substance mostly goes into separating the molecules.

 A few more definitions:  Heat of fusion, L f – the energy per unit mass transferred in order to change a substance from solid to liquid or from liquid to solid at constant temperature and pressure.  Heat of vaporization, L v – the energy per unit mass transferred in order to change a substance from liquid to vapor or from vapor to liquid at constant temperature and pressure.  Latent heat, L – the energy per unit mass that is transferred during a phase change of a substance.