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CHANGES IN TEMPERATURE AND PHASE Holt Chapter 10, Section 3
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10.3 Changes in Temperature and Phase Melting glacier as a result of increasing global temperatures.
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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.
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Subscript p? When is Q positive? When is Q negative?
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Determining specific heat capacity Mass and change in temperature can be easily measured, but not heat. Heat capacity of water is well known: 4.186 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
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The approach is called calorimetry an experimental procedure used to measure the energy transferred from one substance to another as heat. Calorimeter
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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.
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Latent Heat
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PPhase change – the physical change of a substance from one state (solid, liquid, or gas) to another at constant temperature and pressure. RRequires energy to be added or removed Changes U of substance w/o changing its T. PPhase changes require an expansion of definition for heat. HHeat 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.
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Phase changes involve potential energy between particles.
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Energy required to melt a substance goes into rearranging the molecules.
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Energy required to vaporize a substance mostly goes into separating the molecules.
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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.
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