Chp Phase Changes Pg

Characteristics of Phase Changes  When at least two states of a substance are present, each state is described as a phase  Phase change- a reversible physical change that occurs when a substance changes from one state of matter to another  Melting, freezing, vaporization, condensation, sublimation, and deposition are six common phase changes.  When at least two states of a substance are present, each state is described as a phase  Phase change- a reversible physical change that occurs when a substance changes from one state of matter to another  Melting, freezing, vaporization, condensation, sublimation, and deposition are six common phase changes.

Temperature and Phase Changes  The temperature of a substance does not change during a phase change  Temperature remains the same until the phase change is complete  The temperature at which a substance freezes (freezing point) is identical to the temperature at which it melts (melting point)  The temperature of a substance does not change during a phase change  Temperature remains the same until the phase change is complete  The temperature at which a substance freezes (freezing point) is identical to the temperature at which it melts (melting point)

Energy and Phase Changes  During a phase change energy is transferred between a substance and its surroundings  Energy is either absorbed or released during a phase change  Melting is an example of an endothermic change –Endothermic change - the system absorbs energy  During a phase change energy is transferred between a substance and its surroundings  Energy is either absorbed or released during a phase change  Melting is an example of an endothermic change –Endothermic change - the system absorbs energy

Energy and Phase Changes  The amount of energy absorbed depends on the substance –One gram of water absorbs 334 Joules of energy as it melts is its heat of fusion  The heat of fusion –Varies from substance to substance –Heat of “melting”  Fusion is another term for melting  The amount of energy absorbed depends on the substance –One gram of water absorbs 334 Joules of energy as it melts is its heat of fusion  The heat of fusion –Varies from substance to substance –Heat of “melting”  Fusion is another term for melting

Energy and Phase Changes  Freezing is an example of an exothermic change –Exothermic change - the system releases energy to its surroundings  Freezing is an example of an exothermic change –Exothermic change - the system releases energy to its surroundings

Melting  The arrangement of molecules in water becomes less orderly as water melts and more orderly as water freezes  Melting –Attractions between molecules keep them in fixed positions –Heat flows from air to ice –Ice gains energy and molecules vibrate –Some gain enough energy to overcome the attractions and move away –When all can move melting is complete  The arrangement of molecules in water becomes less orderly as water melts and more orderly as water freezes  Melting –Attractions between molecules keep them in fixed positions –Heat flows from air to ice –Ice gains energy and molecules vibrate –Some gain enough energy to overcome the attractions and move away –When all can move melting is complete

Freezing  Freezing –Energy flows from the water to the air in a freezer and the water cools down –As kinetic energy of molecules decrease, the move more slowly –Attractions between molecule take effect –When all molecules have been drawn into orderly arrangement freezing is complete  Freezing –Energy flows from the water to the air in a freezer and the water cools down –As kinetic energy of molecules decrease, the move more slowly –Attractions between molecule take effect –When all molecules have been drawn into orderly arrangement freezing is complete

Vaporization and Condensation  Vaporization - the phase change in which a substance changes from a liquid to a gas  Vaporization is an endothermic process –A substance must absorb energy in order to change from a liquid to a gas  One gram of water gains 2261 joules of energy when it vaporizes –This is its heat of vaporization –Heat of vaporization varies from substance to substance  Vaporization - the phase change in which a substance changes from a liquid to a gas  Vaporization is an endothermic process –A substance must absorb energy in order to change from a liquid to a gas  One gram of water gains 2261 joules of energy when it vaporizes –This is its heat of vaporization –Heat of vaporization varies from substance to substance

Vaporization  Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling point  Evaporation - the process that changes a substance from a liquid to a gas at temperatures below the substance’s boiling point  Water in a closed container –Water evaporates and vapor collects above the liquid –Vapor Pressure - caused by collisions of the vapor and the walls of the container –The vapor pressure increases as the temperature increases  Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling point  Evaporation - the process that changes a substance from a liquid to a gas at temperatures below the substance’s boiling point  Water in a closed container –Water evaporates and vapor collects above the liquid –Vapor Pressure - caused by collisions of the vapor and the walls of the container –The vapor pressure increases as the temperature increases

Boiling  Heat a pot of water –Temperature and vapor pressure increase  When vapor pressure and atmospheric pressure become equal the water boils  As temperature increases water molecules move faster and faster –When it reaches 100 degrees Celsius some molecules below the surface overcome the attraction forces and form bubbles of water vapor –When the bubbles reach the surface they pop and release water vapor into the air –Takes place throughout a liquid  Heat a pot of water –Temperature and vapor pressure increase  When vapor pressure and atmospheric pressure become equal the water boils  As temperature increases water molecules move faster and faster –When it reaches 100 degrees Celsius some molecules below the surface overcome the attraction forces and form bubbles of water vapor –When the bubbles reach the surface they pop and release water vapor into the air –Takes place throughout a liquid

Condensation  Condensation - the phase change in which a substance changes from a gas or vapor to a liquid  An exothermic process  Responsible for the morning dew on grass and the “cloud” on the mirror after you take a shower.  Condensation - the phase change in which a substance changes from a gas or vapor to a liquid  An exothermic process  Responsible for the morning dew on grass and the “cloud” on the mirror after you take a shower.

Sublimation and Deposition  Sublimation - phase change in which a substance changes form a solid to a gas or vapor –Endothermic –Example is dry ice  Goes from solid carbon dioxide to a gas  The gas causes water vapor to condense and that’s how it forms clouds or the fog  Deposition - phase change in which a gas or vapor changes directly into a solid without first becoming a liquid –Exothermic –Opposite of sublimation –Causes frost on cold window glass  Sublimation - phase change in which a substance changes form a solid to a gas or vapor –Endothermic –Example is dry ice  Goes from solid carbon dioxide to a gas  The gas causes water vapor to condense and that’s how it forms clouds or the fog  Deposition - phase change in which a gas or vapor changes directly into a solid without first becoming a liquid –Exothermic –Opposite of sublimation –Causes frost on cold window glass

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