Chapter 23 Changes of Phase Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Kinetic Theory Matter exists in three common states or phases – solid, liquid and gas. A fourth state – plasma – makes up over 90% of our universe. All matter is made up of atoms or molecules that are in constant motion. Conceptual Physics Chapter 23

Macroscopic Properties Microscopic Properties Kinetic Theory Macroscopic Properties Microscopic Properties Solid Low KE levels Slow-moving molecules Retains its own shape Very strong molecular bonds Highly incompressible Small intermolecular spacing Liquid Higher KE levels Faster moving molecules Takes shape of its container Weak intermolecular bonds Slightly compressible Larger intermolecular spacing Gas High KE levels Very fast-moving molecules No intermolecular bonds Highly compressible Very large intermolecular spacing Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Phase Changes In many cases when energy is added to or taken from a substance, the matter will experience a temperature change. If enough energy is added to or taken from a substance, it can undergo a change of state or phase change. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Evaporation Evaporation Evaporation is a change of state from liquid to gas. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Evaporation Molecules in the liquid phase are in continuous motion and undergo collisions leading to energy transfers. Some molecules at the surface of the liquid may gain enough energy from molecules that bump them from below such that the surface molecules are able to break free of the liquid. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Evaporation These molecules undergo a phase change forming a gas. The energy that is taken from the remaining liquid by those molecules undergoing a phase change causes the temperature of the remaining liquid to decrease. Evaporation is a cooling process. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Evaporation We can promote more rapid evaporation by: Warming the liquid Reducing atmospheric pressure Providing dryer air above the liquid Increasing the surface area of the liquid Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Boiling While evaporation only occurs at the surface of a liquid, boiling can take place throughout a liquid. Typically, boiling will be prompted by an outside heat source. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Boiling At standard atmospheric pressure, water will boil when the temperature reaches 100°C. At lower atmospheric pressures, less vapor pressure is required inside the bubbles and boiling occurs at lower temperatures. If enough energy is transferred from the heat source to the molecules in the liquid, tiny bubbles will form beneath the surface. The vapor pressure inside the bubble must be great enough to withstand the water pressure and atmospheric pressure above it or the bubble will collapse. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Boiling The water is in thermal equilibrium! At this point, the water will be cooled by boiling at the same rate that it is being heated by energy from the heat source. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Boiling Pressure cookers are used at higher altitudes where the atmospheric pressure is so low that boiling occurs before water can get hot enough to cook food items. The lid on a pressure cooker can prevent the escape of water vapor and causes the pressure on the surface of the heated liquid to increase, raising the boiling temperature. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Condensation Condensation Condensation is a change of state from gas to liquid. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Condensation Gaseous water vapor molecules in the air can collide with slow moving molecules of a cooler surface. Some of the water vapor molecules may give up enough energy during the collision to undergo a phase change. The gas molecules condense into a liquid (e.g. dew forms on the morning grass). Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Condensation The energy that is transferred to the cool surface by the molecules undergoing the phase change will cause the temperature of the cool surface to increase. Condensation is a warming process. Conceptual Physics Chapter 23

Evaporation and Condensation The effects of rapid evaporation of warm water droplets from your skin after a hot shower can be neutralized by drying off in the shower to take advantage of water vapor condensing on your skin. Evaporation and condensation are opposite processes, but generally occur simultaneously. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Freezing Freezing Freezing is a change of state from liquid to solid. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Freezing When enough energy is taken away from liquid molecules, the molecules will slow to the point where the attractive forces between them will cause the molecules to fuse. The liquid molecules freeze into a solid. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Freezing At normal atmospheric pressure, water at 0°C will turn into a solid when energy is removed from it. The energy released by the liquid molecules that undergo the freezing process is absorbed by the surrounding environment. Freezing is a warming process. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Freezing The freezing process can be hindered if salt, sand or some other impurity is added to the liquid – this causes the freezing temperature to be lowered. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Regelation Applying pressure to solid ice can cause the open-celled hexagonal structure to collapse, reducing the freezing temperature. When the pressure is removed, refreezing occurs. This phenomenon is called regelation. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Regelation The wire will cut through the ice, but its track will refill with ice. The wire and weights fall to the floor, leaving the ice in a single solid piece! Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Changes of Phase The change in the internal energy of a substance causes the change of phase. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Changes of Phase A thermometer in the container reveals a slow increase in temperature up to 0°C. Suppose we have a 1-gram piece of ice at a temperature of -50°C in a closed container, and it is put on a stove to heat. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Changes of Phase The heat of fusion for water is 80 cal/g - for each gram of ice at 0°C that melts, 80 cal of thermal energy must be absorbed by the ice. Not until all the ice melts does the temperature again begin to rise. Once it reaches 0°C, the temperature of the ice remains at 0°C even though heat input continues. Rather than getting warmer, the ice melts. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Changes of Phase Each additional calorie absorbed by the gram of water increases its temperature by 1°C until it reaches its boiling temperature, 100°C. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Changes of Phase The heat of vaporization of water is 540 cal/g. Instead of the heat transfer causing a temperature increase, the water undergoes a phase change. The temperature remains constant while water is boiled away. Conceptual Physics Chapter 23

Conceptual Physics Chapter 23 Changes of Phase When all the water has become steam at 100°C, the temperature begins to rise once more. Conceptual Physics Chapter 23