23 Change of Phase Changes of phase usually involve a transfer of energy.

23 Change of Phase The four possible forms of matter—solid, liquid, gas, and plasma—are called phases. Matter can change from one phase to another. The phase of matter depends on its temperature and the pressure that is exerted upon it.

23 Change of Phase A change in a phase of matter requires an increase or decrease in thermal energy

23 Change of Phase Thermal energy Internal energy Combination of KE + PE Figure 2

23 Change of Phase Phases of Matter Solid Definite volume and shape Liquid Definite volume Undefined shape Gas Undefined volume and shape Plasma Undefined volume and shape Electrically charged gas—High temp! Neon, lightning, stars, auroras, fluorescent bulbs

23 Change of Phase Melting and Evaporation Require Input of Thermal Energy Melting (requires heat of fusion) –Heat of fusion is released when liquids freeze Evaporation (requires heat of vaporization) –Heat of vaporization is released when gases condense

23 Change of Phase Evaporation vs. Condensation Evaporation is a cooling process Evaporation causes the temperature of the liquid to decrease (removes thermal energy from surroundings) Condensation is a warming process Condensation causes the temperature of the surroundings to increase (adds thermal energy to the surroundings)

23 Change of Phase If you heat a solid sufficiently, it will melt and become a liquid. If you heat the liquid, it will vaporize and become a gas. The change in the internal energy of a substance causes the change of phase Energy and Changes of Phase

23 Change of Phase The cloth covering on the sides of the canteen promotes cooling when it is wet. As the faster-moving water molecules leave the cloth, the temperature of the cloth decreases Evaporation

23 Change of Phase When the human body overheats, sweat glands produce perspiration. As the sweat evaporates, it cools us and helps us maintain a stable body temperature Evaporation

23 Change of Phase Pigs lack sweat glands. They wallow in mud to cool themselves Evaporation

23 Change of Phase How does evaporation affect a liquid’s temperature? 23.1 Evaporation Evaporation cools the liquid left behind.

23 Change of Phase Evaporation is a process that cools the liquid left behind Evaporation

23 Change of Phase Evaporation is a change of phase from liquid to gas that takes place at the surface of a liquid Evaporation The increased kinetic energy of molecules bumped free of the liquid comes from molecules remaining in the liquid. Molecules at the surface of the liquid that gain kinetic energy may have enough energy to break free of the liquid. They now comprise a vapor, molecules in the gaseous phase. The average kinetic energy of the molecules remaining behind in the liquid is lowered. This means a drop in temperature!

23 Change of Phase Condensation warms the area where the liquid forms Condensation

23 Change of Phase What about energy flow? condensation is a warming process (warms the surroundings) –Heat flow: from the gas molecules to the liquids and surroundings evaporation is a cooling process (cools the surroundings) –Heat flow: from the liquid and surroundings to the water vapor

23 Change of Phase A solid absorbs energy when it melts. A liquid absorbs energy when it vaporizes. A gas emits energy when it liquefies. A liquid releases energy when it solidifies Energy and Changes of Phase

23 Change of Phase Condensation is the changing of a gas to a liquid. The vapor molecules give up so much kinetic energy that they can’t stay in the gaseous phase. The molecules begin to stick together or condense. Kinetic energy lost by condensing gas molecules warms the surface they strike Condensation

23 Change of Phase a.At high speeds, molecules of water vapor bounce apart and remain a gas Condensation

23 Change of Phase a.At high speeds, molecules of water vapor bounce apart and remain a gas. b.At lower speeds, molecules of water vapor are more likely to stick together and form a liquid Condensation

23 Change of Phase 23.1 Condensation What do these pictures have in common?

23 Change of Phase How does condensation affect temperature? 23.2 Condensation Condensation warms the area where the liquid forms.