Phase Changes Phase change a reversible physical change
Phase Changes Phase change – a reversible physical change that happens when a substance changes from one state of matter to another. • 6 Common Phase Changes 1. Melting 4. Condensation 2. Freezing 5. Sublimation 3. Vaporization 6. Depositition
Characteristics of Phase Changes The temperature of a substance does not change during a phase change. Ex: Water: Melting point and Freezing point are at the exact same temperature. Ice freezes at 0°C. Ice melts at 0°C.
Step 1 Step 2 Step 3 • Energy is either absorbed or released during a phase change. Ex: Ice melting absorbs energy. It is called an endothermic change. Step 4 Where did the heat go? ?
The energy leaves the environment and enters the system. So the surrounding area gets cooler while the ice gets warmer.
Water freezing releases energy. It is called an exothermic change. The energy leaves the system so the environment gets warmer and the system gets cooler.
• The amount of energy absorbed depends on the substance. This energy is called heat of fusion. Think of it as the energy needed to melt a substance. • Every substance has its own heat of fusion.
• Ex: Water has a heat of fusion of 334 Joules (J). One gram of ice absorbs this amount of energy when it melts. • Why would a farmer spray his crops with water before a freeze? ? ?
Answer: As water freezes it releases heat. Adding water right before a freeze slows the drop in temperature and helps to protect the crops from damage.
Melting and Freezing • The arrangement of molecules in water becomes less orderly as water melts and more orderly as water freezes. • As ice melts heat flows from the air to the ice. The ice gains the heat (energy) and causes the molecules to vibrate more quickly. This allows the molecules to leave their “solid” fixed state and move into a liquid state allowing movement of molecules. • When water freezes, heat (energy) flows from the water into the air (exothermic reaction). Molecules slow down until they become fixed into a solid state.
Vaporization • The phase change from a liquid to a gas is called vaporization. This is an endothermic change. • The amount of energy needed to vaporize something is called its heat of vaporization. • One gram of water has a heat of vaporization of 2261 joules (J). Heat of vaporization varies from substance to substance.
• What is the difference between evaporation and boiling? • Answer: Evaporation takes place at the surface of a liquid and occurs at temperatures below the boiling point. If a container is sealed, then vapor pressure can build up. It is caused by the collisions of the vapor on the walls of its container.
• Boiling is when vapor pressure becomes equal to atmospheric pressure. As temperature rises the molecules move faster to overcome the attraction of other molecules. Since water vapor is less dense than liquid water, this causes the vapors to move to the surface and be released. How does the surface area of a liquid affect the rate of vaporization? ?
Answer: The greater the surface area, the faster the water evaporates. • The boiling point of a substance depends of the atmospheric pressure. • Normal boiling point of water is 100°C at sea level. • Boiling point in Denver, CO. is about 95°C. Higher elevations have lower atmospheric pressures.
Condensation: The phase change from a gas or vapor to a liquid. Exothermic change. Ex: Morning dew on grass. Water vapor from the air condenses into drops of liquid on the grass
Sublimation: The phase change from a solid to a gas or vapor. Endothermic change. Ex: Dry ice.
Deposition: The phase change from a gas or vapor to a solid. Exothermic change. Ex: Frost forms on a window. Water vapor in the air comes in contact with a cold window. Enough energy is lost to go from a gas directly to a solid.
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