1. Phase Changes  Recall that matter exists in 3 states or phases: Solid, Liquid, and Gas  A phase change occurs when there is a change in temperature of a substance  Heating (raising temperature) causes particles to move faster and further apart –Ex: melting solid chocolate into liquid when heated over the stove  Cooling (lowering temperature) causes particles to slow down and come closer together –Ex: Evaporated water in the air cools, and condenses into liquid water to form clouds

2. **Particles of matter are constantly in motion. **The higher the temperature, the faster the particles will move. Phase Change Diagram Freezing Melting Condensation Evaporation Solid Liquid Gas

3. Phase Changes:  Freezing – a change from a liquid to a solid state, caused by a lowered temperature  Melting – a change from a solid to a liquid state, caused by a raised temperature  Evaporation – a change from a liquid to a gaseous state, caused by a raised temperature  Condensation – a change from a gaseous state to a liquid state, caused by a lowered temperature  **Sublimation – a change from a solid to a gas without going through the liquid state –Ex: Dry ice evaporating is Carbon Dioxide changing from a solid directly to a gas, skipping the liquid phase

5. Phase Changes  When Matter is heated: –Melting: Solid  Liquid –Evaporation:  Liquid  Gas  When Matter is cooled: –Condensation:  Gas  Liquid –Freezing: Liquid  Solid

6. Water is the exception to the Rule!  Most substances, when freezing, have particles that move closer together  Water actually expands when frozen between 0° and 4° Celsius  Water molecules form a crystalline structure that makes the substance expand slightly  This is why you shouldn’t freeze a water bottle with the lid on  The water will expand in the bottle  This is why ice floats in water  The particles expand, and are actually lighter than the liquid water This is how potholes in the street form

7. Water is constantly undergoing phase changes throughout the Water Cycle Accumulation Underground Runoff Ground water