Phase Changes

1. Phase Changes “It’s just a phase”

2. States of Matter • Solid, liquid and gas (plasma and BEC). • Changes between states are called “phase changes”. • Caused by a change of heat or pressure. More often heat. • HEAT and TEMPERATURE are not the same thing.

3. Temperature • Measures the average kinetic energy of the particles in a substance. • Is measured in Celsius or Kelvin. • Kinetic energy is directly related to the speed of the molecules. • The faster the particles/molecules are moving the higher the temperature.

4. Heat • Heat is a measure of energy (Joules). • HEAT and TEMPERATURE are not the same thing. • Ex. A cold Lake Superior has more heat energy than a boiling pot of water. • For our class, higher temperature means more heat.

5. Solids • Molecules are tightly packed together. • High potential energy – more bonds. • Molecules greatly affected by intermolecular forces. • Particles vibrate in place. • Very Dense. • Not easily compressed.

8. Liquids • Particles are not so tightly packed (liquids flow and can be poured). • Medium potential energy. • Particles are affected by intermolecular forces. • Surface Tension-ability of a liquid’s molecules to “stick” together. • Adhesion-stick to other compounds • Cohesion- stick to self

9. Cool Videos • Number One • Number Two • Number Three

11. Gases • Particles spread out as the container will allow. • High kinetic energy, particles are moving very quickly (1000 km/sec). • Little effect of intermolecular forces. • Low density, can be compressed, very fluid.

13. Cool Videos • Number Two

14. Phase Changes • When a substance changes states. • Requires the input or the removal of energy or change in pressure. • During a phase change the temperature does not change, but the amount of energy does. • Phase Change Diagram.

15. Intermolecular Forces • Interactions between particles that cause them to “stick” together. • Strongest in solids • Weakest in gases. • Strongest when particles move slowly. • During a phase change IMF are either weakened or strengthened.

17. Sublimation • Transformation of a substance to a gas from a solid state with no liquid transition. • Dry Ice does this.

18. Deposition • When a gas transforms into a solid without transitioning through a liquid state. • Ex. Frost forming on windows.

20. Where does all the energy go? • During a phase change energy is added, but the temperature does not increase. • The energy goes toward breaking up weak intermolecular forces between the particles.

21. Some Vocab • Triple Point: Pressure and temperature where a substance exists as a solid, liquid, and gas. • Critical Point: Indicates critical temp and pressure.

22. Critical Temperature • The temperature at and above which no additional pressure can be added to maintain a liquid. • The critical pressure is the highest pressure that will keep a compound a liquid at the critical temperature. • The two correspond to a point on a graph.

23. Helpful Info • Melting point and freezing point are the same thing. It just depends if the substance is getting hotter or colder. • During a phase change temperature does not change, but the amount of heat does.

24. Intermolecular Forces • Intermolecular Forces are the forces of attraction that are between molecules. • This is why lakes form lakes, gold can be melting together, and why any matter can be held together.

25. Phases • Phases is the term scientist use to more properly define solid, liquids and gases. It means the same as the term “state” • Phase is defined as “a part of matter that has uniform properties through out the entire substance”

26. Phases change • As scientists we can change phases. We can change a solid into a liquid and a liquid into a gas. • When two phases exist at the same time it is called equilibrium. • Equilibrium is a dynamic condition in which two opposing changes occur in equal rates in a closed system

27. Equilibrium • To have equilibrium we need to have both a temperature and pressure. • When we have both a measured temperature and pressure two or three states will exist at the same time. • Ice melting into water, water freezing into ice.

28. Phase Diagrams • Scientist have phase diagrams to show exactly the temperature and pressure must be achieved to have a solid, liquid, gas, two phases or all three phases • A phase diagram by definition is “a relationship between physical states that deals with temperature and pressure.”

29. Phase diagram of water. Pressure is in atmopheres (normal pressure is 1 atm) Temperature is in celcius (water boils at 100)

30. Each of the lines represent equilibrium (two states exist at the same time)

31. Point C is called the critical point Critical point: is the point on which liquids and gases are indistinguishable (can’t tell the difference

32. Point A is called the Triple Point The Triple point is the temperature and pressure in which gas, liquid and solid all exist in equilibrium

33. Video • Sublimation • Freezing • Freezing Waves

34. Vapor Pressure • The molecules in a liquid are constantly moving and running into each other. • Sometimes they will “bounce” out of the container they are in. • As a result, a faint vapor will exist over most liquids.

36. Vapor Pressure • Vapor Pressure is the pressure of vapor over a liquid at equilibrium with the atmosphere. • When water boils the vapor pressure exceeds the atmospheric pressure and the molecules leave the liquid with little impediment.

37. Vapor Pressure • So then how do puddles evaporate and why can clothes dry outside when the temperature is below freezing??? • Water molecules are always moving and are always escaping from their container. Heat makes them move faster or slower.