7.2 Temperature and the Phases of Matter pp Mr. Richter

Agenda  Warm-Up  Check and Review HW  Today:  Measuring Temperature  What Temperature Really is  Thermal Expansion  Absolute Zero  Tomorrow:  Phases of Matter  Changing Phase  Monday - Lab

Objectives: We Will Be Able To…  Convert between temperature scales.  Explain the relationship between temperature and the movement of particles in a system.  Describe the relationship between temperature and states of matter.

Warm-Up:  How do you think thermometers work?  Discuss at your table, and write a 1-2 sentence description in your notebook.

Measuring Temperature

 Scientists measure temperature using a thermometer.  Temperature is measured on two different scales:  Fahrenheit (British System)  Celcius (SI)

Measuring Temperature  Fahrenheit:  Water freezes at 32°F  Room temperature is about 72°F  Water boils at 212°F  Celcius (must memorize)  Water freezes at 0°C  Room temperature is about 21°C  Water boils at 100°C

Temperature Conversions  Because most of the US uses the Fahrenheit scale, but scientists use Celcius, temperature conversions are important.  Use the following formulas to convert:

Your Turn 1.Convert 7°F into degrees Celcius. Round to the nearest 0.1 degree. 2.Convert 30°C into degrees Fahrenheit. Round to the nearest 0.1 degree.

What Temperature Really Is

 We saw yesterday that atoms and molecules are constantly moving, randomly vibrating and colliding with each other.  This is true even in solid objects! (They’re just not vibrating very much.)  Temperature is the measure of the kinetic energy of the random motion of atoms.

What Temperature Really Is  The word “random” is important.  You will note that when you throw a rock, it does not heat up, even though it gains kinetic energy.  This is because even though the rock speeds up, the atoms do not speed up their vibrating. The RANDOM kinetic energy of each atom does not change. P. 172

Thermal Expansion

 How do thermometers work?  The “hotter” atoms are, the faster they vibrate.  The more they vibrate and collide with each other, the more they spread out.  This is what happens to the mercury (or other liquid) in thermometer. It expands up the tube.  This is also why bridges have breaks in them

Absolute Zero

 If atoms speed up their random vibration when they heat up, they slow down their vibration when they are cooled.  At Absolute Zero, atoms stop vibrating at all. No movement.  Absolute Zero is really, really cold. There is nothing colder than absolute zero.  °C  °F  Humans have produced temperatures very close to Absolute Zero, but not exactly so far.

Warm-Up  If an ice cube is put into a heated pan, will it immediately turn into water? Why or why not?  Discuss at your table, then write a 1-2 sentence explanation in your notebook.

Phases of Matter

 Three phases of matter are very common:  solid  liquid  gas  Molecules (groups of atoms) are held together by intermolecular forces. They are attracted to each other.  The strength of the intermolecular forces determines whether the matter is a solid, liquid, or gas.

Solids  A solid holds its shape and does not flow.  The molecules vibrate, but mostly stay in one place.  The vibrations are not strong enough to over come the intermolecular forces.

Liquids  A liquid will hold its volume but not its shape.  The molecules vibrate faster, and they change places with their neighbors.  The molecules are still held close together by the intermolecular forces, but the vibrations are strong enough for the molecules to move.

Gases  A gas will not hold its shape or its volume. It will expand to fill any space.  The molecules vibrate very quickly, and break away from their neighbors entirely.  The molecules have enough energy that the intermolecular forces cannot hold them together.

Plasma  At very high temperatures (~10,000 °C), atoms get so excited that individual electrons come loose. This phase is called plasma.  The loose electrons allow plasma to conduct electricity.  This is seen in lightning, nebulas, and more recently, plasma TVs.

Changing Phase

Changing Phase: Definitions  Definitions:  Melting: A solid become a liquid. Molecules gain energy.  Freezing: A liquid becomes a solid. Molecules lose energy.  Evaporating: A liquid becomes a gas. Molecules gain energy.  Condensing: A gas becomes a liquid. Molecules lose energy.  Water melts and freezes at exactly 0 °C  Water boils and condenses at exactly 100 °C

Changing Phase and Energy  It requires a transfer of energy for a substance to change phase.  Ice cannot turn into liquid water unless some heat energy is added.  Similarly, liquid water cannot turn into ice unless some heat energy is removed.  Because of this, phase change takes time.

Changing Phase and Energy  While the ice is changing phase to water, its temperature is not changing.  The ice water has a temperature of exactly 0 °C, no matter how much ice or water is in it!  It will not increase in temperature until all of the ice is turned to water.

Changing Phase: Evaporation  Does water need to be at 100 °C (boiling point) to evaporate?  No! Temperature is the AVERAGE kinetic energy of all of the molecules of water.  Some individual molecules will get more excited than the others and escape as vapor.  The remaining cooler molecules will stay. This is also how sweating works.

Wrap-Up: Did we meet our objectives?  Convert between temperature scales.  Explain the relationship between temperature and the movement of particles in a system.  Describe the relationship between temperature and states of matter.

Homework  p. 175 #1-4