Changes of StateSection 18.3

Demo

• Tape 4 students together with masking tape. (Wrap around several times)

• What state do they represent?

• Are they able to move without breaking the tape?

Demo - continued

• When the students move more and the tape breaks, what state do they represent now?

• What do we call it when this change occurs in a solid?

Changes of State

• Remember that the 3 most common states of matter are solid, liquid and gas.

• A change of state occurs when a substance changes from one physical state to another.

Changes of State• We will be looking at

changes between the 3 common states of matter.

• There are actually six changes of state that can happen as a substance switches from one physical form to another.

Realplayer\18.3\Changes of states of matter - Bing Videos.flv

Solids and Liquids

• Melting is the process of changing any solid into its liquid form.

• In order for this to happen, energy must be added to the solid (an endothermic process).

• Why must energy be added?• To answer this question, we must

consider the solid at the atomic/ molecular level.

Solids and Liquids

• For a solid substance to melt, it must gain sufficient energy to overcome intermolecular attraction.

• As a solid (such as ice) absorbs energy, the individual molecules of the solid vibrate faster and faster, weakening the forces that hold the molecules in their fixed positions.

Solids and Liquids

• Eventually the particles will be vibrating so much that they are able to break free of the attractions to surrounding particles, so they are no longer held rigidly in place.

Solids and Liquids

• Freezing (solidification) is the process of changing any liquid into its solid form.

• In order for this to happen, energy must be removed from the particles in the liquid phase (an exothermic process).

• Why must energy be removed?

Solids and Liquids

• To form a solid, the particles must slow down enough that intermolecular attractions can hold them firmly in place relative to the neighboring molecules. This requires the liquid to be placed in a cooler environment (so it can lose heat to its surroundings).

Solids and Liquids

• Most solids, besides having a more orderly structure than their liquid form, have a slightly higher density than their liquid forms.

• Water is unusual in this respect because as the solid forms, its crystalline structure actually has more space in it than the liquid form does.

Solids and Liquids

• The temperature at which a substance changes from the liquid to the solid phase is known as the freezing point.

• If the change is in the other directions, solid to liquid, then it is called the melting point.

• These two temperatures are the same for any particular substance (the temp. at which it melts is the same temp. at which it freezes).

Realplayer\18.3\Melting and freezing - Bing Videos.flv

Liquids and Gases

• Vaporization is the general term for any liquid changing to the gaseous state.

• Evaporation is the term used for the process of a liquid changing into a gas at the surface of a liquid.

• Evaporation is what cools your skin when you sweat.

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Liquids and Gases

• Recall that temperature is a measure of the motion of the particles in a substance. It is actually a measure of the average motion, since not all particle will be moving with the same speed. images.usatoday.com

Liquids and Gases

• Some of the particles are moving fast enough to break free of the attractive forces at the surface of the liquid – this is evaporation.

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Interesting Fact

• Most people wear an antiperspirant to avoid sweating. These contain compounds that clog the pores of the skin, so sweat cannot be excreted by the body.

• However, sweat is the body’s air conditioner. So when you wear antiperspirant, you cause your body to cool less efficiently.

Liquids and Gases

• Boiling is the term used for the process of a liquid changing into a gas that occurs throughout the liquid.

• Bubbles of the gaseous liquid form anywhere within the liquid that the particles have enough energy to form a gas. This is why you see bubbles forming and rising from the bottom of a pan being heated on a stove.

Liquids and Gases

• Boiling occurs when the pressure inside the bubbles that form (called vapor pressure) equals the pressure outside the bubbles (which is atmospheric pressure).

• The boiling point is the temperature at which boiling will occur for a particular liquid.Realplayer\18.3\Boiling and evaporation - Bing Videos.flv

Interesting Fact

• Atmospheric pressure changes depending upon the altitude of a particular location. Because boiling depends upon atmospheric pressure, being at a lower or higher altitude will affect the temperature at which a liquid boils.

• This is why some recipes have extra instructions for “high altitudes.” When boiling something in water, the water boils at a lower temperature, requiring a longer time to cook the food!

Liquids and Gases

• Condensation is the process of changing a gas back into the liquid phase.

• The condensation point is the temperature at which a gas will become a liquid. It is the same as the boiling point for the liquid.

Realplayer\18.3\condensation time lapse.flv

Liquids and Gases• Condensation is an exothermic process because

it requires the particles to lose energy to their surroundings so they slow down enough that they can become attracted to other particles and begin clumping together, forming a liquid.

Liquids and Gases

• Dew and fog form from condensation when the atmosphere holds more water than it should at a particular temperature.

Liquids and Gases

• Fog is just tiny water droplets suspended in air; it is like a cloud, just at a lower altitude.

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Solids and Gases

• Sublimation is the process in which a solid turns directly into a gas, without going through a liquid phase (like dry-ice).

Solids and Gases

• Dry ice is just another name for frozen carbon dioxide. It is what they frequently use on stage if there is need of a hazy effect.

Solids and Gases

• For sublimation to occur, the particles must gain enough energy to change from a solid to a gas (an endothermic process).

Solids and Gases

Realplayer\18.3\Sublimation - Bing Videos.flv

Realplayer\18.3\Sublimation of Iodine.flv

Solids and Gases

• Deposition (or desublimation) is the process in which a gas turns directly into a solid, without going through a liquid phase.

• Some books and videos call any direct change between solid and gas sublimation. This can be confusing and should be avoided!

Solids and Gases

• This heavy frost formed on all exposed surfaces due to deposition of water out of the air onto cooler surfaces.

Solids and Gases

• Frost that forms on windshields and other surfaces overnight during the cooler months is also an example of deposition.

Application – Changes of State

• The water cycle is an excellent everyday example involving changes of state.

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Interesting Fact• When a sample undergoes a phase change, it

doesn’t change temperature until the phase change is complete, even though heat is being added to or taken away from the sample.

• Where does the energy go?

• When changing from solidliquidgas, the energy is used up breaking the attractions between particles.

Interesting Fact• Example: If you stir ice water so that the

liquid and ice stay well mixed, a thermometer in the mixture will read 32OF (0OC) until either the ice is all melted or the water is all frozen.

Phase Change Diagram

• A phase change diagram is a plot of the change in temperature over time of a substance as heat is added at a constant rate.

• The flat areas correspond to phase changes occurring.

Phase Change Diagram

• The diagonal lines in the diagram indicate times when the substance is all in one state and is warming up (temperature is increasing) as heat is added.

Realplayer\18.3\Change in State.flv

Phase Diagram• A phase diagram indicates what phase a

substance would be in at a particular combination of temperature and pressure.

• A phase diagram for water is shown to the right.

Realplayer\18.3\Phase Diagrams and the States of Matter.flv

References:• http://syd15.files.wordpress.com/2009/02/changes-of-state1.jpg?w=390&h=368• http://www.valdosta.edu/~dnshaner/015.JPG• http://www.eriding.net/media/photos/science/states/050114_rfoster_mp_sci_icecube1.jpg• http://i.ehow.co.uk/images/a04/v7/1c/ice-melt-sprinkle-salt-it_-2.1-800X800.jpg• http://sciencecity.oupchina.com.hk/npaw/student/glossary/img/evaporation.jpg• http://images.usatoday.com/weather/photos/airtemp2.gif?01AD=3Wb8NOrZiEnd1atmZnAgLbZSgbHFxrkRzCzFzOyXT704U0N9x

S0xmPA&01RI=7AB4A5FA1863BBE&01NA=• http://www.school-for-champions.com/Science/images/evaporation.gif• http://0.tqn.com/d/chemistry/1/5/W/W/1/superheating.png• http://itl.chem.ufl.edu/2045/matter/FG05_009.GIF• http://media.photobucket.com/image/dew/electracuted/DewDropFlower780.jpg• http://www.sci.uidaho.edu/scripter/geog100/lect/05-atmos-water-wx/05-17-water-droplets-raindrops.jpg• http://c.photoshelter.com/img-get/I0000iX.wXbzcYzE/s/750/600/Dry-Ice.jpg• http://img.tootoo.com/mytootoo/upload/49/499754/product/499754_fb1c59acd58fc595c461e3695656f862.jpg• http://www.kozmiclazershow.com/SW%20Dry%20Ice.JPG• http://pics4.city-data.com/cpicc/cfiles28057.jpg• http://climate.uvic.ca/climate-lab/front_page_pics/frost_feathers.lrg.jpeg• http://cd7.e2bn.net/e2bn/leas/c99/schools/cd7/website/images/bp-watercycle2.jpg• http://gallery.hd.org/_c/natural-science/ice-melting-in-water-with-thermometer-at-zero-degrees-Celsius-AJHD.jpg.html• http://upload.wikimedia.org/wikibooks/en/b/b7/Phase_Heat_Diagram.png• http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg