Phase Diagram 2

8/13/2019 Phase Diagram 2

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Phase Diagram of Carbon Dioxide

Background

When ice cream trucks drive through towns on hot summer days, how do you

think they keep their products from melting? Refrigeration units are too expensive, sothey pack their ice cream products in dry ice. But why do we use dry ice instead of ice?

Ice taken from a freezer will melt at room temperature to a liuid and eventually !ecomes

a gas when heated. In contrast, dry ice keeps the ice cream cold, and instead of melting, it

su!limes "converts directly from solid to vapor#, which also prevents the ice creamcontainers from getting soggy. $he changing of state for ice and dry ice descri!ed a!ove,

however, only occurs at a pressure of % atm. But what happens to the ice and dry ice at

other temperatures and pressures?

&nother way to show the phase changes of ice and car!on dioxide at a pressure of % atm

can !e illustrated as follows'

Ice is sta!le !elow ()*, water is sta!le !etween ()* and %(()*, and water vapor is sta!le

a!ove %(()*'

+ry ice remains as a solid !elow -)* and changes to fog "gas# a!ove -)*'

+ry ice can also occur in all three phases "solid, liuid and gas#, !ut it is rarelyseen as a liuid. In this experiment, you will see dry ice in all three phases. $he uniue

phase diagramsfor car!on dioxide and water are shown in figure %. & phase diagram

shows the temperatures and pressures at which the various phases "i.e., solid, liuid and

vapor# of a su!stance can exist. Both phase diagrams for water and car!on dioxide have

the same general Y-shape, /ust shifted relative to one another. $his shift occurs !ecausethe liuid phase in the dry ice can only occur at higher temperatures and pressures,

whereas, in ice the liuid phase occurs at lower temperatures and pressures.

0igure %' 1hase +iagrams of Water and *ar!on +ioxide

2triple point3

2triple point3


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$he solid lines on the phase diagram shows the phase changes for a su!stance at

various temperatures and pressures, such as melting, !oiling, freezing, su!limating, etc.

We usually use the terms melting pointand boiling point, rather than melting line or!oiling line, when we are only referring to one specific temperature and pressure.

In this experiment, you will o!serve car!on dioxide at all three phases ando!serve its 2triple point3. $he triple point of a su!stance refers to the specific

temperature and pressure at which all three phases can exist together at euili!rium. $hetriple point is uniue for each su!stance. $he uniue triple point for *45and 654 also

explains the differences in the phase diagrams. $he triple point for' car!on dioxide is

7-)* at 7.5 atm, and water is (.(%)* at (.((8 atm "0igure %#.

Purpose

In the demonstrations and experiment, you will !e a!le to'

o +istinguish su!stances, using their phase diagrams.

o 9nderstand the terms' triple point, su!limation, phase diagram, and standardtemperature and pressure.

o 9se changes in temperature and pressure to determine a su!stances phase and

coordinates on the phase diagram.

o 4!serve su!stances undergoing phase changes at different temperature and

pressures.

o Identify the uniue properties of dry ice and normal ice.

o +istinguish su!stances from their uniue 2triple points3.

Curriculum Outcomes

$his experiment addresses parts of each of these curriculum outcomes'

C30S-1-01 +escri!e the properties of gases, liuids, solids and plasma.

C30S-1-02 9se the :inetic ;olecular $heory to explain properties of gases.

C30S-1-03


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Balloon and twist tie

Safety

o Wear goggles and la! coats.

o *ontact with dry ice can cause !urns. $herefore, it is recommended that' the teacher crushes the dry ice into a powder form for students.

gloves and a scooper are used to transport the dry ice powder into

the pipettes.

the !ul!s of the pipettes are well immersed in the water.

o 0or disposal, allow unused dry ice to su!lime under the fume hood.

Lab Hints for Teacher

o +ry ice can !e purchased from an ice house, packing house, grocery stores andor

hospitals.o $o prepare powdered dry ice, wrap a strong towel around it and smash it

repeatedly on a hard surface.

o =tore the dry ice in a styrofoam cooler with a towel wrapped around it to slow the

evaporation rate.

o 1urchase the dry ice the morning of the la! or the day !efore.

Procedure

Part 1: Teacher-led Demonstrations

Demo !" #hy is it called $dry ice%&

%. 1lace an ice cu!e and a few pellets of dry ice side !y side on a shallow plate.5. &sk the students to predict what will happen to each.

$he students will realize that the ice will eventually melt to form water.

$he ice is changing from a solid to a liuid, which is called melting. In contrast,the dry ice will change directly from a solid to a gas, which is called sublimation.

$he dry ice will completely su!limate long !efore the ice has melted.

Demo '" Dry (ce in coloured )ater

%. 0ill a 7(( ml !eaker @ full of water.5. &dd several drops of food colouring.A. &sk the students what they think will happen when you add some of this white

powder.

. &dd a scoop of dry ice to the water.7. &sk the students to explain what they have o!served.

;any students may think that the dry ice is evaporating, rather than

su!limating. =u!limationmay !e a new concept to them.


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$he dry ice changes directly from a solid to a gas as it a!sor!s the heat

from the surrounding water. $his process is known as sublimation. & white

cloud "fog# is formed that is made up of several tiny !u!!les.


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ConclusionPart 1: Teacher-led Demonstrations

%. 9sing a drawing or words, descri!e how the arrangement of the molecules are

different in the solid and gas phase of the dry ice?

5. 9sing a drawing or words, descri!e how the arrangement of the molecules are

different in the solid, liuid and gas phase of the ice?

A.


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8. &t the triple point "7- )* at 7.5 atm# for the dry ice, descri!e the appearance and

arrangement of the molecules.

-. &t temperature and pressure of 7 )* and % atm "0igure %#, are normal ice and dry

ice at the same phase?

Documents

Phase Diagram 2