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The solid-vapor curve, labeled AB in [link] , indicates the temperatures and pressures at which ice and water vapor are in equilibrium. These temperature-pressure data pairs correspond to the sublimation, or deposition, points for water. If we could zoom in on the solid-gas line in [link] , we would see that ice has a vapor pressure of about 0.20 kPa at −10 °C. Thus, if we place a frozen sample in a vacuum with a pressure less than 0.20 kPa, ice will sublime. This is the basis for the “freeze-drying” process often used to preserve foods, such as the ice cream shown in [link] .

A photograph shows a package with a rocket being launched on the front and a block of pink, white and brown striped solid in a wrapper next to it.
Freeze-dried foods, like this ice cream, are dehydrated by sublimation at pressures below the triple point for water. (credit: ʺlwaoʺ/Flickr)

The solid-liquid curve labeled BD shows the temperatures and pressures at which ice and liquid water are in equilibrium, representing the melting/freezing points for water. Note that this curve exhibits a slight negative slope (greatly exaggerated for clarity), indicating that the melting point for water decreases slightly as pressure increases. Water is an unusual substance in this regard, as most substances exhibit an increase in melting point with increasing pressure. This behavior is partly responsible for the movement of glaciers, like the one shown in [link] . The bottom of a glacier experiences an immense pressure due to its weight that can melt some of the ice, forming a layer of liquid water on which the glacier may more easily slide.

A photograph shows an aerial view of a land mass. The white mass of a glacier is shown near the top left quadrant of the photo and leads to two branching blue rivers. The open land is shown in brown.
The immense pressures beneath glaciers result in partial melting to produce a layer of water that provides lubrication to assist glacial movement. This satellite photograph shows the advancing edge of the Perito Moreno glacier in Argentina. (credit: NASA)

The point of intersection of all three curves is labeled B in [link] . At the pressure and temperature represented by this point, all three phases of water coexist in equilibrium. This temperature-pressure data pair is called the triple point    . At pressures lower than the triple point, water cannot exist as a liquid, regardless of the temperature.

Determining the state of water

Using the phase diagram for water given in [link] , determine the state of water at the following temperatures and pressures:

(a) −10 °C and 50 kPa

(b) 25 °C and 90 kPa

(c) 50 °C and 40 kPa

(d) 80 °C and 5 kPa

(e) −10 °C and 0.3 kPa

(f) 50 °C and 0.3 kPa

Solution

Using the phase diagram for water, we can determine that the state of water at each temperature and pressure given are as follows: (a) solid; (b) liquid; (c) liquid; (d) gas; (e) solid; (f) gas.

Check your learning

What phase changes can water undergo as the temperature changes if the pressure is held at 0.3 kPa? If the pressure is held at 50 kPa?

Answer:

At 0.3 kPa: s g at −58 °C. At 50 kPa: s l at 0 °C, l ⟶ g at 78 °C

Got questions? Get instant answers now!

Consider the phase diagram for carbon dioxide shown in [link] as another example. The solid-liquid curve exhibits a positive slope, indicating that the melting point for CO 2 increases with pressure as it does for most substances (water being a notable exception as described previously). Notice that the triple point is well above 1 atm, indicating that carbon dioxide cannot exist as a liquid under ambient pressure conditions. Instead, cooling gaseous carbon dioxide at 1 atm results in its deposition into the solid state. Likewise, solid carbon dioxide does not melt at 1 atm pressure but instead sublimes to yield gaseous CO 2 . Finally, notice that the critical point for carbon dioxide is observed at a relatively modest temperature and pressure in comparison to water.

Questions & Answers

explain law of definite proportion
MOTIA Reply
yes same idea
DEMGUE
wat?
Advic
what's kinetic energy
Charity Reply
energy possessed by an object in motion
Asare
what is anion?
Hussniz Reply
an anion is a positively charge ion
ade
Thnkz
Hussniz
an anion is a negatively charged ion
Johnson
And anion is a negatively charged ion which move towards the anode which is positively charged.
Ngwesse
a negatively charged ion formed by the gain of an electron(s)
Asare
H
Cyril
What is a electron cause I'm New to chemistry
Joshua
what is matter
Henry Reply
what is pressure
Henry
what is water?
Scarcies Reply
hydrogen gas burned into oxygen gas which forms a polar molecule, has a defenate shape volume but not shape. at 20 degrees Celsius exists as an aqueous solution. Is a solvent that dissolves many solutes into solution. And can act as an acid or a base.
David
HOW CAN ETHANOL BE PREPARED FORM CASSAVA.... (WITH EQUATION)
Obi Reply
by mixing
Henry
it is less dense than water
Erastus Reply
can cooking gas be durable in gallon. ..
mowete
no
PRINCESS
reason pls
mowete
What is happening
Oluchi Reply
Why does ice float on water?
Ngwesse
it is less denser than water
Advic
and why does a boat don't sink in water
DEMGUE
because the boat has a large surface area and the water surface contains molecules that are attracted together by a weak force of attraction
Maxime
brief the water surface has a wall
Maxime
As it cools, olive oil slowly soldifies and froms a solid over a range of temperatures. Which best describes the solid?
Tammie Reply
definition of a sample
ashley Reply
a small part or quantity intended to show what the whole is like
Abdull
What does kj stand for ? or just k. example - 371K
Katr Reply
kilojoules or Kelvin
Annie
what is atom
divinegift Reply
atom is anything that has weight and can occupy space
Neutons
atom is the smallest particle of an element that can neitherbe divided nor destroyed
clin
atom is the smallest indivisible particle of an element.
Igbayima
atom is the smallest particle of an element that can neither be created,subdivided or destroyed
Gbolahan
All wrong
Gift
Do you know that atoms can further be devided
Gift
Go and read 📘 up Dalton atomic theory and who disproved.. Read 📘 up atomic theory
Gift
an atom is the smallest particle of an element that can take part in a chemical reaction
Anyebem
good@anyebem
Henry
what is anion?
Hussniz
what is a structure of atomic
Deren Reply
WATS atomic structure?
mowete
what is the meaning of intermolecular force
Eunice Reply
is the force of attraction that exist between two or more molecules
Johnson
What is a primary standard solution ?
Duval
a known solution
Fiko
Characteristic of a primary standard solution
Duval
pauli's exclusion is based on what?
avdhesh Reply
quantum theory
Charles
Practice Key Terms 4

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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