Colligative Properties





Colligative Properties 3283
Photo by: Fedor Kondratenko

Colligative properties are those properties of solutions that depend on the number of dissolved particles in solution, but not on the identities of the solutes. For example, the freezing point of salt water is lower than that of pure water, due to the presence of the salt dissolved in the water. To a good approximation, it does not matter whether the salt dissolved in water is sodium chloride or potassium nitrate; if the molar amounts of solute are the same and the number of ions are the same, the freezing points will be the same. For example, AlCl 3 and K 3 PO 4 would exhibit essentially the same colligative properties, since each compound dissolves to produce four ions per formula unit. The four commonly studied colligative properties are freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. Since these properties yield information on the number of solute particles in solution, one can use them to obtain the molecular weight of the solute.

Freezing Point Depression

The presence of a solute lowers the freezing point of a solution relative to that of the pure solvent. For example, pure water freezes at 0°C (32°F); if one dissolves 10 grams (0.35 ounces) of sodium chloride (table salt) in 100 grams (3.53 ounces) of water, the freezing point goes down to −5.9°C (21.4°F). If one uses sucrose (table sugar) instead of sodium chloride, 10 grams (0.35 ounces) in 100 grams (3.53 ounces) of water gives a solution with a freezing point of −0.56°C (31°F). The reason that the salt solution has a lower freezing point than the sugar solution is that there are more particles in 10 grams (0.35 ounces) of sodium chloride than in 10 grams (0.35 ounces) of sucrose. Since sucrose, C 12 H 22 O 11 has a molecular weight of 342.3 grams (12.1 ounces) per mole and sodium chloride has a molecular weight of 58.44 grams (2.06 ounces) per mole, 1 gram (0.035 ounces) of sodium chloride has almost six times as many sodium chloride units as there are sucrose units in a gram of sucrose. In addition, each sodium chloride unit comes apart into two ions (a sodium cation and a chloride anion ) when

The freezing point depression of a solution containing a dissolved substance, such as salt dissolved in water, is a colligative property.
The freezing point depression of a solution containing a dissolved substance, such as salt dissolved in water, is a colligative property.

dissolved in water. Sucrose is a nonelectrolyte, which means that the solution contains whole C 12 H 22 O 11 molecules. In predicting the expected freezing point of a solution, one must consider not only the number of formula units present, but also the number of ions that result from each formula unit, in the case of ionic compounds. One can calculate the change in freezing point (Δ T f ) relative to the pure solvent using the equation:

Δ T f = i K f m

where K f is the freezing point depression constant for the solvent (1.86°C·kg/mol for water), m is the number of moles of solute in solution per kilogram of solvent, and i is the number of ions present per formula unit (e.g., i = 2 for NaCl). This formula is approximate, but it works well for low solute concentrations.

Because the presence of a solute lowers the freezing point, many communities put salt on their roads after a snowfall, to keep the melted snow from refreezing. Also, the antifreeze used in automobile heating and cooling systems is a solution of water and ethylene glycol (or propylene glycol); this solution has a lower freezing point than either pure water or pure ethylene glycol.

Boiling Point Elevation

The boiling point of a solution is higher than that of the pure solvent. Accordingly, the use of a solution, rather than a pure liquid, in antifreeze serves to keep the mixture from boiling in a hot automobile engine. As with freezing point depression, the effect depends on the number of solute particles present in a given amount of solvent, but not the identity of those particles. If 10 grams (0.35 ounces) of sodium chloride are dissolved in 100 grams (3.5 ounces) of water, the boiling point of the solution is 101.7°C (215.1°F; which is 1.7°C (3.1°F) higher than the boiling point of pure water). The formula used to calculate the change in boiling point (Δ T b ) relative to the pure solvent is similar to that used for freezing point depression:

Δ T b = i K b m ,

where K b is the boiling point elevation constant for the solvent (0.52°C·kg/mol for water), and m and i have the same meanings as in the freezing point depression formula. Note that Δ T b represents an increase in the boiling point, whereas Δ T f represents a decrease in the freezing point. As with the freezing point depression formula, this one is most accurate at low solute concentrations.

Vapor Pressure Lowering

The vapor pressure of a liquid is the equilibrium pressure of gas molecules from that liquid (i.e., the results of evaporation) above the liquid itself. A glass of water placed in an open room will evaporate completely (and thus never reach equilibrium); however, if a cover is placed on the glass, the space above the liquid will eventually contain a constant amount of water vapor. How much water vapor is present depends on the temperature, but not on the amount of liquid that is present at equilibrium (provided some liquid is present at equilibrium). (At room temperature, the vapor pressure of pure water is about 20 Torr, which is about one-fortieth of the total atmospheric pressure on a "normal" day at sea level.)

If, instead of pure water, an aqueous solution is placed in the glass, the equilibrium pressure will be lower than it would be for pure water. Raoult's law states that the vapor pressure of the solvent over the solution is proportional to the fraction of solvent molecules in the solution; that is, if twothirds of the molecules are solvent molecules, the vapor pressure due to the solvent is approximately two-thirds of what it would be for pure solvent at that temperature. If the solute has a vapor pressure of its own, then the total vapor pressure over the solution would be:

P vap = ⅔ (pure solvent vapor pressure) + ⅓ (pure solute vapor pressure)

Generally, one expects that solutes which are liquids in their pure form (such as ethyl alcohol) will have some vapor pressure of their own, whereas ionic compounds (such as sodium chloride) will not contribute to the total vapor pressure over the solution.

One consequence of this lowering in vapor pressure may be observed in a spilled can of soda. As the water evaporates, the soda becomes more sugar and less water, until the vapor pressure of the water is so low that it barely evaporates. As a result, the spilled soda remains sticky for a long time. Contrast this behavior with that of a water spill.

Osmotic Pressure

Osmosis is the process whereby a solvent passes through a semipermeable membrane from one solution to another (or from a pure solvent into a solution). A semipermeable membrane is a barrier through which some substances may pass (e.g., the solvent particles), and other species may not (e.g., the solute particles). Important examples of semipermeable membranes are the cell walls in cells of living things (plants and animals). Osmosis tends to drive solvent molecules through the semipermeable membrane from the low solute concentrations to the high solute concentrations; thus, a "complete"

Figure 1. Vapor pressure over a benzene-toluene solution, plotted as a function of the fraction of benzene molecules in the solution. The solid curve is the total vapor pressure, while the short-dashed and long-dashed curves are the vapor pressures from the benzene and toluene, respectively. Note that the two dashed curves add up to the solid curve.
Figure 1. Vapor pressure over a benzene-toluene solution, plotted as a function of the fraction of benzene molecules in the solution. The solid curve is the total vapor pressure, while the short-dashed and long-dashed curves are the vapor pressures from the benzene and toluene, respectively. Note that the two dashed curves add up to the solid curve.

osmosis process would be one that ends with the solute concentrations being equal on both sides of the membrane. Osmotic pressure is the pressure that must be applied on the high concentration side to stop osmosis.

Osmosis is a very useful process. For example, meats can be preserved by turning them into jerky: The meat is soaked in a very concentrated salt solution, resulting in dehydration of the meat cells. Jerky does not spoil as quickly as fresh meat, since bacteria on the surface of the salty meat will fall victim to osmosis, and shrivel up and die. This process thus extends the life of the meat without the use of refrigeration.

There are times when one wishes to prevent osmosis when two solutions (or a pure solvent and a solution) are on opposite sides of a semipermeable membrane. Osmosis can be prevented by applying pressure to the more concentrated solution equal to the osmotic pressure on the less concentrated side. This can be accomplished either physically, by applying force to one side of the system, or chemically, by modifying a solute concentration so that the two solute concentrations are equal. (If one applies a pressure greater than the osmotic pressure to the higher concentration solution, one can force solvent molecules from the concentrated solution to the dilute solution, or pure solvent. This process, known as reverse osmosis, is often used to purify water.) A hospital patient receiving fluids intravenously receives an intravenous (IV) solution that is isotonic with (i.e., at the same solute concentration as) his or her cells. If the IV solution is too concentrated, osmosis will cause the cells to shrivel; too dilute a solution can cause the cells to burst. Similar problems would be experienced by freshwater fish swimming in salt water, or saltwater fish swimming in freshwater. The osmotic pressure, like other colligative properties, does not depend on the identity of the solute, but an electrolyte solute will contribute more particles per formula unit than a nonelectrolyte solute.

SEE ALSO Solution Chemistry .

Wayne B. Bosma

Bibliography

Atkins, Peter, and de Paula, Julio (2001). Physical Chemistry , 7th edition. New York: W.H. Freeman.

Brown, Theodore L.; LeMay, H. Eugene; Bursten, Bruce E.; and Burdge, Julia R. (2002). Chemistry: The Central Science , 9th edition. Upper Saddle River, NJ: Prentice Hall.



Also read article about Colligative Properties from Wikipedia

User Contributions:

Megan
Report this comment as inappropriate
Apr 16, 2007 @ 3:15 pm
Thank you! This article really helped me w/ science!
sneha
Report this comment as inappropriate
Nov 10, 2007 @ 7:07 am
plze gie the topic related to 1 bsc general chemistry
leo
Report this comment as inappropriate
Nov 11, 2007 @ 12:00 am
tnx a lot!!
id finish my home work fast thanks for this web...
Sami
Report this comment as inappropriate
Jun 10, 2008 @ 1:01 am
OMG thank-you so much you have no idea how much this helped me with my assignment
Alex
Report this comment as inappropriate
Sep 18, 2008 @ 9:21 pm
Thanks for publishing this webpage. It was a huge help.
fentahun Adeno
Report this comment as inappropriate
Sep 24, 2008 @ 2:02 am
if you add some or simple demonstration for the given note it will become more interested.
angel
Report this comment as inappropriate
Oct 8, 2008 @ 12:00 am
tnx a lot for giving me more information about my chemistry subject (lessons)and its help me a lot.
jhoanna
Report this comment as inappropriate
Dec 15, 2008 @ 2:02 am
thanks for the help... i had finished my final report!....
Madane
Report this comment as inappropriate
Jan 18, 2009 @ 3:03 am
This article really helps for my assignment. Thanks!
Kristal
Report this comment as inappropriate
Feb 3, 2009 @ 12:12 pm
THANX SO MUCH IM DOING A REASEACH PAPER ON FREEZING POINT DRPRESSION THIS WAS REALLY HELPFUL
kariuki
Report this comment as inappropriate
May 3, 2009 @ 7:07 am
The explanation above is excellent. I love it. i wouldn't have gotten it better than i did. keep it up guys.
Gramer
Report this comment as inappropriate
Jun 3, 2009 @ 5:05 am
Thank you so much!
it helped me a lot to prepare for my chemistry exam.
thank you
blende
Report this comment as inappropriate
Aug 14, 2009 @ 6:06 am
thank for the information it really helps a lot for our debate in colligative properties...but may i say this to you ..that you should add more examples...thanks,,,
Shayla
Report this comment as inappropriate
Nov 14, 2009 @ 5:17 pm
This page really did help. Only when I was searching for this information, I found it a bit too late.
crispina
Report this comment as inappropriate
Dec 1, 2009 @ 3:15 pm
i like this article.
but i sought to know more practical applications for each of the colligative properties... anyways, thanks!!
Dave
Report this comment as inappropriate
Jan 6, 2010 @ 5:17 pm
As a none chemist I found this article very interesting but I was looking to find what water freezing point lower limit. I believe there is a maximum concentration of salt that water can keep in solution and this determines the lowest temp that salt treated water can reach. Some years ago I spent a winter in IOWA and was told that salt was not used on the roads because the temperature was regularly below this lowest temperature. The road snow problem was solved by owners being required to fit snow tyres. What is the lowest temp at sea level that salt water solution remains liquid? Best regards Dave
zacharyolang
Report this comment as inappropriate
Jan 8, 2010 @ 12:00 am
SIMPLE, STRAIGHT, SUMMARIZED AND COMPLETE GOOD WORK!!
Ephraim Emeto
Report this comment as inappropriate
Jan 18, 2010 @ 7:19 pm
Am really happy with the articles published here. It has made me gladdened. It is a speed to learning.
Jonny
Report this comment as inappropriate
Jan 20, 2010 @ 12:00 am
Yay this helped me with my lab i bib it no wrry.
Thanks for all the help
Sara
Report this comment as inappropriate
Jan 28, 2010 @ 12:12 pm
Thank you So much for the wonderful information above, it really helped me with chemistry!
Report this comment as inappropriate
Feb 7, 2010 @ 11:11 am
thanks a lot.Now i have finished my topic with the help of this article
Report this comment as inappropriate
Feb 19, 2010 @ 2:14 pm
Thanks a lot for this article . It helped me a lot in clearing my concepts.
Report this comment as inappropriate
Mar 4, 2010 @ 9:09 am
You are simply the best. I have solve a lot of questions because of you.
Report this comment as inappropriate
Mar 16, 2010 @ 3:15 pm
thanx. it helped on my lab report. i will use the site more often
Alex R
Report this comment as inappropriate
Mar 16, 2010 @ 10:22 pm
Thx! This article helped me finish my Inquiry lab! College chem now tht much easier! :)
Report this comment as inappropriate
Mar 25, 2010 @ 8:20 pm
omg thanx so much! this article was uber helpful for my lab report :D
Report this comment as inappropriate
Apr 7, 2010 @ 2:14 pm
why freezing point lowers when Boiling point elevates?
Report this comment as inappropriate
May 4, 2010 @ 8:20 pm
Huge help! Now my homework will be done on time! This really helps with general chemestry. Thanks so much for posting this!
Report this comment as inappropriate
Jun 23, 2010 @ 1:01 am
Will the normal boiling point of an aqueous solution and pure water be the same?
Report this comment as inappropriate
Aug 2, 2010 @ 3:03 am
How can colligative properties of water be of use in villages where people draw water from the boreholes and rivers
Report this comment as inappropriate
Aug 10, 2010 @ 7:07 am
tnx..to this...
i love it..
this article help me in Chemistry,..
Report this comment as inappropriate
Sep 1, 2010 @ 6:18 pm
THANK YOU ..VERY MUCH...NOW,I'VE FINISHED MY ASSIGNMENTS..:)
Report this comment as inappropriate
Sep 6, 2010 @ 2:02 am
THANK YOU
THIS HELPED ME UNDERSTAND,SINCE I CULDNT UNDERSTAND IN CLASS, DUE TO THE FACT THAT THE LECTURER MOVES FAST.
Report this comment as inappropriate
Sep 8, 2010 @ 4:16 pm
what is the formula to find out the maximum amount of sodium chloride that can be dissolved in 100 grams of water at 70c
Matthew
Report this comment as inappropriate
Sep 23, 2010 @ 8:20 pm
This was extremely helpful and very understandable. It helped me more than you can imagine.
Report this comment as inappropriate
Sep 27, 2010 @ 12:00 am
this helped me a lot with my quiz bee review... thanks
Report this comment as inappropriate
Oct 18, 2010 @ 6:18 pm
Thanks i got a lot info for the science fair project
Report this comment as inappropriate
Oct 21, 2010 @ 6:06 am
Very very great thanks to creator of this web page this really help to me please create chemistry topics are organic chemistry and inorganic chemistry ,BYE
Report this comment as inappropriate
Oct 25, 2010 @ 10:10 am
The article is so good. We want the immediate answer when the user asks question about the topic.If the answer is so fast,the publicity becomes more for this article.
Report this comment as inappropriate
Nov 5, 2010 @ 12:00 am
the total osmotic concentration of blood equals 0.3molL. osmotic pressure of blood plasma is 760kpa at 30 degrees celsius.explain what will observe if osmotic pressure is increased to 880kpa
Report this comment as inappropriate
Nov 5, 2010 @ 8:08 am
Thanks you so much for helping us
yes you are it was a huge help and i finish my homework quickly
Report this comment as inappropriate
Nov 18, 2010 @ 11:23 pm
I must be very thankful to everyone who is conducting this page THANK U SO MUCH I had to complete my practice lesson This page was a big help for me
Report this comment as inappropriate
Dec 16, 2010 @ 6:06 am
Thanks for the explanation given you made my research easy
Report this comment as inappropriate
Dec 18, 2010 @ 9:21 pm
Thank you so much! This helped me with my Chemistry assignment!
Report this comment as inappropriate
Jan 8, 2011 @ 12:00 am
This article helped me a lot in understanding colligative properties. Thanks.
Edge
Report this comment as inappropriate
Jan 10, 2011 @ 5:05 am
This article is a helpful one. Thanks. More power. :D
Report this comment as inappropriate
Feb 1, 2011 @ 12:00 am
thanx...
this is easy to understand and helpful..
keep uploading...
Solomon T
Report this comment as inappropriate
Feb 1, 2011 @ 3:03 am
Thanks too much!! This web page realy helpful in terms of getting adequate knowlage.
john mark
Report this comment as inappropriate
Feb 16, 2011 @ 6:06 am
THank you so much it realy help me with my report in chemistry
Report this comment as inappropriate
Feb 21, 2011 @ 1:01 am
salt result and properties when we mix it in to water to dying purpse
Report this comment as inappropriate
Mar 25, 2011 @ 8:20 pm
Thank u so much . but can you do an article on water vapour
Report this comment as inappropriate
Mar 30, 2011 @ 7:19 pm
TNX S this is articles was help me in my report in chemestry
Report this comment as inappropriate
Apr 18, 2011 @ 6:06 am
This article was quite helpful. However, a couple of your values are a little off - in particular the boiling point elevation constant and the freezing point depression constant for water. You have 0.52 and 1.86 respectively, but in fact they are 0.512 (which rounds down to 0.51) and 1.853 (which rounds down to 1.85). There appears to be some rounding errors.
Report this comment as inappropriate
Apr 30, 2011 @ 2:14 pm
Thanks. I find this helpful. I wish u do this for all chemistry topics and if possible little calculation should be added to assist learners.
Amir Hussain
Report this comment as inappropriate
May 13, 2011 @ 9:09 am
The Informations are exellent and very helpfull but I want to ask a question.
At What temperature solvent freezes in a solution?
Does Solvent start freezing at its original freezing point and solution freezes at a lower temperature?
Does only solvent freezes out from the solution or solution completely freezes at lower temperature?
What is the composition of frozen solution? Same as original solution or some super saturated from?
Amir Hussain
Report this comment as inappropriate
May 13, 2011 @ 10:10 am
The Informations are exellent and very helpfull but I want to ask a question.
At What temperature solvent freezes in a solution?
Does Solvent start freezing at its original freezing point and solution freezes at a lower temperature?
Does only solvent freezes out from the solution or solution completely freezes at lower temperature?
What is the composition of frozen solution? Same as original solution or some super saturated from?
Jo
Report this comment as inappropriate
Jul 9, 2011 @ 4:16 pm
This paper was an easy read and more understandable than many complicated science texts.
Report this comment as inappropriate
Aug 17, 2011 @ 6:06 am
how would i know what is the boiling and freezing point of a solution for example, a 4.0 g of sucrose in a 200 g of water??

and also,, what if its given amount had change how would i know??
Report this comment as inappropriate
Aug 31, 2011 @ 10:10 am
Tnx for publishing this webpage...
It help me a lot especially that we are going to have a report about this topic...
Yan-Yan
Report this comment as inappropriate
Oct 1, 2011 @ 8:08 am
Thanks... This is very helpful!It is a big help, especially for us students studying chemistry and has a difficulty in in this lesson... :D
Report this comment as inappropriate
Nov 16, 2011 @ 1:01 am
plz som1 tell me the limitations of colligative properties point wise?
stephanie
Report this comment as inappropriate
Dec 8, 2011 @ 3:15 pm
thanks so much for this webpage it helps me out alot :D
EZRA NGETICH
Report this comment as inappropriate
Dec 11, 2011 @ 4:16 pm
Thanks a lot i have gain more knowledge from this topic but try using graphs when explaining the freezing point depression/bp elevetion
ian
Report this comment as inappropriate
Jan 4, 2012 @ 5:05 am
it help me a lot on my assignment in chemistry in science
Christian
Report this comment as inappropriate
Jan 25, 2012 @ 7:07 am
thanks, it helps me a lot, it gave me more info about my homework :D
MAHMUD ALIYU
Report this comment as inappropriate
Feb 1, 2012 @ 5:05 am
thanks you for provide this for me it really help me in answering my assignment question
Nanda
Report this comment as inappropriate
Feb 8, 2012 @ 11:11 am
Can any body tell me about the osmotic pressure of pure water?
Ibrah
Report this comment as inappropriate
Jul 2, 2012 @ 3:15 pm
Thanks alot but why don't you show graphs and some calculations.
BhavanSingh
Report this comment as inappropriate
Jul 29, 2012 @ 5:05 am
Thanks,for help
I finished my homework with the help of these calculations.
B Kanneh
Report this comment as inappropriate
Aug 3, 2012 @ 11:11 am
Thanks a lot for your publication. I have gained a lot from it.Pls try to include schematic demonstration for each colligative property
jiya
Report this comment as inappropriate
Aug 15, 2012 @ 12:12 pm
Thanks a lot of your publication. I have gained a lot from it.
pooja aher
Report this comment as inappropriate
Aug 28, 2012 @ 10:10 am
Qus:what is the complete defination of OSMOTIC PRESSURE?
darren joseph
Report this comment as inappropriate
Nov 14, 2012 @ 10:22 pm
tnx for the information
:) i have gained a lot from it
;)
KAWANZI ANDREW
Report this comment as inappropriate
Dec 11, 2012 @ 3:03 am
Thanks a lot.u guys have the best explanations ever.i wish u could also explain other subjects like physics,maths.if u have the website to those subjects please link us
kyizinaung
Report this comment as inappropriate
Dec 23, 2012 @ 4:04 am
Thank a lot. These articles getting knowledge me to share my students and my friends.
Manigandan
Report this comment as inappropriate
Jan 29, 2013 @ 8:08 am
Thank you..this site is very easy to understood the colligative property of the solution..i need more notes about vapour pressure.
xuhaib
Report this comment as inappropriate
Feb 26, 2013 @ 2:02 am
Why are colligative properties only four in number?
mina
Report this comment as inappropriate
Apr 5, 2013 @ 12:00 am
THNX a lot for these informations.
i could use them in my class
Tonderai Chidziva
Report this comment as inappropriate
Apr 23, 2013 @ 4:04 am
Typically animal cells burst when frozen. What property of water apparent only as it cools is responsible for this effect in frozen tissue?
Edgar John Mrutu
Report this comment as inappropriate
Nov 10, 2013 @ 3:15 pm
Iam very happy to get a solution of my problem.Thanks alot
Deborah
Report this comment as inappropriate
Nov 11, 2013 @ 4:04 am
This really heled with my research on water properties for biochemistry assignment. Thanks.
KENNETH
Report this comment as inappropriate
May 4, 2014 @ 11:11 am
woow..thx alot.it has helped understand colligative property more..good work
roland
Report this comment as inappropriate
Aug 17, 2014 @ 10:22 pm
THANK YOU SO MUCH FOR THIS INFORMATION ITS HELP ME IN MY REPORT IN CHEMISTRY
Report this comment as inappropriate
Oct 15, 2014 @ 3:15 pm
thanks for help me
it answers I'm put in my preparation of medical entrance
Bob
Report this comment as inappropriate
Oct 29, 2014 @ 10:10 am
This didn't help me at all, it needs WAY MORE information.
Wen
Report this comment as inappropriate
Dec 3, 2014 @ 11:23 pm
This helped me with my science fair this year. At first, I had no clue what colligative properties were, and after reading this article it helped. Thanks

Comment about this article, ask questions, or add new information about this topic:

CAPTCHA


Colligative Properties - Chemistry Encyclopedia forum