<< Chapter < Page Chapter >> Page >

We can measure the freezing point of solutions of various concentrations, and similar to the change in vapor pressure, the experimental data show that the lowering of the freezing point is proportional to the mole fraction of the solute in the liquid phase. This leads us to consider how the presence of the solute disrupts the dynamic equilibrium and produces the change in the freezing point.

To develop a model to understand this, we need another experimental observation. When frozen water is in equilibrium with a salt solution, the solid water is pure water, containing no salt. As you may have read, icebergs floating in the ocean are actually pure water, not frozen salt water. When the salt water from the ocean freezes, the salt remains behind in unfrozen water, producing ice which is pure water without any salt. You can run this experiment yourself by freezing a sample of salt water, removing the ice on the top, and then melting that ice. The water produced is pure, not a salt solution.

This observation raises two questions: why is the solid ice made only of pure water instead of a frozen salt solution? And how does this disrupt the dynamic equilibrium between the liquid and the solid? Let’s focus first on developing a model that helps answer the second question. For a pure liquid in equilibrium with a pure solid, e.g. ice and water at 0 ºC and 1 atm pressure, the rate of freezing and the rate of melting must be equal. The rate of melting depends on the fraction of the molecules in the solid that have sufficient energy to break free of the crystalline structure of the solid, which depends on the temperature. At higher temperatures, more molecules have sufficient energy, and the solid melts. This is why liquids exist at temperatures higher than temperatures where solids exist. The rate of freezing depends on how frequently molecules in the liquid collide with the surface of the solid and are deposited there, losing their energy to the surrounding liquid molecules. This depends on the temperature, but also depends weakly on the pressure applied to the liquid and solid equilibrium.

Consider now adding the solute to the liquid solvent. Since the solute does not enter the solid phase, the rate at which molecules can escape the solid and join the liquid phase does not depend on the presence of the solute. So, the rate of melting is unchanged by the presence of the solute. But we know that the freezing point is lowered by the presence of the solute in the liquid phase. Since the dynamic equilibrium is disrupted by the solute, it must be true that the rate at which molecules enter the solid phase is changed by the presence of the solute. If we don’t change the temperature, adding the solute to the solid-liquid equilibrium causes the solid to melt. So it must be true that, when the solute is present, the rate of melting at the freezing point is greater than the rate of freezing. This means that the rate of freezing of molecules onto the surface of the solid is reduced when the solute is present.

Questions & Answers

what is mutation
Janga Reply
what is a cell
Sifune Reply
how is urine form
Sifune
what is antagonism?
mahase Reply
classification of plants, gymnosperm features.
Linsy Reply
what is the features of gymnosperm
Linsy
how many types of solid did we have
Samuel Reply
what is an ionic bond
Samuel
What is Atoms
Daprince Reply
what is fallopian tube
Merolyn
what is bladder
Merolyn
what's bulbourethral gland
Eduek Reply
urine is formed in the nephron of the renal medulla in the kidney. It starts from filtration, then selective reabsorption and finally secretion
onuoha Reply
State the evolution relation and relevance between endoplasmic reticulum and cytoskeleton as it relates to cell.
Jeremiah
what is heart
Konadu Reply
how is urine formed in human
Konadu
how is urine formed in human
Rahma
what is the diference between a cavity and a canal
Pelagie Reply
what is the causative agent of malaria
Diamond
malaria is caused by an insect called mosquito.
Naomi
Malaria is cause by female anopheles mosquito
Isaac
Malaria is caused by plasmodium Female anopheles mosquitoe is d carrier
Olalekan
a canal is more needed in a root but a cavity is a bad effect
Commander
what are pathogens
Don Reply
In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
Zainab
A virus
Commander
Definition of respiration
Muhsin Reply
respiration is the process in which we breath in oxygen and breath out carbon dioxide
Achor
how are lungs work
Commander
where does digestion begins
Achiri Reply
in the mouth
EZEKIEL
what are the functions of follicle stimulating harmones?
Rashima Reply
stimulates the follicle to release the mature ovum into the oviduct
Davonte
what are the functions of Endocrine and pituitary gland
Chinaza
endocrine secrete hormone and regulate body process
Achor
while pituitary gland is an example of endocrine system and it's found in the Brain
Achor
what's biology?
Egbodo Reply
Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
Lisah
biology is the study of life.
Alfreda
Biology is the study of how living organisms live and survive in a specific environment
Sifune
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Concept development studies in chemistry 2013' conversation and receive update notifications?

Ask