1,2The Behavior of Ions in Aqueous Solution (Water)
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Initial Questions:
What's the secret behind this picture and video? Click here to find out.
Upon completion of the lesson, students will be able to:
Input:
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Both of the above pictures depict the country of Bangladesh, but in slightly different ways. In what way are these two pictures dissimilar?
But what do both pictures suggest Bangladesh has ample access to?
Describe how the following except, which refers to a major problem in Bangladesh, seems counterintuitive based on our analysis of the two pictures:
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Despite its access to several sources of water (see the pictures above), the country of Bangladesh has long been the victim of an inadequate water supply. For much of the early 20th century, the citizens of Bangladesh were forced to drink water infected with harmful bacteria and microorganisms, resulting in millions becoming violently ill, and in some cases dying.
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If you were a scientist, what are some ways you might address this problem? In other words, how would you get clean, usable water to the people of Bangladesh? (Import from other places, desalinate ocean water, distill "tainted" water, chemical products like chlorine, etc.)
What are the drawbacks to using these methods? (very expensive, typically only privileged and wealthy countries can afford access to such methods)
In the 1970's, UNICEF decided to step in and try to solve the problem, but were limited in the amount of funding they could provide. What method do you think they chose to employ to supply the people with water?
To try and correct the problem, UNICEF and a team of scientists installed long tubes in the ground and used them as wells to draw up water from underground and supply the country with a source of clean water. There was just one problem; no one ever bothered to test whether the water was safe to drink! The result was catastrophic. The water was laced with arsenic, a naturally-occurring element deadly upon ingestion, affecting as many as 75 million out of Bangladesh's population of about 120 million. To read more about the arsenic-water debacle in Bangladesh and its current effects, click here.
As chemists, the water-arsenic crisis in Bangladesh brings out a number of interesting questions:
How did the arsenic "get in" the water in the first place?
What things can get in water?
Do some things dissolve better than others in water, or maybe not at all?
What's so special about water that allows it to "mix" with all of these things, one of which is arsenic?
Water - "The Universal Solvent"
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Solvent - the medium in which another substance (the solute) is dissolved.
Recall our example of the arsenic in the Bangladesh water supply. Assuming that the drinking supply contained only water and arsenic, what would be considered the solvent and what would be considered the solute? Why?
Using these two compounds (i.e. arsenic and water), describe a situation where the role would be reversed, meaning that the solvent would become the solute and the solute would become the solvent.
Detailed composition of seawater
| Element Hydrogen H2O Oxygen O2 Sodium NaCl Chlorine NaCl Magnesium Mg Sulfur S Potassium K Calcium Ca Bromine Br |
ppm 110,000 883,000 10,800 19,400 1,290 904 392 411 67.3 |
(referenced from http://www.seafriends.org.nz/oceano/seawater.htm)
Looking at the chart above and the definition provided, why might water be considered "the universal solvent?"
In the Bangladesh story described earlier, what was the solvent and what was the solute?
But how did it get in there? And why don't we see these elements?
Countless chemical reactions occur largely among substances dissolved in water, or, chemically speaking, in aqueous solution. So, it's very important to understand what happens to compounds once they go into water.
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What do these two pictures have in common?
What is the difference between these two pictures?
Suppose we were to mix table salt, or NaCl, and water, in the same fashion as the picture below.
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If we mixed this solution vigorously, what would result?
Could we see the salt in the water? Where does it go?
Real question: What is happening at the molecular level between salt and water?
Sodium Chloride is an ionic compound → molecules that consist of charged ions with opposite charges.
One of the ions has a positive charge → cation
One of the ions has a negative charge → anion
The old adage still applies, "opposites attract." The molecules are held together by electrical attraction resulting from the oppositely charged ions.
Recalling the picture from earlier (shown below), what would be considered the cation and what would be considered the anion? How can you tell?
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In order for a compound such as NaCl to dissolve into solution, it has to be separated from the oppositely charged ion that surrounds it (Na+ must be separated from Cl- and vice versa).
Compounds that are considered ionic dissolve very well in water...and for good reason!
Earlier we asked the question, What's so special about water that allows it to "mix" with all of these things?
Water is unique in that it is a polar molecule, that is to say one end of the molecule is slightly positive and the other end of the molecule is slightly negative.
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In this particular case, the oxygen end of the molecule has a slightly negative charge while the other end of the molecule (the side containing the two hydrogen's) has a slightly positive charge. Thus, water is considered a polar molecule.
Looking at the picture above, what might you say water is capable of doing to an ionic compound once it is placed in aqueous solution (remember, opposites attract)? What might the water do to sodium chloride if the two were to mix?
Click here to see what happens at the molecular level when our initial example, sodium chloride, reacts with water.
So, a water molecule can attract a positive ion to its negatively charged end, or it can attract a negative ion to its positively charged end. Hence the reason so many "foreign" ionic compounds tend to be found in water.
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In summary:
NaCl is an ionic compound, meaning it is composed of two ions each with an opposite charge.
The ion with a positive charge is called the cation, which in this case would be Na+
The ion with a negative charge is called the anion, which in this case would be Cl-
When an ionic compound is put in water, it dissolves very readily and forms solution because of the polar nature of water.
Water is polar, meaning one end of the molecule is slightly positive while the other end of the molecule is slightly negative. These slightly positive and negative charges become attracted to the anions and cations now in the water. Opposites attract, forming a solution.
Closure:
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The graphic above is a picture of arsenic sulfide, one of the more common ways arsenic presents itself in the environment. Earlier we discussed the problem of a noticeable amount of arsenic being found in the Bangladesh water supply.
Assuming arsenic sulfide is ionic, why is it not surprising that large amounts of arsenic were found dissolved in the Bangladesh water supply?
If the formula for arsenic sulfide is As2S4, what would it look in ionic form? What is the cation? What is the anion?
What is happening at the molecular level between arsenic sulfide and water?
What special characteristic of water makes this possible?
Application (Hands-on/Minds-on):
To find out more about why water is considered such a good solvent and its unique polar structure, click here. After reading and manipulating the applet, please answer the multiple choice questions found at the bottom of the page.
We've already established that water is great at dissolving ionic compounds. Click here for an interactive activity that takes a closer look at the relationship between ionic compounds and water. Keep in mind the following questions while performing this virtual lab:
- Of the three substances listed, which dissolve in water?
- What substances would be considered ionic based on these observations?
- Often knowing whether a compound is ionic or not can provide many clues to its chemical structure and properties. Based on this activity, what is a simple way to get an idea of whether an unknown compound is ionic or not? What reasoning would you use to justify your answer?
The Arsenic water crisis in Bangladesh has forced scientists to come up with new ways to clean the contaminated well water. One particular idea now being implemented in parts of Bangladesh is the method of "Ion exchange." An ion exchange, as defined by Remco Engineering, is
"a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins."
Click here to view an interactive presentation of the ion exchange process similar to the one being implemented in Bangladesh. Focus in particular on slides IV and V but be sure to read through the entire presentation. While viewing this presentation, keep in mind the following questions:
- What form does arsenic initially take before the ion exchange occurs?
- What is the resin site (labeled R) initially attached to in "anion exchange" reaction?
- What does the arsenic become attached to after the ion exchange takes place?
- Based on the study done by the Environmental Protection Agency (EPA), how effective is the ion exchange process at removing arsenic?
- Based on the detailed photos provided of various ion exchange sites across the country, what, if any, drawbacks do you think are associated with this methods use?
- Carbon tetrachloride, or CCl4, is an organic compound with equally damaging effects to the human body upon exposure and ingestion. However, the method of ion exchange is of no use in purifying water supplies contaminated with carbon tetrachloride. Why is this the case? What is the primary principle at work that allows this process to take place and purify a contaminated water supply?
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Photos, Videos, and Resources consulted, unless stated otherwise, borrowed from the following (in chronological order):
1 http://english.people.com.cn/200604/17/images/0416_A67.jpg
3 http://www.photoatlas.com/photo/bangladesh_bangabandhu_bridge.jpg
4 http://www.galaxybd.com/holidays/bd_photos/bangladesh-map1.jpg
5 http://web.mac.com/empower2/iWeb/The%20Empowerment%20Channel/Humanity_files/WATER31.
6 http://joel.mawhorter.org/priorities/child_with_dirty_water.jpg
7 http://www1.istockphoto.com/file_thumbview_approve/1892614/2/istockphoto_1892614_water_molecule.jpg
8 http://www.dkimages.com/discover/previews/778/196765.JPG
9 http://www.chemistryland.com/CHM107/Water/WaterTableSalt.jpg
11 http://www.chemistryland.com/CHM107/Water/WaterTableSalt.jpg
12 http://academic.brooklyn.cuny.edu/biology/bio4fv/page/image15.gif
13 Chemistry and Chemical Reactivity; Kotz and Treichel, 5th edition, page 150.
14 http://www.gc.maricopa.edu/earthsci/imagearchive/realgar.jpg
15 http://www.ehponline.org/members/2006/114-7/innovations.jpg
