polarity – Fountain Magazine

Like Dissolves Like

Louima Cunningham — Fri, 01 Jan 2016 00:00:00 +0000

Can you imagine a cup of coffee, sugarless at the top but intensely sweet at the bottom? Likewise, imagine a bowl of soup without salt at the top but over-salted at the bottom. Would these be enjoyable? We owe the joy of uniformly sweetened coffee or perfectly seasoned soup, as well as several vital life processes, to “dissolution.”

The process of dissolution occurs when a solute (solid, liquid, or gas) is placed in a solvent (also solid, liquid, or gas) and dissolves to form a solution which is a homogeneous mixture. A homogeneous mixture is uniform in composition and properties throughout. Pure substances, whether they are elements or compounds, are rarely found in nature. Most materials we encounter are mixtures of two or more substances. A mixture of sand and salt is not homogeneous, while a mixture of salt and water forms a homogeneous mixture (or solution) when enough water is added to dissolve all of the salt present. Likewise, gases dissolve in liquids to form solutions. Fish can survive in water by using their gills to extract the dissolved oxygen. Much of the world around us is made up of aqueous solutions. The oceans and our blood are only two of many examples.¹

Aqueous solutions are the solutions in which water is the solvent. Water is called the “universal solvent” because it dissolves more substances than any other liquid. The kidneys and water’s solvent properties play an essential role in keeping us alive and healthy. The kidneys are responsible for filtering out substances that enter our bodies from the foods and drinks we consume. However, the kidneys have to get rid of these substances when they accumulate. This is where water helps out: being such a great solvent, water washing through the kidneys dissolves these substances and sends them on their way out of our bodies.2

Polarity is the property whose job is to ensure water to be such an excellent solvent. Water molecules have a polar arrangement of oxygen and hydrogen atoms—one side (hydrogen) has a positive electrical charge and the other side (oxygen) has a negative charge. This allows the water molecule to become attracted to many polar molecules as well as less-polar molecules. Water can become strongly attracted to a polar compound, such as salt (NaCl); this allows it to disrupt the attractive forces that hold the sodium and chloride ions in the salt compound together, and, thus, dissolves it.²

Similarly, glucose, which is a carbohydrate and the most important simple sugar, dissolves in water because polar water molecules attach to the glucose molecules. The six O-H (hydroxyl-) groups in glucose are the polar centers of a glucose molecule. The oxygen in each -O-H has a slight negative charge, and the hydrogen end of the -O-H has a slight positive charge. They are attracted to the water molecules by dipole-dipole forces. When the attractive forces of the water molecules for the glucose exceeds the attractive forces between the glucose and its neighboring glucose molecules, the water can pull the sugar molecule out of the crystal. It is said that water has “dissolved” the sugar molecule. This process continues until either the sugar is completely dissolved or the unattached water molecules are exhausted; in other words, when “the solution is saturated.”³

“Like dissolves like” is an expression used by chemists to state that polar solvents dissolve polar solutes; non-polar solvents dissolve non-polar solutes.For example: water is polar, oil is non-polar. Water will not dissolve oil. Water is polar, and table salt (NaCl) is ionic, which is extremely polar. According to the rule of thumb, like dissolves like, and polar dissolves polar; thus, water dissolves table salt.⁴

Polar water molecules are attracted to ions (atoms or groups of atoms with a charge) where “cations” are ions with a positive charge and “anions” have a negative charge. Most ionic compounds have high solubility in water, which means that large concentrations of those compounds can dissolve before the capacity for water molecules to isolate the ions is exceeded. The point at which Na+ and Cl-, for example, would begin to precipitate a salt in seawater is termed “saturation.” For NaCl (the mineral “halite”), this only occurs when seawater evaporates and is reduced to about 10% of its original volume. Besides Na+ and Cl- ions, which account for over 85% of the total dissolved solids (salts), seawater contains other important ions such as Mg2+, Ca2+, K+, Sr2+, B+3, SO42-, HCO3-, Br-, and F-, where the positive and negative charges are balanced. As a sign of mercy in its creation, seawater is electrically neutral; otherwise, the flow of current from the sea would be shocking!⁵

All other dissolved substances in seawater are at very low concentrations (parts per million or billion, ppm or ppb: 10-6 to 10-9). This includes important nutrients such as phosphate and nitrate that are cycled by organisms (ions called “bio-limiting”) and essential for life. Metals are also found in trace concentrations. There are about 9 million tons of gold dissolved in seawater, which is about equal to all the gold mined on earth throughout history.5

The evaporation of about 81-96% of the mass of seawater produces a predictable sequence of mineral salts (as minerals become saturated at a certain point) such as CaCO3 (calcite), CaSO4 (gypsum), NaCl (halite), and the K+ and Mg2+ salts (w/ SO42- and Cl-). There is enough salt in the ocean to cover the earth’s land with a layer 170 meters thick. Such natural deposits from ancient oceans are called “evaporites.”

Even though most ionic compounds are highly soluble in water, there are some that are insoluble (or very slightly soluble). Soluble substances can form a solution of at least 0.1M (0.1 moles per liter) at 25 C while insoluble substances cannot.For an ionic substance to dissolve in water, there are two competing factors that determine the enthalpy of the solution ΔHsol, which is the enthalpy (energy) change when a solute is dissolved in a solvent (which is water, in this case):

The lattice energy LE, or the energy of the formation of the crystal between infinitely separated ions. LE is proportional to the charges of its ions. This value is always positive, as energy is required to separate the ions.
The hydration energy of gaseous ions, which is the enthalpy change when gaseous ions dissolve in sufficient water to give an infinitely dilute solution. These values are always negative, as energy is released upon the hydration of ions.

As an example, the LE of calcium carbonate (CaCO3, or calcite), which is insoluble in water, is so large, a great amount of free energy would be required to break the strongly attracted ions apart, and this energy has to come from the enthalpy of hydration. However, the enthalpy of hydration is not large enough to overcome the large lattice energy, hence it does not dissolve in water and exists as a solid (a precipitate form).⁶

It would be very hard to predict whether a precipitate is formed in an aqueous reaction if there were not a number of patterns in the data obtained from measuring the solubility of various salts.These patterns form the basis for the solubility rules⁷ which can guide predictions of whether a salt will dissolve in water. This sense of order and harmony in creation is an exceptional gift to human beings, for it makes our lives much easier.

References

The Tiniest Captains of the Ocean

Louima Cunningham — Mon, 01 Mar 2010 00:00:00 +0000

The date of invention for compass still is not known with certainty. Some historians think that it was invented in China around 900 BC, while others claim that it was around 100 AD. The use of a magnetized needle as a navigation tool, however, was not until twelfth century. This brief information can be found from history books, after a quick search on the history of compass. However, it cannot be considered complete, since it does not mention the nation that has been using nanometer size magnets to find their directions for millions of years. They are the navigators of deep oceans and small ponds utilized with a technology that took thousands of years of humankind to discover. They are magnetotactic bacteria.

In the early 1970s, a young graduate student, Richard Blakemore, observed an interesting group of bacteria in a mud sample collected from Eel Pond in Massachusetts. These bacteria were migrating through a certain edge of the microscope slide. Rotating the slide did not affect their motion; they were still moving through north. Repeating the experiment in the dark also showed that it was not light that affected the bacteria’s swimming direction. The experiments left one possible explanation to the directed motion of the bacteria and that was a crazy one-that they were sensing the magnetic field of the earth! It wasn’t hard for Blakemore to place a magnet next to the microscope slide and to prove that the crazy idea was indeed true. The bacteria were attracted by the south pole of the magnet and repelled by the north pole of it. This was the beginning of a new field of an interdisciplinary research, which attracted many scientists from very different fields such as, microbiology, physics, geophysics and paleogeology.

To understand how magnetotactic bacteria feel the magnetic field, it would be useful to check one’s transmission electron micrograph (Figure 1). The chain of magnetite (iron oxide) crystals forming a specialized organelle, called magnetesome, can be seen easily. The length of the whole chain is around one micron and each small crystal is around 50-60 nanometers. Each small crystal can be considered as a small magnet. These small magnets are aligned in a way that they support each other and the chain becomes a strong magnet. A lipid bilayer membrane surrounds this chain and holds them together. Forming chains on a straight line is not an expected behavior for small magnetite crystals. If they are produced synthetically, they accumulate together to form an aggregate. The pathways of chain formation in magnetesome are still an open question.

Magnetotactic properties are not limited to a certain species. There are many different bacteria that have magnetotactic properties. Therefore the magnetesomes may differ in size, length and even in chemistry. Some magnetotactic bacteria have greigite (iron sulfide) minerals instead of magnetite. Alsothere are magnetotactic bacteria that have more than one magnetesomes. (Figure –2)

Magnetesomes passively align bacteria parallel to the geomagnetic field but do not exert a force on bacteria to change their speed. Aligned with the magnetic field, the bacteria decide to either move towards south or north. Interestingly, almost all magnetotactic bacteria in northern hemisphere are north seeking, almost all magnetotactic bacteria in southern hemisphere are south seeking, and magnetotactic bacteria living around the geomagnetic equator consist of almost equal number of bacteria of each magnetic polarity. The bacteria in northern and southern hemispheres may have different polarities but they have one thing in common; they both move downwards. The geomagnetic field is not exactly parallel to the earth’s surface except around the geomagnetic equator. As it may seem in figure-3, the magnetic field lines are tilted up and down respectively in southern and northern hemispheres. Therefore the north seeking bacteria in northern hemisphere end up at the bottom of the water and so do the south seeking bacteria in southern hemisphere. Most of the magnetotactic bacteria cannot survive in atmospheric oxygen levels, so sensing vertical position and moving downwards, where oxygen concentration is low, is crucial for them.

Being north seeking or down seeking, i.e. polarity, is a genetic property for magnetotactic bacteria. Almost all progenies (descendants) of a north-seeking cell are also north-seeking cells. However this requires partition of magnetesomes to each daughter cell during the division. Occasionally, daughter cells may have no magnetesomes or they maybe too small to have a magnetic moment, so they develop their own magnetesomes. When these progenies develop their own magnetesomes they may have either polarity. Therefore any natural population of magnetotactic bacteria has less than 0.5% “wrong” polarity members. Wrong is written in quotation marks because without that “mistake” magnetotactic bacteria would live only in one hemisphere of the earth.

Magnetotactic bacteria are not the only creatures that can sense geomagnetic field. For example birds, also, can sense geomagnetic field and find their direction while they are migrating. However, birds use other factors such as sun and the horizon to find their direction and their body is much more complex. Also they do not sense magnetic field in a mechanical way like magnetotactic bacteria do, but probably they have some complex chemical ways of magnetic reception. That is another research field by itself and this article’s volume is not enough to go into it.

Everything, every entity that we see around us calls us to reflect upon their Creator. The ones that we see with electron microscopes or high-tech tools are not exceptions. Magnetotactic bacteria synthesizes magnetite crystals from scratch and then puts them in an order like beads hitched on a string. Could it be possible for them to manage this incredible task on their own accord and without acting in the name of God? To this day we cannot even understand the basic principles of that process to its complete degree in order to mimic it. In fact, the more we understand the more we esteem and at awe we become from the great craft of Supreme Artist.

Auroras (northern lights), with their beautiful colors, make long winter nights more bearable in Arctic Circle. They are emitted by charged particles, something very harmful for living creatures if ever reached to earth’s surface and trapped in earth’s geomagnetic field. (Figure-4) While thinking about magnetotactic bacteria I remember those beautiful scenes I had seen in pictures that had caught my awe and wonder and can’t help but express deep gratitude once more to the Creator and Sustainer of it all, of us all, who guides the tiny little cells to more livable environments with the very same geomagnetic field that He protects us from harmful solar winds. Extraordinary is the ordinaries we are surrounded by!

Ahmet Uysal is a PhD candidate in Physics at Northwestern University, Evanston, IL.

References

Richard P. Blakemore, 1982, “Magnetotactic bacteria”. Ann. Rev. Micrbiol. 36:217-238
Dirk Schüler, 2008, “Genetics and cell biology of magnetesome formation in magnetotactic bacteria”. FEMS Microbiol. Rev., 32:654-672
Arash Komeili, 2007, “Molecular mechanisms of magnetesome formation”. Annu. Rev. Biochem. 76:351-366
Thorsten Ritz, Salih Adem, and Klaus Schulten, 2000, “A model for photoreceptor-based magnetoreception in birds”. Biophysical Journal. 78:707-718
http://www.birdgeo.com/images/CTE1810.jpg
http://solar-center.stanford.edu/images/solar-wind-magfield_b.gif