My notes and thoughts from Biology 111, for Monday, September 8, 2008. The entire series can be found here.
We left off on Friday discussing the second important emergent property of water, the property of temperature moderation.
On Friday, we began with the third emergent property of water that is critical to biology.
3. Solid form of water is less dense than the liquid form
In other words, ice floats. First we took a quick look at what generally defines each state of matter at room temperatures (we didn’t delve into plasmas etc)
|States of Matter|
|Constant Shape, Constant Volume||Constant Volume, Changing Shape||Changing Volume, Changing Shape|
So we can say that generally speaking, the state of matter is dependent on its density and the fixity of its bonding. Ordinarily, the solid state of matter is more dense than the liquid state, and this unusual property of water has a very important consequence for life.
(More of this lecture, below the fold)
Water is its most dense at 4°.
Because ice floats, the top of a body of water will freeze first in the winter. Because of the moderation of temperature, water (in this case, ice) is a good insulator from heat. What this means is that the floating ice on a body of water hinders the escape of heat from below the ice, and the ice then acts as a thermal blanket, preventing the underlying water from freezing solid. This gives life below the ice a place to continue living, even in the coldest of arctic winters.
4. Solvent Ability
Before we can discuss water’s ability to act as a solvent, we need to define some terms.
Solution – A homogeneous mixture of 2 or more different substances
Solvent -> The dissolving agent.
Solute -> The substance that dissolves
So when you salt a pot of water on the stove, the water is the solvent, the salt the solute, and the mixture is a solution.
Water has the ability to dissolve more different substances and in greater quantity than any other solvent. (It’s not universal however, or it would dissolve the container that holds it, etc.)
The difference in salinity between the body and the seawater is why salt water tastes so salty to us, despite the fact that the salt concentration is only 3.5%.
Salt is an ionic compound, and water dissolves it because of the charges of each of the ions involved. Water’s polarity is attracted by its slightly charged poles to the charges of the ions. Because the Sodium is positively charged (a cation), the slight negativity at the Oxygen end of a water molecule is attracted to the Sodium. Conversely, the Chlorine ion is negatively charged (an anion), so the slightly positively charged hydrogen end of the water molecule is attracted to the Chlorine. The water sort of forces the salt ions apart through hydrogen bonding, and so the Sodium is separated from the Chlorine.
The solubility of sugars (for instance) works a little differently, and we cover that a bit later on.
Polarity and electrical charge are the keys to water solubility, and water’s polarity and hydrogen bonding are what make it such an excellent solvent.
Oils on the other hand, are non polar molecules. That’s why oil will separate from water.
Oil doesn’t repel water so much as it just doesn’t dissolve in water.
We say that Oils are hydrophobic.
Hydrophilic -> Attractive to water (NOT interchangeable with water soluble, though! –> Cotton is hydrophilic but not water soluble. Cotton and starch are entirely made of cellulose (a sugar). It’s components are water soluble. So we can say that being non-water-soluble is an emergent property of cellulose.
Hydrophobic -> Not attractive to water.
At this point we moved on to pH.pH —> A measure of the concentration of Hydrogen ions in a solution.
H+ is a Hydrogen atom that has lost an e–, and what remains is just a p+
We denote H+ concentration thus: [H+] (where the brackets mean “concentration of” whatever is between them, in this case Hydrogen ions).
Some notes about the scale:
The pH scale is a negative logarithmic scale, meaning that each increase of 1 unit on the scale is a decrease of H+ concentration by a factor of 10. So a solution with a pH of 8 has ten times less [H+] relative to [OH–] than distilled water. A solution with a pH of 9 has ten times less [H+] relative to [OH–] than the previous solution, and 100 times less than distilled water.
A small change in the pH number can mean a big change in the balance of [H+] to [OH–]. At 7, [H+] = [OH–], and the solution is said to be neutral. I find it helpful to think of the pH scale as a balance type of scale. When a solution is neutral, it balances evenly between the [H+] and the [OH–].
Thinking about it, this makes sense, as ideally all the H+ and OH– in water should be joined together as H2O.
Also note that the body needs to be regulated relatively closely. Normally, the pH of human blood is between 7.35 and 7.45. A few tenths of a unit of pH one way or the other is fatal. Too many H+ or OH– floating around would make for a very bad day.
One last note: [H+] in distilled water is 10-7 mols/L. This is where the 7 comes from in the pH of water, and the scale is based on that.
1 mole = 6.02 x 1023 This number is known as Avogadro’s number, and is a handy little number to keep in your back pocket.
The lecture ended there, and we picked it up on Wednesday.
From whence came the art:
The first image is of our textbook, Biology, Eighth Edition, by Campbell & Reese et al.
Other images by me and are licensed under the Creative Commons Attribution- NonCommercial- Share Alike 3.0 License.