The chemistry of solutions always has to start with a discussion of the structure and properties of water. We live on a planet that is covered 2/3 by water. Water makes up the majority of our body weight (60%+). Water is the most important solvent used in chemistry for this reason. Many chemical reactions are water based (aqueous in nature) and almost all biochemical reactions are done in water since most biological systems are also water based. So what makes water so important and useful?
A water molecule (H2O), is made up of three atoms - one oxygen and two hydrogen.
Water is Polar In each water molecule, the oxygen atom attracts more than its "fair share" of electrons. The oxygen end "acts" negative. The hydrogen end "acts" positive. This causes the water to be POLAR. However, water is neutral (equal number of e- and p+) - Zero Net Charge
Hydrogen Bonds Exist Between Water Molecules Formed between a highly Electronegative atom of a polar molecule and a Hydrogen. One hydrogen bond is weak, but many hydrogen bonds are strong. Negative Oxygen end of one water molecule is attracted to the Positive Hydrogen end of another water molecule to form a HYDROGEN BOND.
What are the Properties of Water?
- Cohesion
- Adhesion
- High Specific Heat
- High Heat of Vaporization
- Less Dense as a Solid
Cohesion
Cohesion is the attraction between particles of the same substance (why water is attracted to itself). This results in a high Surface tension (a measure of the strength of water’s surface). It also produces a surface film on water that allows insects to walk on the surface of water.
Adhesion
Adhesion is the attraction between two different substances. Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and cotton. Capillary action is the process by which water molecules will "tow" each other along when in a thin glass tube. This is the process by which plants and trees remove water from the soil, and paper towels soak up water.
In summary, both cohesion and adhesion are the result of water's ability to form hydrogen bonds with itself and other molecules. This ability stems from water's high polarity and is one of the biggest reasons water has such unique properties.
High Specific Heat
The specific heat of a substance is the amount of heat needed to raise or lower 1g of the substance by oC. Water resists temperature change, both for heating and cooling. Water can absorb or release large amounts of heat energy with little change in actual temperature. At sea level, pure water boils at 100oC and freezes at oC. The boiling temperature of water decreases at higher elevations (lower atmospheric pressure). For this reason, an egg will take longer to boil at higher altitudes. The high boiling point of water (similar sized molecules are normally gases at room temperature) is also due to its ability to form hydrogen bonds.
High Heat of Vaporization
The Heat of Vaporization (ΔHvap) is the amount of energy to convert 1g or a substance from a liquid to a gas. In order for water to evaporate, hydrogen bonds must be broken. Water's heat of vaporization is 540 cal/g. In order for water to evaporate, each gram must GAIN 540 calories . This is a very high heat of vaporization for a little molecule. Because of the need for so much energy to evaporate, as water leaves the surface from which it is evaporating and removes a lot of heat with it. You feel this as a cooling effect on your skin. The earth benefits from this high heat of vaporization as well. Water vapor forms a kind of global ‘‘blanket” which helps to keep the Earth warm. Heat radiated from the sun warms the surface of the earth and is absorbed and held in by the vapor.
Less Dense as a Solid
One of the most important properties of water stemming from the hydrogen bond networks of water is its becoming less dense as it freezes. Ice is less dense as a solid than as a liquid (ice floats). Because ice floats, lakes and oceans do not freeze from the bottom up but rather the ice floats to the top where it can be melted. If oceans froze from the bottom up, with the ice sinking as it formed then they would stay frozen and life on this planet would cease to exist as we fell into a perpetual ice age.
Liquid water has hydrogen bonds that are constantly being broken and reformed. Frozen water forms a crystal-like lattice whereby molecules are set at fixed distances.
Solutions & Suspensions
Water is usually part of a mixture. There are two types of mixtures:
- Solutions
- Suspensions
Solutions
Ionic and polar compounds disperse as ions in water forming solutions. The ions spread out until they are evenly distributed. The Substance that is being dissolved is called the SOLUTE. The Substance into which the solute dissolves is called the SOLVENT.
Suspensions
Substances that don't dissolve but separate into tiny pieces and are supported in solution by water are called suspensions. Water as a function of its density keeps the pieces suspended so they don't settle out. Milk is an example of a suspension.
Aqueous Solutions
Solutions in which the solvent is water are called Aqueous Solutions. Solutions are a common medium in chemical experiments. Concentrations in solutions are expressed as the number of moles of solute in the solution per Liters of the total solution. This unit of concentration is called Molarity and is abbreviated M.
$$ \text{Molarity } = {\text{moles of solute} \over \text{liters of solution}} $$
Let's do a sample calculation: What is the molarity when 0.75 mol is dissolved in 2.50 L of solution?
$$ \text{M } = {0.75 \text{ mol} \over 2.50 \text{ L}} = 0.30 \text{ M solution} $$
One of the most common uses of solution concentration in laboratory work is the process of dilution. To dilute a solution is to make it less concentrated by the addition of more solvent. In chemistry, we don’t just add solvent randomly but in a controlled or calculated manner. The dilution of solutions is calculate in the following way:
Dilution of Solutions
If Moles of solute before dilution = moles of solute after dilution
then M x V in liters before dilution = M x V in liters after dilution
or
\begin{align} \text{M}_1\text{V}_1 & = \text{M}_2\text{V}_2 \\ \text{where} \\ \text{M}_1 & = \text{ Molarity before dilution} \\ \text{V}_1 & = \text{ Volume of solution before dilution} \\ \text{M}_2 & = \text{ Molarity of solution after dilution} \\ \text{V}_2 & = \text{ Volume of solution after dilution} \end{align}
This is easiest way to understand this process is by doing an example problem:
Suppose in lab you have a stock solution of NaCl that is 5.00M in concentration, I need a series of 10 mL total volume NaCl solutions from 4.0M to 1.0M in concentration varying by 0.5M. What amount of stock solution will I add to each 10mL flask to make my solutions?
\begin{align} \text{M}_1 & = 5.00\text{M (the concentration of the stock solution)} \\ \text{V}_1 & = \text{ ? (This is what you are trying to calculate: how much stock solution you need to dilute.)} \\ \text{M}_2 & = 4.0\text{M (This is the concentration of the solution you want to make)} \\ \text{V}_2 & = 10\text{mL (This is the volume of the solution you want to make)} \end{align}
$$ (5.0\text{M})(\text{V}_1) = (4.0\text{M})(10\text{mL}) $$
$$ \text{V}_1 = (4.0\text{M})(10\text{mL}) / (5.0\text{M}) = 8.0\text{mL} $$
So what this answer indicates is that we should take 8.0 mL of the stock (5.0M) solution and dilute it with water until we reached a total volume of 10 mL. The resulting solution would have a total molarity of 4.0M.
Try to complete the rest of the dilutions on your own and see if you get the correct answers: Gary let's make this a pop up answer box.
| M2 | V1 |
| 3.5 M | 7.0 mL |
| 3.0 M | 6.0 mL |
| 2.5 M | 5.0 mL |
| 2.0 M | 4.0 mL |
| 1.5 M | 3.0 mL |
| 1.0 M | 2.0 mL |
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