How exactly do substances dissolve to make a solution?
As you may already know, majority of the solutions are dissolved in water. So the question is, why is water such an excellent solvent?
Water is classified as the universal solvent because of its unique structure.
The hydrogen atoms in water are bonded at an angle of 104.5°. Further, the electrons are distributed in an uneven manner. In other words, if you were to look at the molecule structure of water, the negative electrons are distributed or found mainly around the oxygen leaving a slight positive charge on the hydrogen “arms”. This allows a charge difference making the water bond called a Polar bond.
The Polar bond essentially means that there is a difference in positive and negative charge. And due to this difference, the molecule makes it easily to attract other opposite charges. For example, the slight positive charge on the hydrogen atoms of water allows it to bond with other negative charges such as chlorine atoms from NaCl in solution.
Determining if a bond is a polar bond, covalent or ionic bond.
In order to determine if a bond is a polar bond, covalent bond or an ionic bond, one must look at the electronegativity of the atom.
Electronegativity is defined as the tendency for an atom to attract or pull electrons to itself. The higher the electronegativity value, the stronger (or the higher) the ability for the atom to pull the electrons to itself. One can look at electronegativity as the ability of a person giving a strong hug. An element with high electronegativity value will have a strong hug, thus pulling electrons to itself. However, an element with low electronegativity value will have a weaker hug, thus not being able to hold electrons to it.
So, how do we determining the bond type?
At this stage, you have probably heard of the two main types of bonding, covalent and ionic bonding. These two bonds are the “extremes” of bonding in the bonding character of how electrons are shared. Covalent bond is when there is an absolute equal sharing of electrons, while an ionic bond is when there is a total transfer of electrons to the other element. Polar bond is somewhere in between.
To determine the type of bond, take the difference between the electronegativity value of the elements.
HF – Hydrogen fluoride has Hydrogen and Fluorine.
Hydrogen has electronegativity of 2.1 while fluorine has an electronegativity value of 4.0.
- Take the higher value and subtract it to the lower value
4.0 – 2.1 = 1.9
- Use the table and look at the bond type.
- HF is an ionic bond.
Table in determining if a bond is covalent, polar or ionic.
|Electronegativity difference & bond character|
|0||<0.4||0.4 – 1.7||> 1.7|
|Nonpolar Covalent||Mostly covalent||Polar covalent||Mostly Ionic|
OH – Hydroxide
Oxygen – 3.3
Hydrogen – 2.1
Subtract the larger number to the smaller value: 3.3 – 2.1 = 1.2
OH is a polar bond – meaning the electrons are NOT evenly distributed.
Dissolving oil in water? The rules in mixing solutions
Not everything can be mixed and dissolved into solution. Some substances are readily dissolved into solution such as salt into water, while other substances aren’t such as oil in water. But does that mean that oil can never be dissolved in any solution while salt can be dissolved in to every type of liquid?
The rules in mixing solutions are quite simple.
- Polar molecules will form solutions with other polar molecules.
- NaCl and water. Go check it out!
- Non-polar molecules will form solutions with other non-polar molecules.
- Oil and another form of oil.
- Describe and give examples of various types of solutions. Include: all nine possible types
- Describe the structure of water in terms of electronegativity and the polarity of its chemical bonds.
- Explain the solution process of simple ionic and covalent compounds, using visual, particulate representations and chemical equations. Include: crystal structure, dissociation, hydration
- Interpreting a a solubility curve of a pure substance in water and differentiate among saturated, unsaturated, and supersaturated solutions.
- Explain how a change in pressure affects the solubility of gases.
- Explain freezing-point depression and boiling-point elevation at the molecular level. Examples: antifreeze, road salt…
- Include: grams per litre (g/L), % weight-weight (% w/w), % weight-volume (% w/v), % volume/volume (% v/v), parts per million (ppm), parts per billion (ppb), moles per litre (mol/L) (molarity)
- Prepare a solution, given the amount of solute (in grams) and the volume of solution (in millilitres), and determine the concentration in moles/litre. (Molarity)
- Solve problems involving the dilution of solutions.
Include: dilution of stock solutions, mixing common solutions with different volumes and concentrations