How Water Works

Intermolecular forces (IMFs) are the forces of attraction between molecules (where as intramolecular forces are the bonds between atoms in a molecule.) IMFs are what keep molecules stuck together in solid or liquid form and are a big factor in melting and boiling points. The stronger the forces pulling molecules together, the more energy it takes for them to pull apart.

A polar molecule is a molecule with an uneven distribution of valence electrons. This creates positively charged areas and negatively charged areas in the molecule. A Dipole-Dipole force is when the positive area of one molecule attracts the negative area of another molecule, creating an attractive force between molecules (an intermolecular force!)

See the video below: ammonia (NH3)

The ammonia in the example above is an example of hydrogen bonds, the strongest type of intermolecular force. H-bonds are a specific kind of dipole-dipole force. A hydrogen bond is only between hydrogen atoms and either nitrogen, oxygen, or fluorine, because of the large electronegativity difference between hydrogen atoms and N, O, or F.

Since water is composed of hydrogen and oxygen, it creates hydrogen bonds. These strong attractive forces are the reason water has so many amazing properties. H-bonds are the reason ice floats and snowflakes appear as hexagons. H-bonds are what make water such a good solvent, and they are what allow this funny little lizard to do what he does:

This little guy, known as a Basilisk lizard makes use of water’s high surface tension, to dart across it. Water has high surface tension, because water molecules form very strong bonds with each other, called, you guessed it, hydrogen bonds. The molecules on the surface of liquid water actually have a stronger attraction to each other than those below the surface. The lizard is so fast and flat footed, he can get across before the surface tension created by the attractive forces breaks. Pretty cool, huh?

The way water dissolves certain substances is a bit more complicated, but no less cool. Haven’t you ever wondered at the chemistry behind your daily cup of coffee, with extra sugar please? Because you have hydrogen bonds to thank for that.

The sugar we use as sweetener is a molecular solid. Its individual sucrose molecules are held together by fairly weak IMFs. The sugar dissolves when the sucrose molecules break the bonds they have with each other and instead form intermolecular bonds with the water molecules.

Salt (NaCl) is a different matter. It’s an ionic compound, and there are actually no IMFs between ionic compounds, only covalent compounds. Ionic compounds are held together by electronegativity, but IMFs can still affect them. Ionic compounds are composed of positively and negatively charged ions, known as cations and anions respectively. NaCl dissolves in water when the Na+ cations are attracted to the negative region of the water molecule (the oxygen atoms) and the negative Cl- anions are attracted to the positive region (the hydrogen atoms). This intermolecular force is known as an Ion-Dipole force.

http://www.visionlearning.com/en/library/Chemistry/1/Water/57

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch18/soluble.php

https://www.youtube.com/watch?v=Mp1R4Lxoj5c