2.2: Water

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Polar Covalent Bonds

H₂O is a polar covalent molecule. The Bonds between the H atoms and the O atom arise from sharing electrons. These shared electrons form to satisfy the octet rule. However, oxygen is a “selfish” sharer. This electronegative aspect of oxygen means that the electrons of the H₂O molecule preferentially associate near the oxygen atom, creating partial charges. We indicate this by placing a δ^–near the O and δ⁺‘s near the H atoms. These partial charges make the H₂O polar.

File:Electrostatic Potential.jpg

The electron cloud around a water molecule lingers around the oxygen molecule to render it partially negative. Red illustrates the partial negative end of the molecule while blue indicates the partial positive.

Because of this polarity, H₂O molecules arrange in a highly structured way. Use the following simulation to explore polarity of molecules.

These weak associations that arise from the polar: polar attractions are referred to as Hydrogen Bonds (H-bonds). While independently weak, the summation of all the H-bonds is very strong. These associations give rise to the special properties of water: surface tension, cohesion, adhesion, and high specific heat capacity.

Hydrogen Bond Simulation

Polar materials mix with polar materials. Things that can dissolve in water are also polar and referred to as being hydrophilic (hydro = water, philic = liking). Non-polar substances do not interact or mix with polar solvents and are referred to as hydrophobic (hydro = water, phobic = hating). Since carbon and hydrogen share electrons equally, organic compounds are non-polar. Oil is a hydrocarbon that does not mix well with water or vinegar. Vinegar, however, is a polar compound that interacts with water. Detergents are called amphiphilic (amphi = both; philic = liking) because they have portions that are non-polar and portions that are very polar. Detergents can, therefore, dissolve in hydrocarbons and water. Water alone cannot effectively remove oil from your skin but a detergent can dissolve the oil and carry it away in water.

The common detergent in soap – Sodium Lauryl Sulfate

Surface Tension

Surface tension presents as an invisible film that encompasses the surface of water. The attractive forces arising from the intermolecular cohesion holds the surface of water together.

This paperclip would sink if it broke through the surface of the water.

This water strider is not on top of the water because it is light. It has not broken through the surface of the water and is, therefore, on top of the water.

Solutions

Water is an excellent solvent of other polar compounds. Table salt (NaCl) ionizes readily in water. The δ^– O associate around Na⁺ while the δ⁺ H associate with the Cl^–. If NaCl is dissolved in H₂O, what do you think happens to the intermolecular interactions between water molecules? What do you think would happen to the H-bonds? Would you expect there to be a difference in the surface tension? How do you think this explains the difference of boiling or freezing?

Running on Water

Green Basilisk in Tortuguero National Park

Basiliscus plumifrons (green basilisk)

Common Basilisk

Basiliscus basiliscus (common basilisk)

Lizards of the genus Basiliscus have the nickname “Jesus Christ lizard” for their very special adaptation regarding water.

In the face of danger, these lizards run on their hind legs across the water to escape predation. Their hind legs have long toes that help in increasing surface area to distribute their weight so they can propel themselves on the surface of the water. They do not sink because the surface tension of the water is not broken by the large surface covered by their feet. After about 4.5m, they lose sufficient momentum to propel themselves on the water surface and break through. H-bonds enable this adaptation.

Video from National Geographic

What If?

The namesake of these lizards walked on water but was also able to turn water into wine. Wine is a solution of ethanol (11%) in water. Ethanol has a polar end and a non-polar end.

How successful would the basilisk run on wine?
Why?
How could you test this without necessarily using a lizard?