2.2 Properties of Water
Key Takeaways
Water is polar because its bent shape keeps the partial charges of its polar covalent oxygen-hydrogen bonds from canceling.
Hydrogen bonds between water molecules produce cohesion, adhesion, surface tension, capillary action, a high specific heat, a high heat of vaporization, and ice that floats because the solid is less dense than the liquid.
About 4.2 joules raises 1 gram of liquid water by 1 degree Celsius, so a large energy change causes only a small temperature change and stabilizes organisms and climates.
Evaporative cooling, as in sweating, removes heat because vaporizing water absorbs energy as hydrogen bonds between molecules are overcome.
Water dissolves polar and ionic solutes, and the hydrophobic effect clusters nonpolar molecules; the bonds inside one water molecule are polar covalent, while hydrogen bonds are between molecules.
2.2 Properties of Water
Water is a polar molecule, and the properties in this section follow from that fact. Each oxygen-hydrogen connection inside one molecule is a polar covalent bond. Oxygen pulls the shared electrons closer, so oxygen carries a partial negative charge and each hydrogen carries a partial positive charge. The molecule is bent, with a bond angle of about 104.5 degrees, so the partial charges do not cancel. Carbon dioxide is the useful contrast: its bonds are polar, but the molecule is linear, so the pulls cancel and the molecule has no lasting poles. Water's bend leaves a positive pole and a negative pole.
Hydrogen bonds are between molecules
Inside one water molecule the bond stays polar covalent. A hydrogen bond is the attraction from a hydrogen on one water molecule to the oxygen of a neighbor. Each water molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through electron pairs on oxygen. In liquid water those bonds break and re-form constantly. They are weaker than the covalent bonds inside a molecule, yet large numbers of them produce the behavior below. The usual trap swaps the labels. The internal bond is polar covalent. The attraction between molecules is the hydrogen bond.
Properties hydrogen bonds explain
| Property | What it means | Biological payoff |
|---|---|---|
| Cohesion | Water molecules stick to one another | Water columns can hold together |
| Adhesion | Water sticks to other polar materials | Water clings to cellulose in cell walls |
| Surface tension | The surface resists stretching | A water strider can stand on a pond |
| Capillary action | Water rises along a narrow tube | Movement through fine spaces |
| High specific heat | A large energy change causes a small temperature change | Body temperature and coastal climates change slowly |
| High heat of vaporization | Becoming vapor absorbs a large amount of heat | Sweat cools the skin |
| Ice floats | Solid water is less dense than liquid water | Surface ice insulates water beneath it |
Cohesion is water sticking to water. Adhesion is water sticking to a different polar material, such as cellulose. Surface tension is cohesion at the surface: molecules at the top are pulled toward their neighbors, so the surface behaves like a weak skin. Capillary action needs both adhesion and cohesion. Adhesion draws water up the wall of a thin tube, and cohesion tugs the column along. In plants, those attractions help keep xylem sap continuous while evaporation from leaves pulls the column upward. Capillary rise by itself, in a tube the width of a xylem cell, is only a short distance. It does not by itself lift water to the top of a tall tree.
Specific heat and evaporative cooling
Specific heat is the energy required to raise the temperature of a stated mass by a stated amount. For liquid water, about 4.2 joules raises 1 gram by 1 degree Celsius. Much of the added energy strains hydrogen bonds between molecules before the molecules move much faster, and faster motion is what a thermometer reads as higher temperature. A large energy change therefore causes only a small temperature change. Coastal water warms and cools more slowly than nearby land. Inside an organism, the same resistance protects cells from heat released by metabolism.
Heat of vaporization is the energy needed to convert liquid to gas. Water's value is high because molecules must overcome hydrogen bonds before they leave as vapor. The surface left behind loses that heat. The name for the result is evaporative cooling.
One gram of sweat
About 4.2 joules warms 1 gram of liquid water by 1 degree Celsius. Turning that same gram into vapor near skin temperature absorbs roughly 2,400 joules. Divide 2,400 by 4.2 and the quotient is about 570. Evaporating 1 gram absorbs about as much heat as warming that gram by about 570 degrees Celsius would, if the water could stay liquid through such a climb. The hydrogen bonds between water molecules make that energy cost large, so the skin cools. This cooling is a vaporization effect. It is separate from specific heat, which describes temperature change while water stays liquid. Water's specific heat is high, so the liquid resists temperature change. The large heat loss in sweating comes from breaking intermolecular hydrogen bonds as water becomes vapor.
Why ice floats
In liquid water, hydrogen bonds constantly break and re-form, and molecules can pack fairly close. When water freezes, hydrogen bonds lock into an open lattice, so molecules sit farther apart on average. Ice has a density of about 0.92 grams per milliliter, compared with about 1 gram per milliliter for liquid water, so the solid floats. A pond can freeze at the surface while liquid water remains below. Freezing does not break the polar covalent bonds inside each molecule. Those internal bonds stay intact in ice and in liquid water.
Solvent behavior and the hydrophobic effect
Water is a solvent for polar molecules and for ionic compounds. The partial charges on water surround ions: oxygen ends face positive ions, and hydrogen ends face negative ions. Sugars dissolve because hydroxyl groups can hydrogen-bond with water. That is why blood plasma can carry dissolved salts and glucose. Fats do not dissolve freely, because they cannot offer the same polar contacts.
Hydrophobic substances, such as oils and membrane tails, mix poorly with water. Water molecules keep hydrogen-bonding with one another and crowd nonpolar molecules into clusters. That clustering is the hydrophobic effect. It comes from water holding onto water, not from a new covalent bond inside the oil. Van der Waals contacts inside a cluster are real but weak.
Important
Inside one water molecule, oxygen and hydrogen are joined by polar covalent bonds. Hydrogen bonds are attractions between molecules. Cohesion, high specific heat, evaporative cooling, and floating ice come from those intermolecular hydrogen bonds.
Ice floats in a pond through winter. Which explanation matches the structure of solid water?
Ice floats because partial charges disappear during freezing and every solid with no partial charges is less dense than water.
Van der Waals forces inside a single water molecule expand when the molecule freezes, making each molecule larger than the liquid.
Freezing breaks the polar covalent bonds inside each water molecule and packs the atoms into a denser ionic crystal that still floats.
Hydrogen bonds hold solid water in an open lattice, so ice is less dense than liquid water.
During hard exercise, sweat evaporates and the skin cools. Why does this evaporation remove heat?
Liquid water has a low specific heat, so a tiny energy loss causes a huge temperature drop while the sweat is still liquid.
Vaporizing water absorbs a large amount of heat because hydrogen bonds between water molecules must be overcome.
Capillary action lifts liquid water into the air with no energy cost, so adhesion to air warms the surrounding skin.
Sweat cools the skin by forming new polar covalent bonds inside each water molecule, and that bond formation pulls heat out of the body.
A diagram shows two neighboring water molecules. Which labels fit the oxygen-hydrogen connection inside the first molecule and the attraction to the second molecule?
The connection inside the molecule is ionic electron transfer, and the attraction between molecules is a nonpolar covalent bond.
The connection inside the molecule is a polar covalent bond, and the attraction between molecules is a hydrogen bond.
The connection inside the molecule is a hydrogen bond, and the attraction between molecules is only a brief van der Waals force with no partial charges on water.
Both interactions are hydrogen bonds, and neither one involves unequal sharing of electrons.
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