# Chapter 2 Water

## 2.1 The reason for everything

1. polar covalent bonds between O and H create an asymmetric distribution of electrons in a water molecule – with two excess electrons on the O side of the molecule and two deficient electrons on the hydroden side.
2. This asymmetry gives water molecules the property of 1) cohesion – the attraction of water molecules to other water molecules and 2) adhesion – the attraction of water molecules to substances that are charged or have charge asymmetries including ions, polarized molecules, and charged surfaces such as glass.

## 2.2 Consequences

### 2.2.5 Humans regulate body temperature using evaporative cooling

The temperature of a system is the average kinetic energy of the particles in the system. Remember that $$KE = \frac{1}{2}MV^2$$, where $$M$$ is mass and $$V$$ is speed. Water is liquid because the $$KE$$ of the water molecules is small enough that the molecules arethey are able to interact and form hydrogen bonds. This keeps the water molecules relatively close to each other. By contrast water vapor (water molecules in a gas phase) have too much kinetic energy to interact and form hydrogen bonds. In air, water molecules are far apart. Because water molecules in the liquid phase are highly attracted to each other, water has a high heat of vaporization, which means it takes a relatively large amount of energy to transition the water from a liquid to gas phase (“Heat” is a form of energy not a measure of temperature).

If heat energy is transferred into water, the average $$KE$$ (and therefore, temperature) increases. With enough heat transfer, the fastest water molecules will have enough $$KE$$ that they cannot interact with other water molecules and they evaporate from the surface as water vapor – that is, they have transitioned from a liquid to a gas phase. Because the water molecules that evaporate are the molecules with the highest $$KE$$, the average $$KE$$ of the remaining water is lowered. The opposite occurs as heat is transferred out of moist air. The average $$KE$$ of the water molecules decreases and water molecules are, on average, closer together (the relative humidity increases). If enough heat is transferred out, the water moleculees with the smallest $$KE$$ can interact with each other forming hydrogen bonds and condensing into liquid drops. The result is rain, clouds, and fog. If a cold surface cools the air at it’s surface, the result is condensation.

Humans and other mammals generate a tremendous amount of heat from the many chemical reactions that occur in our cells. This heat maintains our body temperature well above that of our surrounding environment. When skeletal muscles are working at a high rate (high power), they generate excess heat, which increases the average $$KE$$ of the water (and other) molecules in the body. In humans, sweat glands are stimulated to secrete water onto the surface. The water molecules in the sweat with the highest $$KE$$ evaporate as long as the air is not saturated with water vapor. This evaporation lowers the average $$KE$$ (and thus temperature) of the remaining water in the body. And because water’s high heat of vaporization, this evaporation is a very effective mechanism for cooling. Sweat glands are a highly unusual, but very effective, method of cooling in mammals.

Sweating only cools if water can evaporate from the surface – sweating itself doesn’t cool the body. This means that as air becomes more saturated with water vapor (that is, increased relative humdity), water is less likely to evaporate because the there is less and less more “room” in the air for water molecules. High relative humdity is not just a function of the east coast of the U.S. in the summer. Evaporation from a human running on a treadmill in a closed room with poor ventillation and air circulation can rapidly increase the relative humidity of the air surrounding their body and decrease the effectiveness of evaporative cooling.

### 2.2.6 Water has high surface tension

Consider a sheet of rubber. If I pull on a rubber sheet, I transfer mechanical energy into sheet and stretch the material (literally pulling atoms further apart). The stretched atomic structure of the rubber material resists being stretched – the interatomic forces pull back. This internal resistance is stored elastic strain energy. When I let go of the rubber sheet, this stored elastic strain energy is the source of the work to pull the stretched sheet back to its starting size. The stored elastic strain energy can also be transferred to kinetic energy, say to shoot a ball across a room.

Because of the attraction of water molecules to each other, a water surface (at a water-air, water-oil, or water-wax interface) effectively acts like a rubber sheet – that is it takes energy to stretch the surface and a stretched surface of water stores elastic strain energy. The energy required to stretch a surface of water is the surface tension. Becuase of the extreme attraction of water molecules to each other, water has high surface tension.

Unless opposed by a larger force, water molecules spontaneously re-arrange to maximize water-water interactions and minimize surface area. Because of water’s high surface tension, it takes a large force (or high work) to keep water from minimizing surface area at any air-water, air-oil, or air-way interface. Some consequences of this are

1. Rain drops are spherical. A sphere has the distinction of being the 3D geometry with the smallest surface to volume ratio (an infinite number of objects of different shape can have the same volume. Of these infinite objects, the sphere has the smallest surface area). And, water spread onto a wax surface will “bead up” into spheres.

2. small animals can walk on water. When a small animal (generally insects but also small vertebrates such as basilisk lizards) steps onto the surface of water, the animal’s weight pushes down on the surface, transferring energy (work) into the surface, and stretches it, forming a dimple on the surface. The stretched surface resists being stretched and pulls back, which creates an upward force that can balance the weight of the animal if the animal is small enough. A bigger animal tranfers enough mechanical energy in the surface to stretch the water enough to break it, but if the animal can move the foot out of the dimple before the surface breaks, then it can walk or run on water. Big animals transfer too much mechanical energy into the stretched surface to walk or run on water – they simply cannot move the feet out of the dimple fast enough (before the surface breaks). One exception is a human water skiing on bare feet (or on water skis, which is cheating).

3. Emulsification by bile acids. The stomach churns food and this mechanical energy breaks lipids into many, many small droplets, which has much more surface area, relative to the volume of lipid, than if the small droplets all coalesced into a single lipid ball. In other words, the presence of many small lipid droplets in water requires a high input of mechanical energy to maintain the high water surface area. The stomach empties its contents into the small intestine which is the sight of lipid digestion and the lipid digesting enzymes can only bind to lipids at the surface of a lipid drop. Because many drops have more surface area than a single lipid ball, lipid digestion is faster if the lipid content is maintained as a bunch of drops. But there is no churning in the intestine and no more mechanical energy to maintain the lipid as drops. Instead of churning the contents to maintain the drops, the liver and gall bladder secrete bile salts into the small intestine. The bile salts contain amphipathic molecules that act as surfactants, which lower the surface tension of the water and reducing the energy required to maintain the lipids as many drops.

4. Lung alveoli function. Air moves in and out of the lungs via respiratory tubes that form a respiratory tree with small, balloon shaped structures called alvoli (sing. alveolus) at the tips. Gas exchange occurs across the wall of the alveolus. A watery layer occurs between the air and the plasma membranes of the alveolar cells. Because the alveolus contains a pocket of air, the watery layer has a stretched water surface (water-air interface). The surface tension of the watery layer is pulling against this stretching, which, if unbalanced by an opposing force, would have the effect of collapsing the thin layer of water into a small drop of water, which would collapse the alveolus itself. The opposing force is the relatively low fluid pressure in the closed cavity surrounding the lung (the pleural cavity), which pulls the lung out in all directions. Because of the extreme attraction of water molecule’s to each other, giving water the property of high surface tension, this outward force would not be high enough to resist the inward, collapsing force by the water surface if the water layer were pure water. But the outward force is high enough because specialized cells secrete surfactants into the water layer, which lowers its surface tension.