365 Days of Climate Awareness 30 - Water

 

365 Days of Climate Awareness

30 - Water

(This post wound up rather longer than I planned, but...water's a pretty big deal, actually.)

Water is a molecule made of three atoms: two hydrogens and one oxygen, arranged in a roughly Mickey Mouse ears-type pattern, the larger oxygen (O) atom being the head, and the two smaller hydrogen (H) atoms being the ears. The hydrogen atoms form a 104.45-degree angle between them.

The hydrogen-oxygen bonds are technically covalent, which means the atoms share their electrons equally, but in reality, the oxygen atom pulls much more strongly on the electrons and attains a slightly negative charge, since the electrons tend to spend more time there. The hydrogen atoms, by contrast, have a slight positive charge because of the proton in the nucleus, while electrons spend comparatively little time on that end of the molecule.

This results in a hugely important property of water molecules: they are polar, with the negative

Water molecule.

charge on the oxygen and and the positive charge on the hydrogen ends. Water molecules form weak ionic-type bonds, called hydrogen bonds, with other charged molecules, such as salt ions (for example, table salt: sodium (Na+) and chlorine (Cl-) goes easily into solution). Even more importantly, water molecules also form hydrogen bonds with other water molecules, resulting in a liquid which coheres strongly to itself. This means water has an extremely high heat capacity: 1000 cal/kg-K, which means 1000 calories of heat (not the same as dietary calories) are required to heat one kilogram of water 1 degree Kelvin (Celsius). By comparison, the specific heat capacity of lead is 310 cal/kg-K. Water can absorb an awful lot of heat. This is one of the basic governing features of global weather patterns.

A solid becoming a liquid does not immediately change temperature. It first passes from the solid state to a liquid state at the same temperature. If more heat is added, only then does it resume gaining temperature. The freezing/melting point for water is 0 degrees Celsius (at one atmosphere of pressure). For ice at 0°C to become liquid water, heat must be added to break the crystal lattice and give the molecules enough energy to begin moving more freely. The specific heat of fusion of water--how much energy is needed to accomplish this--is 79.7 kcal ("kilocalories") per kilogram. When water freezes and becomes ice, it releases that same amount of heat.

One other critical feature of ice: it is less dense than water. When water molecules are frozen into their crystal lattice (there is more than one geometry of crystal ice lattice, by the way, and the different types of ice they produce all behave a little differently), the molecules spread out. The molecules of liquid water are closer together, and therefore denser. For this reason ice floats. It's why we get frozen surfaces of lakes, ponds, rivers and some of the ocean. It's why ice cubes and icebergs float (about 90% of each is below the surface, unless you use a lot of ice and they're stacked up in the glass). It's also why glaciers float on the ocean and become ice shelves.

A similar process of energy absorption takes place when water evaporates. The water at any given temperature (it does not need to be the boiling point. Puddles evaporate at air temperature) becomes water vapor at the same temperature, after having absorbed enough heat--the latent heat of vaporization, 539 cal/g water--for that liquid to vaporize. The opposite happens when vapor condenses: the molecules release that much heat as they lose energy and become liquid. This is called the latent heat of vaporization. In water it is 539 kcal/kg, the amount absorbed (released) when water evaporates (condenses).

Some asides you might find interesting:

The boiling point of a liquid is not not the temperature at which it evaporates. Water can evaporate from 0° Celsius liquid (just not quickly). (Water molecules can vaporize from solid ice too, a process called sublimation.) The boiling point is defined as the temperature at which, for a given air pressure, the substance can no longer exist as a liquid and it must become a gas. For water, at one atmosphere pressure, this is 100° Celsius.

Evaporation is how sweat cools off the body. Simply expelling the water onto the skin makes no difference to the body's temperature. It's only when the sweat absorbs heat from the skin and becomes vapor, that it acts to cool a person.

Conversely to how molecules of ice occasionally vaporize, vapor can become solid, a process called deposition. This is common, and is how frost forms on windows. It's also how, starting with a tiny droplet of water which froze around dust or pollen, snowflakes grow.

Tomorrow: water vapor and clouds.

Be well!