Properties of Water and the Hydrologic Cycle

Water's Unique Properties and the Hydrologic Cycle

Water's Polarity

• Almost all unique properties of water stem from the angle between its hydrogens and oxygen atoms.

• This bent structure causes electrons (which have a negative charge) to cluster more around the oxygen than the hydrogen atoms.

• Result: A slight negative charge around the oxygen and a slight positive charge around the hydrogens.

• This uneven distribution of charge is called polarity.

• Consequence: Water is a polar molecule, making it want to stick to itself and giving it various interesting properties.

States of Water and Phase Changes

• Water on Earth generally exists in three states:

  • Liquid: Between $0^ ext{o} ext{C}$ and $100^ ext{o} ext{C}$.

  • Solid (ice): Below $0^ ext{o} ext{C}$.

  • Gas (water vapor): Above $100^ ext{o} ext{C}$.

• A fourth state, plasma, exists but is not relevant to this discussion.

• Changes of State (Phase Transitions):

  • Freezing: Liquid to solid.

  • Melting: Solid to liquid.

  • Boiling / Evaporation: Liquid to gas (evaporation is the process that produces vapor).

  • Condensation: Gas to liquid (e.g., frost on windows on a cold morning).

  • Sublimation: Solid to gas directly (e.g., ice cubes shrinking in a freezer over time).

  • Desublimation: Gas to solid directly (e.g., frost forming on a windshield or grass when water vapor freezes on a surface colder than the air; this is distinct from freezing rain).

Water as a Solvent

• Water is an excellent solvent, particularly for polar and ionic molecules.

  • Mechanism: Due to its polarity, water can surround charged ions. For example, in salt ($NaCl$), the positive hydrogen ends of water molecules surround a negative chloride ion ($Cl^-$), and the negative oxygen end surrounds a positive sodium ion ($Na^+$), effectively pulling them apart and keeping them separate.

  • Example: Salt (an ionic molecule) dissolves well in water.

• Water is not a good solvent for fats (non-polar molecules).

  • Example: Cooking oil floating on water.

  • Biological Importance: This property is crucial for life, as cell membranes are made of fatty layers (lipids) that water doesn't dissolve, allowing cells to maintain their internal environment and regulate what enters and exits.

Specific Heat of Water

• Definition: Specific heat is the amount of energy required to raise the temperature of $1$ gram of a substance by $1^ ext{o} ext{C}$.

• Water has a very high specific heat; it is about $10$ times higher than copper.

  • This means it takes significantly more energy to change the temperature of water compared to many other substances.

• Implications for Climate and Environment:

  • Water bodies (oceans, lakes) change temperature very slowly.

  • Moderating Effect: Coastal areas experience milder climates due to the moderating effect of large water bodies. For example, beaches in summer are cooler because the ocean water, even in July, remains relatively cool.

  • Sea Breezes: Land heats up faster than water during the day, causing air above the land to rise. This draws cooler air from over the ocean towards the land, creating a pleasant sea breeze. At night, the land cools faster than the water, and the pattern reverses, with breezes blowing from land to sea.

Latent Heat and Energy Transfer

• Latent Heat: The energy required or released when a substance changes from one state to another (e.g., solid to liquid, liquid to gas) without a change in temperature.

• Types of Reactions in Terms of Energy:

  • Endothermic Reactions: Take energy (heat) out of the environment, thereby cooling it.

    • Example 1: Evaporation (e.g., sweat): When sweat evaporates from the skin, it absorbs latent heat from the body, cooling the skin.

    • Example 2: Swamp Air Conditioner: Pulling air through a wet towel causes water to evaporate, absorbing heat and blowing cooler air. This is often used in greenhouses to cool plants in summer.

  • Exothermic Reactions: Give off heat, thereby warming the environment.

    • Example: Freezing: When water freezes, it releases latent heat. This phenomenon can be used to prevent other things from freezing. For instance, putting a barrel of water in a root cellar can keep produce from freezing; as the water begins to freeze, it releases heat that helps keep the cellar above freezing temperatures.

Density of Water and Ice Flotation

• Cold water sinks: Generally, colder water is denser and sinks. This property is a major driver of global oceanic currents.

• Ice floats: Unlike most substances, water becomes less dense as it freezes.

  • Why this is crucial for life: If ice sank, lakes, rivers, and oceans would freeze from the bottom up, likely preventing the existence of life as we know it.

  • Cold water at the bottom not freezing: While cold water sinks, it is densest at about $4^ ext{o} ext{C}$. Below this temperature, as it approaches $0^ ext{o} ext{C}$ and freezes, it becomes less dense and floats to the surface. Therefore, the very bottom of bodies of water might not freeze, or if it does, the ice would float up.

  • Salinity's role: In oceans, salinity also plays a significant role in water density and circulation, alongside temperature.

The Hydrologic (Water) Cycle

• The water cycle describes the continuous movement of water on, above, and below the surface of the Earth.

• Key Principle: There is a fixed amount of water on the planet; it merely changes states and locations (reservoirs).

• Major Reservoirs (Storage Areas):

  • Oceans: The largest reservoir, containing vast amounts of saltwater.

  • Ice Caps and Glaciers: Significant reservoirs of freshwater (Earth is currently in a glacial period with ice at both poles).

  • Groundwater: Water stored in aquifers beneath the Earth's surface (freshwater).

  • Atmosphere: Water vapor, clouds.

  • Freshwater Lakes and Rivers.

  • Soil Moisture.

  • Living Organisms: Water stored within plants and animals.

  • Other: Less significant reservoirs include steam from volcanoes and geothermal vents.

• Major Fluxes (Water Movement):

  • Evaporation: Water changing from liquid to gas, primarily from oceans and land surfaces.

  • Transpiration: Water evaporated from plants' leaves.

  • Evapotranspiration: The combined process of evaporation from land surfaces and transpiration from plants.

  • Condensation: Water vapor in the atmosphere changing back into liquid droplets or ice crystals, forming clouds.

  • Precipitation: Water falling from the atmosphere to the Earth's surface (rain, snow, hail, sleet).

  • Surface Runoff: Water flowing over the land surface into streams, rivers, lakes, and eventually oceans.

  • Infiltration: Water seeping into the ground to become groundwater.

  • Rivers and Streams: Acts as both reservoirs and fluxes, constantly moving water across landscapes.

• Creation of Freshwater: The primary mechanism for creating freshwater is evaporation from the ocean. When saltwater evaporates, it leaves the salts behind, producing pure water vapor that eventually precipitates as freshwater on land. Evapotranspiration on land also involves freshwater.

• The water cycle is essentially a closed loop in terms of fluxes, ensuring water is continually recycled throughout the planet.