This section of Healing Earth helps you to begin answering the following set of questions that were posed at the end of the case study:

Closer Look

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The USGS Water Science School is an interactive website all about water and a good resource for investigating in more depth the topics presented in this section.

  • what are the properties of water that make it essential for life on Earth?
  • what is the current condition of the planet’s water resources?
  • what are the major human impacts on water quality and quantity in the world today?

Water is so common on our planet that its extraordinary properties are often overlooked. It comprises nearly 70% of the mass of the human body and up to 95% of plant body mass. Water helps make the Earth habitable for organisms and has an essential role in physiological processes. These incredible features of water all begin with the molecule.

The Structure of Water

water molecule
Figure 1: Water Molecule. The water molecule is comprised of two hydrogen (H) atoms and one oxygen atom. The oxygen atom has 8 electrons, and each H has 1 electron. The H atoms bond to the oxygen by sharing a pair of electrons in what is called a covalent bond. In each pair of shared electrons, one electron is contributed by the H (black) and one from the oxygen (red).1

Water is a molecule (H2O) that contains two hydrogen atoms each sharing a pair of electrons with an oxygen atom (see Figure 1). When atoms share electrons in this way, a covalent bond is created. These bonds are essential to living organisms.

In water molecules, oxygen and hydrogen atoms share electrons unequally. Electrons, which always carry a negative charge, are drawn more strongly to oxygen atoms. Because the shared electrons spend more time circling the oxygen nucleus and less time circling the nuclei of the hydrogen atoms, the water molecule becomes polarized with distinct negative (oxygen) and positive (hydrogen) ends. This property is referred to as molecular polarity.

Since positive and negative charges attract each other, polar water molecules align when they approach each other—the positive hydrogen end of one molecule is attracted to the negative oxygen end of a second molecule. This attraction is called a hydrogen bond (see Figure 2). Water molecules are loosely bound together by weak hydrogen bonds, which gives water its liquid property. If hydrogen bonds were stronger, water would be a rigid, rather than a fluid substance.

Closer Look

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This paper published at the E.R. Johnson Research Foundation, University of Pennsylvania presents a more in depth description of water structure and properties. 

hydrogen bonds
Figure 2: Hydrogen Bonding. There are 5 water molecules, each consisting of one oxygen atom (red ball) and two hydrogen atoms (white balls). The bonds holding the H2O molecules together are weak hydrogen bonds. These weak bonds give water its fluid structure. 2

A drop of water contains trillions of water molecules held together by weak hydrogen bonds. The tendency of water molecules to bond is called cohesion. The fact that water molecules cohere using weak hydrogen bonds is what gives water its fluid consistency.

Water’s polarity also makes it adhesive, which means that it will be attracted to other types of molecules with positive and negative charges. Think of how trees transport water from the soil up into their high branches without any kind of pump. Water molecules enter the root and adhere to the molecules that make up the walls of the plant’s conductive tissues, called the xylem cells, which are shaped like tiny drinking straws. Water evaporates into the air at the opposite end of the xylem—the leaf—causing an upward, cohesive pull on the entire column of water and replacing the water that was vaporized.

Looking Ahead

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Later in this chapter, you will learn beautifully flowing, life-giving fluidity of water makes possible what many religions of the world see as the spiritual significance of water.

Cohesion and adhesion are some of water’s most remarkable qualities. They are strong enough to work against the force of gravity, allowing water to move to the top of a tree hundreds of feet high. This movement is called capillary action and is a key way plants transport fluids through their bodies.

Water Solvency

A solvent is a substance that is able to dissolve other substances. Water is a universal solvent due to its ability to dissolve most all other polar substances. Have you ever added table salt to water before boiling pasta or rice? If so, you probably noticed that the salt eventually seemed to ‘disappear’.

Looking Ahead

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In the Water and Spirituality section below you will learn that the solvency of water makes possible what many religions of the world see as the spiritual significance of water in its purifying and healing quality.

sodium dissolved in water
Figure 3: Sodium dissolved in water. The positively charged sodium ion (Na+) in table salt is attracted to the negatively charged oxygen-end of H2O molecules more than it is attracted to the negatively charged chloride ion (Cl-) of table salt. Therefore, NaCl is easily dissolved into water. Here you see how the oxygens of 6 water molecules are attracted to the sodium, and how the sodium is no longer bonded to its chloride. 3

 Table salt (NaCl) is made of ions that, because of their charges, readily separate and dissolve into water. Salt dissolves in water because the water molecules form a more favorable attraction to the salt ions than the salt ions’ attraction to one another (see Figure 3). Molecules or parts of molecules that are attracted to water molecules are called hydrophilic (i.e., “water loving”).

Many nutrients that support life are hydrophilic. Because water is a universal solvent, it is an essential medium for transporting vital nutrients. Water can also transport many substances harmful to life, such as pesticides. Water’s solvency makes it a useful cleaning agent, but it also makes possible the contamination of our essential water supply.

Water’s Three Physical States

Water is the only substance on Earth that naturally occurs in three physical states: solid, liquid, and gas (see Figure 4). Depending on temperature and atmospheric pressure, water can change from one state to another, a process called physical phase change. Because of this, some geographical regions of the world experience humidity, rain, snow, or even a combination of all three.

phase change
Figure 4: Phase Changes of Water. This figure demonstrates how the addition of heat can convert solid ice to liquid water through the process of melting, and with additional heat, the liquid water can be converted into gas (water vapor). Likewise, cooling converts liquid water to solid ice through the process of freezing. In general, moving up the diagram from bottom to top indicates adding heat/energy, while moving from top to bottom the diagram indicates releasing heat/energy. Moving between gas and solid directly takes more energy than moving from solid to liquid to gas. 4

Closer Look

closer_look_video

Water molecules in water vapor have few hydrogen bonds and more space between them, making vapor light and less dense than water or ice. While the H₂O molecules are closer together in liquid water than in solid ice, there are fewer hydrogen bonds in liquid water than in the rigid lattice of ice. Therefore, water is fluid whereas ice is solid. This video demonstrates the three states of water molecules.

Hydrogen bonds are again the key. The number of bonds between molecules determines whether water will be a solid, liquid, or gas. In the solid state, water molecules have the maximum number of hydrogen bonds (4 per molecule), giving water the rigid characteristic of ice. In its liquid state, water has fewer hydrogen bonds, which accounts for its less-structured, fluid character.

As water changes from solid to liquid to gas, hydrogen bonds are broken, giving water molecules more freedom of movement. This cycling of water through its states is the basis for the hydrologic cycle that is essential for life on earth, purifying water and distributing it across land masses. This important cycle is discussed in the hydrologic cycle section below.

Water Density

Water is one of the few substances on Earth that is less dense as a solid than a liquid. As ice, water molecules form four hydrogen bonds that lock them into a rigid crystalline structure. In this state, the water molecules are actually further apart than when they are in a liquid state. This is why water expands as it freezes and is less dense than the surrounding liquid water. Because solid water is less dense, ice floats on the surface of a lake in winter and insulates the water below from freezing, providing a vital benefit to aquatic organisms. If water in its solid form was denser than water in its liquid form, lakes and ponds would freeze solid to the bottom during winter, and no longer provide viable habitats.

As it is, fish and other species in polar and temperate regions have evolved to live in lakes when outdoor temperatures fall below the freezing point of water (32ºF, 0ºC). These aquatic organisms possess a relatively wide range of tolerance for temperature changes throughout the year. However, the range of tolerance that species have adapted to is being threatened by temperature shifts induced by global climate change. More will be said below and in Chapter 6 about the effects of global climate change on water.