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2.3 Water

Do you ever wonder why scientists spend time looking for water on other planets? Water is essential to life; even minute traces of it on another planet can indicate that life could or did exist on that planet. Water is one of the more abundant molecules in living cells and the one most critical to life as we know it. Approximately 60 to 70 percent of your body is made up of water. Without it, life would not exist.

Structure of Water

The hydrogen and oxygen atoms within water molecules form polar covalent bonds. The shared electrons spend more time associated with the oxygen atom than the hydrogen atoms. There is no overall charge to a water molecule, but there is a slight positive charge (δ+) on each hydrogen atom and a slight negative charge (δ-) on the oxygen atom. In a solution of pure water, the partial negative charge on the oxygen is attracted to the partial positive charges on other water molecules, forming hydrogen bonds between water molecules. The hydrogen bonds between water molecules are responsible for all the properties of water.

 

Polar covalent bonds shown in planetary model, 3D model, & structural formula
Figure 2.3.1 Polar Covalent Bonds in a Water Molecule. Image by OpenStax, CC BY 4.0

Properties of Water

The properties of water include:

Water is Cohesive
Water Stabilizes Temperature
Water Expands Upon Freezing
Water is a Versatile Solvent

Exercise 2.3.1

Text Description
Match each property of water with the correct description.
  1. Water is _____ because the oxygen atom is partially negatively charged, and the hydrogen atoms are partially positively charged.
  2. _____ substances dissolve easily in water, while _____ substances do not.
  3. Water stabilizes _____ because it can absorb a lot of energy before it changes state (i.e., from liquid to gas).
  4. Water is an excellent _____ because it can dissolve polar and ionic compounds.
  5. _____ is caused by hydrogen bonds holding water molecules together. This also causes _____, which allows objects to float on the surface of the water, held up by all the hydrogen bonds.
  6. _____ is the attraction between water molecules and other molecules.
Possible answers:
  • hydrophobic
  • Cohesion
  • polar
  • Hydrophilic
  • Adhesion
  • surface tension
  • solvent
  • temperature

Answers:

  1. Water is polar because the oxygen atom is partially negatively charged, and the hydrogen atoms are partially positively charged.
  2. Hydrophilic substances dissolve easily in water, while hydrophobic substances do not.
  3. Water stabilizes temperature because it can absorb a lot of energy before it changes state (i.e., from liquid to gas).
  4. Water is an excellent solvent because it can dissolve polar and ionic compounds.
  5. Cohesion is caused by hydrogen bonds holding water molecules together. This also causes surface tension, which allows objects to float on the surface of the water, held up by all the hydrogen bonds.
  6. Adhesion is the attraction between water molecules and other molecules.

Buffers, pH, Acids, and Bases

The pH of a solution is a measure of its acidity or alkalinity. You have probably used litmus paper, treated with a natural water-soluble dye as a pH indicator, to test how much acid or base (alkalinity) exists in a solution. You might have even used some to ensure the water in an outdoor swimming pool is treated correctly. This pH test measures the number of hydrogen ions in a given solution in both cases. High concentrations of hydrogen ions yield a low pH, whereas low levels of hydrogen ions result in a high pH. The overall concentration of hydrogen ions is inversely related to its pH and can be measured on the pH scale (Figure 2.3.5). Therefore, the more hydrogen ions present, the lower the pH; conversely, the fewer hydrogen ions, the higher the pH.

The pH scale ranges from 0 to 14. A change of one unit on the pH scale represents a change in the concentration of hydrogen ions by a factor of 10, and a change in two units represents a change in the concentration of hydrogen ions by a factor of 100. Thus, small changes in pH represent significant changes in the concentrations of hydrogen ions. Pure water is neutral. It is neither acidic nor basic and has a pH of 7.0. Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (7.1 to 14.0) is alkaline. The blood in your veins is slightly alkaline (pH = 7.4). The environment in your stomach is highly acidic (pH = 1 to 2). Orange juice is mildly acidic (pH = approximately 3.5), whereas baking soda is basic (pH = 9.0).

pH scale with examples. See Figure 2.3.5 Description
Figure 2.3.5 The pH scale measures the amount of hydrogen ions (H+) in a substance. Modification of work by Edward Stevens, CC BY 4.0
Figure 2.3.5 Description
  • Acidic examples below 7.0 include gastric acid (1), lemon juice (2), orange juice (3), tomato juice (4), black coffee (5), urine (6)
  • Neutral example includes distilled water (7)
  • Alkaline examples above 7.0 include seawater (8), baking soda (9), Milk of magnesia (10), ammonia solution (11), soapy water (12), bleach (13)

Acids are substances that provide hydrogen ions (H+) and lower pH, whereas bases provide hydroxide ions (OH–) and raise pH. The stronger the acid, the more readily it donates H+. For example, hydrochloric acid and lemon juice are very acidic and readily give up H+ when added to water. Conversely, bases are those substances that readily donate OH–. The OH– ions combine with H+ to produce water, which raises a substance’s pH. Sodium hydroxide and many household cleaners are very alkaline and give up OH– rapidly when placed in water, increasing the pH.

Most cells in our bodies operate within a very narrow window of the pH scale, typically ranging only from 7.2 to 7.6. If the body’s pH is outside of this range, the respiratory system malfunctions, as do other organs. Cells no longer function properly, and proteins break down. Deviation outside of the pH range can induce coma or even cause death.

So, how is it that we can ingest or inhale acidic or basic substances and not die? Buffers are the key. Buffers readily absorb excess H+ or OH–, keeping the body’s pH in the narrow range. Carbon dioxide is part of a prominent buffer system in the human body; it keeps the pH within the proper range. This buffer system involves carbonic acid (H2CO3) and bicarbonate (HCO3–) anion. If too much H+ enters the body, bicarbonate will combine with the H+ to create carbonic acid and limit the decrease in pH. Likewise, if too much OH– is introduced into the system, carbonic acid will rapidly dissociate into bicarbonate and H+ ions. The H+ ions can combine with the OH– ions, limiting the increase in pH. While carbonic acid is an essential product in this reaction, its presence is fleeting because it is released from the body as carbon dioxide gas each time we breathe. Without this buffer system, the pH in our bodies would fluctuate too much, and we would fail to survive.

Exercise 2.3.2

Drag each item to its corresponding substance type.

Text Description

Substances that make up a pH solution: Acid, Base, and Neutral

Draggable items: pH equal to 7, pH greater than 7, pH less than 7, Water, Bleach, Orange juice, Alkaline, Readily donates H+, Readily donates OH-, High concentrations of hydrogen ions, Low concentrations of hydrogen ions


Answers:

  • Acid: pH less than 7; Orange juice, High concentrations of hydrogen ions, Readily donates H+
  • Base: pH greater than 7; Alkaline, Bleach, Low concentrations of hydrogen ions, Readily donates OH-
  • Neutral: pH equal to 7; Water

 

Text Description
Select all of the statements that are true about buffers.
  1. Are a strong acid or a strong base.
  2. Help maintain homeostasis.
  3. Absorb excess H+ or OH-
  4. Are uncommon in biological systems.

Correct answer(s): b. Help maintain homeostasis.; c. Absorb excess H+ or OH-

2.2 Water” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Water molecules exhibit cohesion” from  Chemistry for Biology 1190 Students Copyright © by Julia Wong is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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