The pH of a solution indicates its acidity or alkalinity. \[\ce{H_2O(I) \leftrightharpoons H^+ (aq) + O^- (aq)} \nonumber\] litmus or pH paper, filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator, to test how much acid (acidity) or base (alkalinity) exists in a solution. You might have even used some to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures the concentration of hydrogen ions in a given solution. Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H+) ions and hydroxide (OH-) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H30+). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water. The concentration of hydrogen ions dissociating from pure water is 1 × 10-7 moles H+ ions per liter of water. Moles (mol) are a way to express the amount of a substance (which can be atoms, molecules, ions, etc), with one mole being equal to 6.02 x 1023 particles of the substance. Therefore, 1 mole of water is equal to 6.02 x 1023 water molecules. The pH is calculated as the negative of the base 10 logarithm of this concentration. The log10 of 1 × 10-7 is -7.0, and the negative of this number (indicated by the “p” of “pH”) yields a pH of 7.0, which is also known as neutral pH. The pH inside of human cells and blood are examples of two areas of the body where near-neutral pH is maintained. Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH number, whereas low levels of hydrogen ions result in a high pH. An acid is a substance that increases the concentration of hydrogen ions (H+) in a solution, usually by having one of its hydrogen atoms dissociate. A base provides either hydroxide ions (OH–) or other negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water molecules. The stronger the acid, the more readily it donates H+. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids. Conversely, strong bases are those substances that readily donate OH– or take up hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly alkaline and give up OH– rapidly when placed in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are able to thrive in it. The pH scale is, as previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure \(\PageIndex{7}\)). Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH of 1 to 2. So how do the cells of the stomach survive in such an acidic environment? How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of the human stomach is completely replaced every seven to ten days. 
Link to Learning Watch this video for a straightforward explanation of pH and its logarithmic scale. So how can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers readily absorb excess H+ or OH–, keeping the pH of the body carefully maintained in the narrow range required for survival. Maintaining a constant blood pH is critical to a person’s well-being. The buffer maintaining the pH of human blood involves carbonic acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, as shown in Figure \(\PageIndex{8}\), excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs. This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood’s pH. Likewise, if too much OH– is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy. Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer “heartburn” after eating. The unique properties of water that contribute to this capacity to balance pH—as well as water’s other characteristics—are essential to sustaining life on Earth.
Which of the following is not an important characteristic of water in biology? Possible Answers:
High density as a liquid, and low density as a solid
Correct answer: Low boiling point Explanation: The properties of water make it essential to life. Cohesion refers to its ability to form hydrogen bonds, attracting the molecules together and contributing to its high surface tension. Adhesion refers to water's attractive properties to other substances, and helps processes like absorption through the xylem. Solid ice is less dense than liquid water, allowing life to exist below the frozen surfaces of lakes and ponds. The polarity of water is essential for numerous biological processes and makes it a good solvent for most biological molecules. Finally, the high specific heat of water makes it resistant to temperature change, allowing life forms to maintain relatively constant internal temperatures. The high specific heat and surface tension of water contribute to its high boiling point, helping to keep it in liquid form for most biological processes.
Which property or properties of water is/are most important for water transport in plants? Possible Answers:
Low density of solid water (ice)
Correct answer: Cohesion and adhesion Explanation: Adhesion refers to the attraction between water molecules and foreign particles or surfaces. Cohesion refers to the attraction between one water molecule and another. Adhesion of water to the cell walls of the xylem vessels and cohesion of water molecules to one another allow for water transport in plants. This is also known as capillary action. The high heat capacity, low solid density, and polarity of water, as well as its use as a solvent, are all essential to the role of water in supporting life in other ways.
Which of the following properties of water are beneficial to life? I. High specific heat II. Evaporative cooling III. Ice floats in lakes IV. Dissolves many polar molecules Possible Answers:
Correct answer: I, II, III, and IV Explanation: The high specific heat of water means that it requires a large amount of heat to raise the temperature of water. This helps the temperature of the environment from becoming too hot or too cold. Also, humans are about 66% water, thus this property of water helps us regulate our body temperature too. Evaporative cooling helps prevent bodies from overheating. We have evolved to take advantage of this property of water, perspiring through our skin to cool it down during exercise. The fact that ice floats protects lakes from further cooling by cold wind. The fact that water is such a good solvent for polar molecules allows the chemistry of life to occur. Recall that life uses four main biomolecules, and dehydration synthesis and hydrolysis reactions occur constantly.
What is the best definition for osmosis? Possible Answers:
Diffusion of water through a selectively permeable membrane.
Diffusion of solute particles to a hypertonic region.
Diffusion of solute particles to a hypotonic region.
Active transport of solute particles.
Correct answer: Diffusion of water through a selectively permeable membrane. Explanation: In osmosis, water diffuses through selectively permeable membranes to regions where the concentration of solute is higher (hypertonic). Osmosis is not the movement of the solute particles.
If a cell is dropped into a solution that is hypertonic to the cell, what happens? Possible Answers:
Water diffuses out of the cell.
The cell has an equilibrium of water in and water out.
No change in diffusion occurs.
Water diffuses into the cell.
Correct answer: Water diffuses out of the cell. Explanation: A hypertonic solution has a higher concentration of solute than the solute in the cell. Water diffuses to the higher concentration of solute, so water diffuses out of the cell, and the cell shrivels up. One way to think of this is that water follows "salt," as salt is the most common biological solute.
If a cell is dropped into a solution that is hypotonic to the cell, what happens? Possible Answers:
The cell has an equilibrium of water in and water out.
Water diffuses into the cell.
No change in diffusion occurs.
Water diffuses out of the cell.
Correct answer: Water diffuses into the cell. Explanation: When the solution is hypotonic to the cell, the concentration of solute outside the cell is less. The water diffuses to the higher concentration inside the cell and causes the cell to swell with water, at it may burst.
What type of bonds are formed in the water molecule which contribute to its chemical properties in living systems? Possible Answers:
Nonpolar covalent bonds between oxygen and hydrogen
Ionic bonds between oxygen and hydrogen
Polar covalent bonds between oxygen and hydrogen
Correct answer: Polar covalent bonds between oxygen and hydrogen Explanation: The bond between oxygen and hydrogen in water molecules involves the sharing of electrons in which the oxygen atom pulls the electrons towards it more than the hydrogen pulls the electron. This results in a slight negative charge on the oxygen and a slight positive charge on the hydrogen. The bond is not ionic because electrons are not totally transferred, rather they are shared.
Which of the following would the strongest type of bond present in a sample of water? Possible Answers:
Dipole-dipole interactions
Van der Waals interactions
Correct answer: Covalent bond Explanation: The correct answer is covalent bonds because they are the strongest of all bonds present in a water sample. Although hydrogen bonding is present in water it is not the strongest bond in a sample of water. The bonds that make up the water molecule themselves are strongest. Ionic bonds do not exist in water.
What type of bonds in water allow it to engage in cohesion? Possible Answers:
Correct answer: Hydrogen bonds Explanation: Hydrogen bonds are the intermolecular forces that allow it to engage in cohesion. Ionic bonds are strong bonds within a molecule between a cation and anion. Polar covalent bonds are bonds within a molecule in which there is a slight charge on the elements. Nonpolar covalent bonds are bonds within a molecule in which there is no charge on the elements.
What is the best description of an acid? Possible Answers:
A neutral compound that mixes with oil
A compound that takes up
A compound that releases
A compound that releases
Correct answer: A compound that releases ions. Explanation: Acidic compounds and solutions release hydrogen (
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Which of the following are properties of water that are important for the survival of organisms
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