Water and pH

The importance of water to life, and therefore biology, cannot be understated. It covers over 70% of the Earth and is the most abundant compound in living things. All living things on Earth depend upon water to survive. Water is required for many essential reactions within cells, such as cell respiration and photosynthesis, which we will discuss in a later module.

Water is a simple but unique molecule that is tasteless, odorless, and transparent. Its chemical formula is H₂O. It has hydrogen atoms that are covalently bonded to an oxygen atom. What makes water unique, and so important for life, are the interesting characteristics, or properties, that water displays as a result of its structure. First, we will discuss the structure of the water molecule, and then its unique properties.

The Water Molecule

Water is a neutral molecule, meaning that it has the same number of protons as electrons. Look at the images that shows a water molecule above. The red represents oxygen and the white hydrogen.

Even though water is neutral, its electrons are unequally distributed among the oxygen and hydrogens that make it up. The oxygen atom, with its eight positively charged protons, has a strong pull on the negatively charged electrons; this makes the probability of finding those electrons near the oxygen greater than finding them near the hydrogen atoms.

Look at the picture below. You can see that the oxygen end of the water molecule takes on a slightly negative charge and the hydrogen ends of the water molecule take on slightly positive charges. Water is therefore a polar molecule.

cellwater2

Properties of Water

Have you ever wondered why a water bug can walk across the water, but a frog cannot? It’s an interesting question that relates directly to properties of water! Many of the properties of water are related to each other. All of the properties we will discuss are important to life on Earth.

Polarity

Remember how a water molecule has an unequal distribution of charges? We call this phenomenon polarity because the molecule has a positive pole on one end and negative pole on the other. Water molecules’ positive and negative charges cause opposite ends of water molecules to attract each other like tiny magnets. Polarity of water is important to life on Earth in a number of ways, which are explained below.

Hydrogen Bonding

Hydrogen Bonding is a unique feature of water that is made possible by polarity. Hydrogen bonds are weak, attractive bonds that exist between individual water molecules. Hydrogen bonds are easily broken and re-formed. This picture shows water forming hydrogen bonds with itself. Hydrogen bonding of water molecules allows for several unique chemical properties important for life:

cellwater3

Water is called the “Universal Solvent”

Water’s polarity also makes it a very good solvent. This is biologically helpful because it means that water can transport or hold onto dissolved substances for organisms (salt, food).

Because water is polar itself, when it comes into contact with other polar or ionic substances, it is able to fit in between the atoms that make up that substance, dissolving it. In other words, these substances can mix. Salt or rubbing alcohol will dissolve in water and are therefore called hydrophilic, or “water loving.” Take a look at what happens when table salt (NaCl) is mixed with water. In the salt solution, water is the solvent while salt (NaCl) is the solute.

Water cannot dissolve non-polar substances, such as oil, or fats, and will often show a separation from them acting as if it is “squeezing” them together. This is called the hydrophobic effect (“water fearing”). This effect is very important in the formation of cell membranes.

Solid Water is Less Dense than Liquid Water

Hydrogen bonds between water molecules expand as the water freezes. This creates extra “space” in solid water (ice) and makes it less dense than liquid water. Translation: ice floats

This property is significant to living things because it means that bodies of water do not freeze solid. Instead, a layer of solid water (ice) forms nearer the top, insulating the organisms at the lower layers. Take a look at the diagram below to see this temperature separation:

High Heat of Vaporization

Liquid water does not evaporate until it reaches relatively higher temperatures that can break the hydrogen bonds within it. This makes it a good cooling agent for living things; as water evaporates it carries heat away.

High Specific Heat

Water has a high specific heat, which means that it is slow to undergo changes in temperature. This allows very large bodies of water to maintain their temperatures to sustain living things despite weather changes. It also helps your body to maintain a relatively stable temperature.

Large bodies of water (lakes and oceans) help regulate temperatures of nearby land masses. This is the reason why winters are colder in the middle of the U.S. compared to coastal cities.

Acids, Bases and pH

Water molecules can dissociate, or come apart, into two ions:

H₂O → H⁺ + OH^–

water → hydrogen ion + hydroxide ion

Scientists have devised a scale to measure the concentration of hydrogen ions (H⁺) and hydroxide ions (OH^–) that exist in solutions. The scale is called the pH scale. The pH scale ranges from 0 to 14.

Neutral solutions have equal amounts
of H⁺ and OH^– ions. Neutral solutions have a pH of 7.
Acids, or acidic solutions, have a greater amount of H^+, and are assigned a number between 0 and 7 on the pH scale. The lower the number, the more acidic the solution.
Bases, or basic solutions, have a greater amount of OH^–, (or less H⁺) and are assigned a number between 7 and 14 on the pH scale. The higher the number, the more basic the solution.

pH is measured on a logarithmic scale. This means that the difference of one pH unit (example: between 3 and 4) means that there is a tenfold difference in H⁺ or OH^– concentration.

Take a look at the picture on the right that shows the pH scale and common substances with each pH level.

All living organisms have a set range of pH levels that are optimal for their growth and development. Sudden changes in pH levels can harm organisms. The pH inside human cells must be kept close to neutral in the range of 6.5 to 7.5. If a chemical with a lower or higher pH enters the cell, it could disrupt chemical reactions and cells’ homeostasis. Substances with a very low (strong acid) or very high (strong base) pH cause chemical burns. To combat this, cells contain weak acids and weak bases called buffers which work to neutralize substances inside cells that are too acidic or too basic.

Acid rain is is a concern for biologists because it can harm living things. It damages trees at higher elevations and can create an acidic environment for organisms in streams and lakes. It results when rain combines with waste chemicals in the atmosphere from cars or power plants (sulfur dioxides or nitrogen oxides). Acid rain has a pH around 4. Take a look at what acid rain can do to a forest.

Test Yourself

A student measured the pH in a pond over a three year period, before, during and after a factory opened in a neighboring city. At what time was the pond the most acidic? (Answer: year 3, lowest pH is the most acidic)