All life exists within the context of its environment. Each environment is characterized by its biological, physical, and chemical properties. Since organisms are adapted to a specific environment, radical changes in these conditions often result in injury to the individual or possibly extinction of the species. Recent reports of declining frog populations, for example, have been correlated with increased ultraviolet radiation from the sun (specifically UVB). Chemical reactions that take place inside of an organism are dependent upon both internal and external chemical and physical properties. We will explore some of these properties in today’s lab.
Although water is generally regarded as a stable compound, individual water molecules are constantly gaining, losing, and swapping hydrogen atoms. This process is represented by the following chemical reaction:
Pure water with a pH of 7 has equal numbers of hydrogen and hydroxide ions at any given moment. Water is considered to be a neutral substance. The pH of any solution can be determined by calculating the total concentration of hydrogen ions in the solution.
A mole is a term used to describe the quantity of something. If you have a mole of paperclips, that means you have paperclips. This is similar to the way we use the word “dozen.” We know that if you have a dozen paperclips, it means you have 12 of them.
1 dozen = 12
1 mole = 6.02×1023
Scientists measure acidity using the pH scale. The pH scale ranges from 0 to 14, and the numbers represent the concentration of hydrogen ions–1 in the substance. For example, battery acid, with a pH of 1, has 1×10 moles of hydrogen ions per liter of solution. Ammonia, which is a very basic substance with a pH of 12, has 1×10–12 moles of hydrogen ions per liter of solution. The more acidic the solution, the more hydrogen ions it contains.
Excessive changes in pH can cause metabolic and ecological problems. For example, the pH of your blood is carefully kept between 7.35 and 7.45. Any deviation above or below this range will result in alkalosis or acidosis, and both conditions can be deadly. Acid rain, on the other hand, can dissolve toxic metals from the soil particles into the soil solution and impair plant growth. As we will soon see, plant health is a factor that quickly affects most other life forms on the planet.
Following the instructions given by your teacher, measure the pH of each solution using pH paper. Record the pH of these items below.
Buffers are molecules that resist changes in pH. They can take up and release excess hydrogen ions in a solution and therefore prevent drastic changes in pH, regardless of whether acid or base is added to the solution. The net result is that the pH of the solution remains relatively stable (until the buffer is overwhelmed). Buffers are commonly found in dissolved minerals, soils, and in living organisms.
For example, buffers can play a protective role in lake ecosystems. In a lowland lake, acid rain causes very little fluctuation in pH because these lakes are typically high in organic molecules that act as buffers. A lake with little buffering capacity, such as a high alpine lake low in organic molecules, will experience a much greater change in overall pH as a result of acid precipitation.
Record the pH of your buffered and unbuffered solutions after each addition of 10 mL of hydrochloric acid to each beaker.
| Table 1. Buffered and Unbuffered Solutions | ||
|---|---|---|
| Volume (mL) of 0.001 M HCl | ph | |
| Buffered Solution | Unbuffered Solution | |
| 0 | ||
| 10 | ||
| 20 | ||
| 30 | ||
| 40 | ||
| 50 | ||
Illustrate the buffering capacity of each solution by graphing your results below. Place the volume of HCl on the x-axis and the pH value on the y-axis. Don’t forget to give your graph a title.
You can download this graph paper template to complete this portion.
Bicarbonate ions act as a powerful buffer in your blood. They are created when carbon dioxide (CO2), produced during cellular respiration, reacts with water:
Notice that hydrogen ions are also generated, which increases the acidity of blood and decreases the pH. In your body, the hydrogen ions are absorbed by hemoglobin molecules on your red blood cells. Meanwhile, the bicarbonate ions circulate in the blood plasma, preventing rapid pH changes. As your blood circulates past the metabolizing cells, more and more CO2 enters your bloodstream and turns to bicarbonate ions. By the time the blood reaches the lungs, it is full of bicarbonate and hydrogen ions. The bicarbonate and hydrogen ions now combine and the reaction goes from right to left, releasing the CO2, which is now breathed out. In this exercise, you will bubble CO2 into tap water and demonstrate the change in pH as carbonic acid is formed. You will use the pH indicator phenol red, which turns yellow in acidic conditions and magenta (red-purple) in basic conditions.
| Initial Solution | Final Solution |
|---|---|
| pH | |
| Color |
Water is a fascinating molecule whose chemical structure is pretty much responsible for life on earth. Chemical reactions that take place inside of a cell exist in an aqueous environment consisting principally of water. The primary characteristic of the water molecule that imparts its many unique qualities is the simple fact that water is a polar molecule. When considering the polarity of water, you must first remember that chemical bonds occur when 2 or more atoms “share” electrons. In the water molecule, the oxygen atom ‘hogs’ the electrons it shares with the hydrogen ions. Because the electrons are closer to the oxygen atom, that side of the water molecule ends up being partially negative while the hydrogen side of the molecule ends up being partially positive. This makes water an excellent solvent. Substances that are hydrophilic (love water) are usually polar or ionic molecules themselves, and will dissolve readily in water. Substances that are hydrophobic (hate water) are usually nonpolar molecules, and will not dissolve in water. Nonpolar substances will dissolve in a nonpolar solvent such as oil. Surfactants are special molecules that are both hydrophilic and hydrophobic. They allow water and oil to mix. Soaps and detergents are both examples of surfactants.
