Hot Colors eTrip: A PhD in pH
Pre- and Post-Trip Lesson Plan
The students will:
- Participate in graphic presentation to explain the concept of concentration.
- Be able to explain the meaning of pH.
- Measure the pH levels of different substances and rank them relative to each other.
- Adequate quantities of drinking water and sugar
- 4 containers of greatly different sizes: gallon, quart, cup, ¼ cup
- Adequate quantities of distilled water, lemonade, vinegar, tomato juice,
- antacid, baking soda, and household ammonia
- 7 equal-sized containers
- Litmus paper and corresponding pH scales
The thermophiles of Yellowstone live in hot springs which vary widely in both temperature and pH levels. Although temperature is a concept familiar to students, an understanding of pH will also be helpful.
The chemical formula for water is H2O, which indicates that water is a compound containing two hydrogen atoms and one oxygen atom. When water is a liquid, the molecules may break down into two forms, H and O-H. Lone H atoms have a positive electrical charge of one, so chemists describe them as H+. Pure water must have a net charge of zero, meaning that O-H must be "minus one" in charge (written as OH-). H+ and OH- are called "ions." Positive ions are called "cations," and negative ions are called "anions."
The level of acidity depends on the concentration of H+ per unit volume. The concept of the concentration of H+ per unit volume may be explained by the following analogy. If a fixed amount of sugar is added to different volumes of tea, the sweetest tea is the one with the smallest volume because the same amount of sugar has to spread through more tea in the larger volumes. It is the same with acidity-the most acidic water is that which has the most H+ per unit volume.
Acids and bases are opposites and are terms that have meaning only in the presence of water. Acidity is the measure of how much free H+ is present per unit volume, while basicity is the measure of the amount of OH- present per unit volume.
When an acid is added to water, the hydrogen in the acid's molecule is easily separated, resulting in a large extra quantity of H+. For example, if you were to pour hydrochloric acid (HCl) into pure water, the HCl would break apart into two atoms-hydrogen and chlorine (H and Cl). The lone H atom has a positive charge, and the Cl has a negative charge. A sudden infusion of HCl would result in the water gaining many more H+ cations and becoming acidic. When a base is added to water, it gives up the OH- or hydroxide anion.
Potential of hydrogen or "pH" is the measure of how acidic or basic something is. It is measured on a scale of 0-14, where "0" is extremely acidic and "14" is extremely basic. The pH scale is a logarithmic, not a linear, relationship. Calculated mathematically, pure water has a pH reading of 7. Under normal conditions, the pH level of most natural systems is between 5 and 9. Some of the Yellowstone hydrothermal areas have a much wider range.
The pH of water is determined by vegetation type and quantity, the type of underlying rocks, the quality of water entering from another source, and human activity. Air pollution from automobiles and factories can contribute to acid rain, which can precipitate into surface water and lower its pH. Lime in agriculture and household products can greatly reduce the acidity of water and raise its pH value.
Litmus paper is paper that has been treated which a special chemical. This chemical changes color when it is exposed to an acid or a base. The color can then be compared to an accompanying scale to give a measurement of pH.
Procedure - Experiment 1
The instructor will:
- Assemble the necessary materials prior to class.
- Enlarge the pH scale accompanying the litmus paper on a color copy machine, making a copy for each group of students prior to class.
- Show students the four containers of different sizes (gallon, quart, cup, ¼ cup) and select a student to fill each container to near capacity with drinking water.
- Select students to measure four identical amounts of sugar and add to each of the four containers.
- Ask students to hypothesize which container will have the sweetest water and ask them to explain why. Be sure students understand the concept of concentration.
Procedure - Experiment 2
The instructor will:
- Explain that the chemical formula of water is H2O, meaning that water is composed of two hydrogen atoms and one oxygen atom.
- Refer to the sugar experiment in Procedures 3-5 and explain acidity or basicity works the same way as the sugar, except that in the case of acidity, the "sugar" is H+ and in the case of basicity, the "sugar" is OH-. Reiterate that acidity and basicity are measures of concentration of either H+ or OH- present in water per unit volume.
- Divide students into pairs.
- Present each pair with enough litmus strips to test each container. Explain that a new piece of litmus paper should be used for each substance.
- Instruct students to insert end of litmus strips into each container and remove immediately.
- Direct students to compare the color of each item with their pH scale and record the corresponding number on the opposite end of the litmus strip.
- When all containers have been tested, direct students to prepare their own scale, showing the pH level of each substance. Beside each substance, they should write "H+" if more H+ cations are present. Write "OH-" if more OH- anions are present.
Student pH charts
National Science Standards for Grades 5-8