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Students will experiment with the relationship between pH, volume, concentration, and the relationship between acids and bases while practicing their critical thinking skills in an inquiry based experiment. This lesson is written using the 5E Learning Model. Learning Outcomes:
Curriculum Alignment:1.01 Identify and create questions and hypotheses that can be answered through scientific investigations. 1.02 Develop appropriate experimental procedures for:
1.04 Analyze variables in scientific investigations:
1.05 Analyze evidence to:
1.06 Use mathematics to gather, organize, and present quantitative data resulting from scientific investigations:
1.08 Use oral and written language to:
4.01 Understand that both naturally occurring and synthetic substances are chemicals. 4.03 Explain how the periodic table is a model for:
4.06 Describe and measure quantities related to chemical/physical changes within a system:
Classroom Time Required:Approximately 6 class periods (~50 minutes each) are needed, however, some things can be assigned as homework to decrease the time spent in class. Materials Needed:
**** Instead of making the paper you can also purchase pH Hydrion paper with a wide scale to use in the experiment. Explore/Explain:1 piece of citrus fruit per group (it is best if the groups have different types of citrus fruit), plastic knives, pH paper (made in Engage stage- can also be bought), fruit juicer, 5 equal sized containers per group, distilled water, copies of pH experiment worksheet, graph paper, overhead of pH summary chart Extend:
|
| pH | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| Color |
Very Acidic Acidic Neutral Basic Very Basic
See Figure 1 and 2 to see a color range (1) of a universal indicator (2).
Recall that the value of pH is related to the concentration of H+ (\(H_3O^+\)) of a substance. pH itself is approximated as the cologarithm or negative logarithm of the \(H^+\) ion concentration (Figure 3).
\[pH \approx -log[H_3O^+] \tag{3}\]
A pH of 7 indicates a neutral solution like water. A pH less than 7 indicates an acidic solution and a pH greater than 7 indicates a basic solution. Ultimately, the pH value indicates how much H+ has dissociated from molecules within a solution. The lower the pH value, the higher concentration of H+ ions in the solution and the stronger the acid. Likewise, the higher the pH value, the lower the concentration of H+ ions in the solution and the weaker the acid.
The color change of a pH indicator is caused by the dissociation of the H+ ion from the indicator itself. Recall that pH indicators are not only natural dyes but also weak acids. The dissociation of the weak acid indicator causes the solution to change color. The equation for the dissociation of the H+ ion of the pH indicator is show below (Figure 4).
\[HIn + H_2O \rightleftharpoons H_3O^+ + In^- \tag{4}\]
with
- \(HIn\) is the acidic pH indicator and
- \(In^-\) is the conjugate base of the pH indicator
It is important here to note that the equation expressed in figure 4 is in equilibrium, meaning Le Chatelier's principle applies to it. Thus, as the concentration of \(H_3O^+\) (H+) increases or decreases, the equilibrium shifts to the left or right accordingly. An increase in the \(HIn\) acid concentration causes the equilibrium to shift to the right (towards products), whereas an increase of the \(In^-\) base concentration causes the equilibrium to shift to the left (towards reactants).
pH indicators are specific to the range of pH values one wishes to observe. For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used to indicate pH ranges of about 8 to 10, 4.5 to 6, and 6 to 7.5 accordingly. On these ranges, phenolphthalein goes from colorless to pink, methyl red goes from red to yellow, and bromothymol blue goes from yellow to blue. For universal indicators, however, the pH range is much broader and the number of color changes is much greater. See figures 1 and 2 in the introduction for visual representations. Usually, universal pH indicators are in the paper strip form.
It is important to note that the pH scale is a logarithmic scale: hence an increase of 1 pH unit corresponds to a ten times increase of \(H_3O^+\). For example, a solution with a pH of 3 will have an H+ (\(H_3O^+\)) concentration ten times greater than that of a solution with a pH of 4. As pH is the negative logarithm of the H+ (\(H_3O^+\)) concentration of a foreign substance, the lower the pH value, the higher the concentration of H+ (\(H_3O^+\)) ions and the stronger the acid. Additionally, the higher the pH value, the lower the H+ (\(H_3O^+\)) concentration and the stronger the base.
pH indicators can be used in a variety of ways, including measuring the pH of farm soil, shampoos, fruit juices, and bodies of water. Additionally, pH indicators can be found in nature, so therefore their presence in plants and flowers can indicate the pH of the soil from which they grow.
Nature contains several natural pH indicators as well: for example, some flower petals (especially Roses and Hydrangeas), certain fruits (cherries, strawberries) and leaves can change color if the pH of the soil changes. See figure 7.
(7)
In the lemon juice experiment, the pH paper turns from blue to vivid red, indicating the presence of \(H_3O^+\) ions: lemon juice is acidic. Refer to the table of Universal Indicator Color change (figure 1 in the introduction) for clarification.
The household detergent contained a concentrated solution of sodium bicarbonate, commonly known as baking soda. As shown, the pH paper turns a dark blue: baking soda (in solution) is basic.Refer to the table of Universal Indicator Color change (figure 1 in the introduction) for clarification.
Here is a closer look of the pH papers before and after dipping them in the lemon juice and cleaning detergent (Figure 10):
neutral acidic neutral basic
Figure 10:
Here is a simple demonstration that you could try in the lab or at home to get a better sense of how indicator paper works. Make sure to always wear safety glasses and gloves when performing an experiment!
Materials
- 1 cabbage
- cooking pot
- white paper coffee filters
- strainer
- water
- a bowl
Procedure
- Peal the cabbage leaves and place them into the pot.
- Add water into the pot, making sure the water covers the cabbage entirely.
- Place the pot on the stove and allow to cook at medium heat for about 30 to 35 minutes.
- Allow it to cool, then pour contents into the bowl using the strainer.
- Soak your coffee filters in the cabbage juice for about 25 to 30 minutes.
- Allow the filters to fully dry, then cut them into strips.
- Now start your pH testing (starts out blue, changes to green [basic], and red [acidic]).
1. A hair stylist walks into a store and wants to buy a shampoo with slightly acidic/neutral pH for her hair. She finds 5 brands that she really likes, but since she never took any introductory chemistry classes, she is unsure about which one to purchase. The first has a pH of 3.6, the second of 13, the third of 8.2, the fourth of 6.8 and the fifth of 9.7. Which one should she buy?
Answer: The brand that has a pH of 6.8 since it's under 7 (neutral) but very close to it, making it slightly acidic.
2. You decide to test the pH of your brand new swimming pool on your own. The instruction manual advises to keep it between 7.2-7.6. Shockingly, you realize it's set at 8.3! Horrified, you panic and are unsure whether you should add some basic or acidic chemicals in your pool (being mindful of the dose, of course. Those specific chemicals are included in the set, so no need to worry about which one you have to use and (eek!) if they are legal for public use). Which one should you add?
Answer: Since the goal is to lower the pH to its ideal value, we must add acidic solution to the pool.
3. Let's say the concentration of Hydronium ions in an aqueous solution is 0.033 mol/L. What is the corresponding pH of this solution, and based on your answer identify whether the solution is acidic, basic or neutral.
Answer: Using the formula \(pH \approx -log[H_3O+]\)
pH= -log[0.033]= 1.48 : The solution is highly acidic!
4. Now let's do the inverse: Say you have a solution with a pH of 9.4. What is the H30+ ions concentration?
Answer: [\(H_3O^+\)]= 10-9.4= 3.98E-10 mol/L. Seem too low to be true? Think again, if the pH is >7, the solution will be basic, hence the hydronium ions will be low compared to the hydroxide (OH- ions).
5. A more trickier one: 0.00026 moles of acetic acid are added to 2.5 L of water. What is the pH of the solution?
Answer: M=n/L : Macetic acid= 0.00026/2.5 =1.04E-4 mol/L
pH= -log[1.04E-4]= 3.98
Outside Links
References
- General Chemistry: principles of modern applications. 9th ed. New Jersey: Pearson Education,Inc, 2007.
- Utah State University Extension: Understanding your watershed. Geiger, J and Mesner, N. Department of Agriculture, June 2005.
- Nature: International weekly journal of science (425, 365). Oceanography: Anthropogenic carbon and ocean pH. Caldeira, K and Wickett, M. September 2003.
- Kain Escobar (UCD), Solesne Blosse (UCD), Kasey Nakajima (UCD)
