pH of Common Materials

How to Proceed

Read through the introductory materials below.
Open the Unit 1 Experiment Answer Sheet and complete the following Experiment exercises this unit:
Experiment 1 Exercise 1 – The Scientific Method (~30-45 min)
Experiment 1 Exercise 2A – pH of Common Materials (~30-45 min)
Experiment 1 Exercise 2B – pH and Buffers (~45-60 min)
Save your completed Unit 1 Experiment Answer Sheet and submit it no later than Sunday midnight CT.
The Scientific Method – Introduction

The Scientific Method is the basis for almost all scientific research. If you click on the Unit 1 Overview page, you can read about how the Scientific Method is conducted. You can also read about the process in your book on pp 14-17. One area of confusion often involves the difference between a hypothesis and a prediction. This is because many people use these terms interchangeably, but in fact, they are different. Here is how your book discerns the two:

Hypothesis – an answer to a question or explanation of an observation (p 14).

Prediction – an expected outcome if our hypothesis is correct; often worded as “if…then” (p15).

The purpose of this first exercise is to have you use the Scientific Method yourself. We will use the following web site. Be sure that you can access it and use it:

Glencoe/McGraw Hill. No date. The Scientific Method
http://www.glencoe.com/sites/common_assets/science/virtual_labs/ES01/ES01.html (Links to an external site.)

When you are ready to begin, use the instructions in the Unit 1 Experiment Answer Sheet and work through the exercise.

pH of Common Materials – Introduction

This unit we are also learning about some of the chemistry that is important in biological systems, such as pH. Be sure you have read pp 32-33 in your book and our online lecture this unit before beginning this exercise. The pH scale ranges from 0 to 14; a pH less than 7 is considered acidic and a pH greater than 7 is basic. The pH scale is logarithmic, which means that a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4 and a hundred times more acidic than a solution with a pH of 5.

Acids and bases are not necessarily a bad thing. Many of the materials that we handle and eat and drink everyday vary in pH. Some of these materials are safe to handle, such as “weak” acids (e.g., soda, coffee). Stronger acids (e.g., battery acid) and bases (e.g., ammonia) can be quite caustic and damaging. One way to measure the pH of liquids is to use pH indicator paper; paper that turns a particular color depending on the pH of the solution. Anyone with a swimming pool or hot tub is probably familiar with such paper.

We will use a virtual lab to examine the pH of common solutions that you might have around the house. You will use the following website; be sure you are able to access and use it:

Glencoe/McGraw Hill. No date. pH of Common Solutions
http://www.glencoe.com/sites/common_assets/science/virtual_labs/E22/E22.html (Links to an external site.)

When you are ready to begin, open the Unit 1 Experiment Answer Sheet and follow the instructions to complete this exercise.

Buffers – Introduction

As you saw in the previous exercise, the pHs of common solutions vary across the pH scale! Yet our body is constrained to work within a very narrow pH range. Small changes in pH can alter the function of biologically important molecules such as enzymes, by breaking hydrogen bonds and denaturing these proteins. For this reason, in most organisms (such as ourselves), pH is very closely regulated. pH can be kept relatively constant by the use of buffers, chemicals which can absorb or release hydrogen ions to maintain a relatively steady pH.

In most vertebrate animals, blood pH must be maintained between 7.35 and 7.45. There are several biological buffers that work to maintain this pH; one of the more important being the carbonic acid – bicarbonate system:

H2O + CO2 <–> H2CO3 <–> H+ + HCO3-

In the reactions above, the double headed arrows indicate that each step is reversible. If carbon dioxide (CO2) levels increase in our blood, it can combine with water to form carbonic acid (H2CO3), which can break down to form bicarbonate (HCO3-) and hydrogen ions. This would shift the pH towards the acidic end. If the acidity levels become too high, the whole process will reverse, such that hydrogen ions are removed and carbon dioxide is produced; thereby shifting the pH towards the alkaline end. This is only one example of a biological buffer; there are several other systems involved, but they all operate in a similar manner.

The purpose of this exercise is to help you understand the chemistry of buffers. Be sure that you have read through the material on pp 32-33 in your book and this unit’s online lecture on The Chemistry of Life. For this exercise, you will use the following website (be sure your speakers are on):

McGraw-Hill Education. No date. Buffers
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf (Links to an external site.)

You may need to download and install a plugin to use this simulation, so test this simulation early in the unit in case you run into problems. When you are ready, open the Unit 1 Experiment Answer Sheet and follow the instructions there to complete this exercise.

WEEK 1 EXPERIMENT ANSWER SHEET Please submit to the Week 1 Experiment dropbox no later than Sunday midnight.

SUMMARY OF ACTIVITIES FOR WEEK 1 EXPERIMENT ASSIGNMENT

· Experiment 1 Exercise 1 – The Scientific Method

· Experiment 1 Exercise 2A – pH of Common Solutions

· Experiment 1 Exercise 2B — pH and Buffers

Experiment 1 Exercise 1: The Scientific Method

Be sure that you have read over the introductions to this week’s Experiments activities before starting. When ready, open the following website:

Glencoe/McGraw Hill. No date. The Scientific Method http://www.glencoe.com/sites/common_assets/science/virtual_labs/ES01/ES01.html

Scenario

You have been tasked to design the most efficient compost pile possible; one that can take organic waste material and quickly break it down into a form that can be applied as mulch. A compost pile typically involves:

· Green materials (e.g., fresh grass clippings, fresh leaves)

· Brown materials (e.g., dried grass, twigs, hay, dried leaves)

· Water

· Aeration

The efficiency of a compost pile is measured by how quickly organic matter is decomposed and this efficiency is dictated by the proper combination of the components listed above. Unfortunately, you do not know where to begin! Fortunately, you have a compost simulator that will allow you test a variety of compost designs before you have to construct your actual compost pile.

The purpose of this exercise is to use the Scientific Method to determine the best design for the most efficient compost pile. Note that the parameters that can be varied (using the slider bar) in our simulator are:

· Brown to Green Balance: 100% Green material, 100% Brown material or a combination of both

· Water Concentration: 0 to 100% water

· Number of turns per Month: 0 to 8 turns per month (the greater the number of turns the greater the aeration)

Hypothesis

We will start with the hypothesis that “an efficient compost pile needs lots of green material, a lot of water and a lot of aeration to be efficient”.

Question

1. Based on the on the hypothesis above and knowing the design parameters, write a reasonable prediction if the hypothesis is correct. Be sure to word it as an “If…then” statement (2 pts).

Procedure

A. Conduct an experiment (Experiment 1) to test the hypothesis above using the simulation program.

a. Set the design criteria using the sliders for Brown to Green Balance, Water Concentration and Number of Turns per month. Be sure to use settings based on the hypothesis; this is what you are testing.

b. Record your design criteria in Table 1 below for Experiment 1.

c. Click on the Calendar (Sept 1) in the simulation to start the experiment. When complete, record the Efficiency Meter reading.

Table 1. Design criteria and experiment results (2 pts)

	Brown to Green Balance	Water Concentration	Number of Turns per Month	Efficiency (High, Medium, Low)
Experiment 1
Experiment 2
Experiment 3 (Optional)

Questions

2. Was your prediction correct? If not, why do you think so (2 pts)?

3. Write an alternative hypothesis regarding an alternative compost pile design (2 pts).

4. Conduct another Experiment (Experiment 2) to test your new hypothesis using new design criteria (Click Reset to start over). Enter the necessary information in Table 1 above. What was the result of Experiment 2 (2 pts)?

Optional

If your second design was still not very efficient, conduct another Experiment and record your design criteria and results in the Table above.

Experiment 1 Exercise 2A: pH of Common Solutions

Be sure that you have completed your text book readings, have read through the online lecture and have read the introductory material for the Week 1 Experiment before starting. First, answer the following questions:

Questions

1. What is the definition of an acid? Your definition should include more than just a pH range. Provide one example of an acid. Cite your sources. (2 pts).

2. What is the definition of a base? Your definition should include more than just a pH range. Provide one example of a base. Cite your sources. (2 pts).

Open the pH simulation below to begin:

Glencoe/McGraw Hill. No date. pH of Common Solutions http://www.glencoe.com/sites/common_assets/science/virtual_labs/E22/E22.html

Procedure

A. Record the six substances shown across the top of the screen in Table 2 below (e.g., antacid, shampoo, battery acid, soft drinks…).

B. Enter a predicted pH value for each solution and a brief explanation for your choice.

Table 2. Predicted and measured pH values and your explanations (6 pts).

	Substance	Predicted pH	Explanation for Prediction	Measured pH
1
2
3
4
5
6
Optional additional solutions
7
8
9
10
11
12

C. Next, use the pH paper to measure the pH of each of the six solutions.

a. Click on the lose end of pH paper and drag into the first test tube.

b. It should change color. Drag the piece of paper over to the dispenser and use the color chart to estimate the pH. Record the measured pH in the Table above.

c. Use the up and down arrows beneath the name of the substance and set the value to the one you determined using the pH paper.

d. Repeat this procedure for the remaining five substances.

D. When you have recorded your pH values and set the counter to indicate the measured pHs, click on Check to see how you did. If necessary, retest any solutions you got wrong.

E. This simulation has twelve different solutions. Feel free to test them all if you would like. This is not required though! Click on Reset if you are interested.

F. When you are done testing the pHs, answer the questions below.

Questions

3. Which of your substances tested are considered an acid (1 pts)?

4. Which of your substances tested are considered a base (1 pts)?

5. What surprised you most about your results in this activity (1 pts)?

Experiment 1 Exercise 2B: Buffers

Before beginning, answer the following question:

Question

1. What is a buffer and briefly, how do they work? Cite your source (2 pts)?

Procedure

Open the buffer simulation below to begin (if necessary, copy the web address and past it into your browser). Be sure your speakers are turned on.

McGraw-Hill Education. No date. Buffers http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf

A. Listen to the Introduction. If you need to listen to it again, reload the page.

B. Next, click on the Add Strong Acid, H+ button. Pay attention to the bars in the graph. They correspond to the level of the components in the beaker. You will need to watch them carefully.

Questions

2. Why does the green bar in the graph drop? Why does the purple bar in the graph rise? Explain what is occurring chemically (4 pts).

3. In the simulation shown, what happens to the pH in the beaker when HCl is added? How do you know this based on what you see in the graph (2 pts)?

4. What will happen to the pH if HCl is added after all of the acetate is used up? (1 pts)?

Procedure (continued)

C. Next, click on the Add Strong Base. OH-.

Question

5. What is formed when sodium hydroxide is added and how does this affect the pH (4 pts)?

 

Week 1 Experiment Grading Rubric

Component	Expectation	Points
Experiment 1 Exercise 1	Demonstrates an understanding of the Scientific Method and an ability to apply it (Table 1, Questions 1-3)	10 pts
Experiment 1 Exercise 2A	Demonstrates an understanding of pH and how it applies to your everyday life (Table 2, Questions 1-5).	13 pts
Experiment 1 Exercise 2B	Demonstrates an understanding of pH and the effect of buffers (Questions 1-5)	13 pts
TOTAL		36 pts

 

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