Experiment:
The Basics of Acids and Bases

Introduction:

You have already learned that many solutions can be classified as acidic, basic or neutral.  Recall the characteristics of these substances.  Acidic solutions taste sour (something we do not test in our chemistry lab!), turn blue litmus paper red and can conduct electricity.  Basic solutions also conduct electricity; they have a bitter taste and turn red litmus paper blue.  Neutral solutions are neither acidic nor basic.  They may or may not conduct electricity, they do not cause a change in either red or blue litmus paper and they may taste salty or have no flavor at all.  

How would you define an acid? As a substance that turns blue litmus paper to red?  This definition has limited usefulness because what is really important is to understand WHY the litmus paper turns color or WHY an acid neutralizes a base or WHY acid rain destroys plants or damages buildings.  To explain chemical behavior of acids and bases, it is important to know how they are similar and how they behave on a molecular level.

In this experiment, you will not be able to see the molecules but you will gather additional information that will help you classify solutions as acidic or basic.  Then, you will examine the chemical formulas of these substances to find structural similarities.  

Materials:

• test tubes and test tube racks

• 10 mL graduated cylinders

• distilled water

• glass stirring rods

• red and blue litmus paper

• pH paper

• goggles and aprons

• PART I solid substances:

  1. sodium bicarbonate

  2. ascorbic acid

  3. sodium hydroxide pellets

  4. magnesium oxide (magnesia)

  5. sodium chloride

  6. potassium nitrate

  7. magnesium ribbon

• PART II solutions:

  1. 0.1 M nitric acid

  2. 0.1 M sulfuric acid

  3. 0.1 M acetic acid

  4. 0.1 M hydrochloric acid

  5. 0.1 M sodium acetate

  6. 0.1 M sodium carbonate

  7. 0.1 M ammonia

  8. rubbing alcohol

Procedure:

PART I:

1. Get 8 clean test tubes all of the same size.  Measure 3 mL of distilled water in a graduated cylinder.  Pour into the first test tube.  Now, add distilled water directly into the other 7 test tubes so that they "match" the first - "eyeball" them.

2. Add a sample of each of the solid substances from the list above (NOT including the magnesium ribbon!) to each of the test tubes with the distilled water.  Use a sample the size of a match head.  Gently tap the bottom of each test tube to mix the contents.  Let sit for a bit.

3. Take TWO pieces each of BLUE and RED litmus paper and tear it into a total of 8 pieces - place on a dry paper towel. 

4.  In the first test tube, test the distilled water on the RED and BLUE litmus paper and the pH paper.  Dip the stirring rod into the distilled water and then put a drop of it onto a piece of BLUE litmus paper, put a drop onto the RED litmus paper and then a drop onto the pH paper. For the pH paper, compare the color to the "key" on the plastic box holding the pH paper. Record your results for each paper on the data table.  Think about this: What results SHOULD you see?

5. Now, for each of the solid substances dissolved in water: 

• Dip the stirring rod into the solution and then put a drop of it onto a piece of BLUE litmus paper.  Record your results ("no change" or "turned red") onto your data table.

• Then, using the same technique, put a drop of the solution onto a piece of RED litmus paper.  Record your results ("no change" or "turned blue") onto your data table.

• Then, using the same technique, put a drop of the solution onto a piece of pH paper.  Compare the color to the "key" on the plastic box holding the pH paper.  Record your results onto your data table.

6. In the 8th test tube of distilled water, drop a TINY piece of magnesium.  Let it sit for at least 5 minutes.  Perform the same litmus tests and pH tests as you did above.  Record your results onto your data table.

7. Now, take a TINY piece of magnesium and drop it into each of the test tubes that you tested above.  Again, let sit for 5 minutes.  As you let the magnesium react, be sure to summarize your observations about what is happening in the test tube on your data table for each substance.

8. When you are done with all of the solid samples, discard the contents.  For any substances that did not completely dissolve, discard into the trash can with the plastic liner first.  Then, rinse THOROUGHLY each test tube and then give a final rinse with distilled water.

PART II:

1. To each of the 8 cleaned test tubes, add 2 mL of each of the solutions listed above.  Again, use the same technique to measure the approximate volume as you did in step 1 above.

2. Repeat the same litmus and pH tests as you did in Part I.  Record your results onto your data table.

3. Repeat the same magnesium test as you did in Part I.  Record your results onto your data table.

3. Discard the solutions in the test tubes.  Rinse thoroughly and INVERT in your test tube rack.  Return cleaned rack up to the front.

4. Clean your lab bench area.  Be sure that NO litmus paper gets put into your sink!

Data Table:

Prepare the data table BEFORE you come to lab following the directions below.

• Using Microsoft Word,  type your name, date and block period at the top of the page.  Then, RIGHT JUSTIFY!
• Make the page PORTRAIT.
• Then, copy/paste DATA TABLE BELOW from the website into your Word document in preparation for the lab.  After your have copy/pasted the table, make the table take up the whole page by clicking once on the table in the lower right corner and drag the corner to the bottom right of the page.  This way, you will have enough space to hand-write in your data in the experiment.  Make sure the data table uses the whole page!
• In the lab, be sure to use blue or black PEN (NO PENCIL) when you complete this data table.  NEVER WITE-OUT data.  Enter data carefully so that you make no mistakes.  

Analysis:

1. On your data table, in the column marked formula, determine the chemical formula for each substance.  If you don't know it, get on the internet or use a dictionary to find it out.

WORD-PROCESS THIS NEXT SECTION OF THE LAB REPORT.

2. List the samples that are ACIDIC according to your LITMUS TESTS.  

• How many substances indicate that they are acids?

• What substance do all these have in common?

• Did all the acids react with magnesium?  Explain.

• What was the pH range?

• Based on your data results, write a definition for an acid.

3. List the samples that are BASIC according to your LITMUS TESTS.  

• How many substances indicate that they are bases?

• What substance do all these have in common?

• Did all the bases react with magnesium?  Explain.

• What was the pH range?

• Based on your data results, write a definition for a base.

4. List the samples that are NEUTRAL according to your LITMUS TESTS.

• How many substances indicate that they are neutral?

• What substance do all these have in common?

• Did all the acids react with magnesium? Explain.

• What was the pH range?

• Based on your data results, write a definition for a neutral substance.