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**Lesson 6.3: What is an acid?

LOOKING BACK

  1. What did we figure out last class and what question needs to be answered next?

  2. How does answering this question fit into our investigations of determining why shellfish are dying?

  3. What knowledge or procedures can we use from the last lesson to investigate pH?

Part 1: Investigating SOLID Substances

Materials:  Forceps, well plate, Containers with 8 solids, pH paper

  1. Determine the layout of your well plate for the 8 samples to be tested. 

  2. You will have two samples at the table you start at.

  3. Place into the designated well a small sample of each substance (about the size of a rice grain) using a spatula. Do NOT touch the chemicals.

  4. Test the substances by using forceps to place a small piece of pH strip on the surface of the solid substance, dispose of the pH paper, and record the pH value and color in the “TEST 1” column.

  5. Move to the next table where you will find the next two samples. 

  6. Keep your samples in the well plate for TEST 2.  Do NOT clean your samples.

Data Table 1:

Substance Chemical Formula pH Prediction <7 or =7 or >7 Test 1 - pH ACID or BASE?
Citric solid \(HC6H7O7\) <7 None N/A
Lactic solid \(HC3H4O3\) <7 None N/A
Sodium Chloride \(NaCl\) =7 None N/A
Epsom Salt \(MgSO4\) =7 None N/A
Vitamin C \(HC6H7O6\) <7 None N/A
Cream of Tartar \(HC4H5O6\) <7 None N/A
Calcium Hydroxide \(Ca(OH)2\) >7 None N/A
Sugar \(C12H22O11\) =7 None N/A

  1. Compare the results from the data table from last class and today’s data table. 

  2. What is different about the substances we tested in the two experiments? 

  3. How could we retest our solid substances to determine if they were actually acidic? Check with your teacher before beginning part 2

Part 2: Retesting the Substances

Materials:  Well plate, solid samples, pH paper, water, spatula, stirring rod

Procedure: 

  1. Re-do the previous investigation making the change that was discussed in step 6.

  2. Record observations under the “Test 2 (aqueous)” column in the data table above.

  3. When complete, throw away used pH papers and rinse out your well plate. Make sure lab area is clean and wash hands. 

Substance Chemical Formula pH Prediction <7 or =7 or >7 Test 1 - pH ACID or BASE?
Citric solid \(HC6H7O7\) <7 2.0 A
Lactic solid \(HC3H4O3\) <7 2.0 A
Sodium Chloride \(NaCl\) =7 No [H+] Neutral
Epsom Salt \(MgSO4\) =7 No [H+] Neutral
Vitamin C \(HC6H7O6\) <7 2.0 Acid
Cream of Tartar \(HC4H5O6\) <7 4.0 Acid
Calcium Hydroxide \(Ca(OH)2\) >7 11.0 Basic
Sugar \(C12H22O11\) =7 No [H+] Neutral

Part 3. DEFINING AN ACID

  1. Using your observations from the two investigations today (data in Tables 1 and 2), what can we say we have figured out about acids?

The Acids need to be aqueous to be identifiable by universal indicators.

Need to be dissociated in the water for the pH paper to identify the ions.

  1. The equation for the dissociation of HCl gas in water is written at the top of the diagram below.  What part of dissociation is the diagram representing: a, b or c?

A. The HCl gas in the air.

B. The HCl was initially placed in water.

C. The H+ ions and the Cl- ions dissociated in water.

The graph shows (B), where the HCl gas is now in the water, but not yet dissolved.

  1. Explain the reasoning for your choice.

The HCl particles are still bonded while surrounded by water, therefore they are not dissociated yet, neither are they out of the water, so the answer must be (B)

  1. What will happen to the HCl when the acid dissociates?

The acid will dissociate into H+ ions and Cl- ions. The water will then surround the ions with either their positive or negative side, depending on the ion, but for the H+ ions, the water bonds with them strongly enough to create H3O+ instead of just being dissolved.

  1. Here are a few Lewis structures of the acids we investigated.  Circle or identify the hydrogens that could dissociate from the acid into the water.

  1. The hydrogen ion, H+, interacts with water to create the hydronium ion.  The Lewis structure of the reaction is modeled below. Draw the hydronium ion using lines for the covalent bonds in the box provided. Include the bracket and charge in your answer. 

Hydronium ion with covalent bonds 

_____________
|    H      |
|    |      |
| H - O - H |
-------------

  1. Using what we have figured out today about acids and remembering the polarity of water, create a model that shows the dissociation of HCl in water.  Draw three solo water molecules, one H+ as a hydronium ion and one Cl- ion.
 _______________________________
|                               |
| HCl ->   H+   +   Cl-  H - O  |
|          |                 |  |
|      H - O - H             H  |
 -------------------------------

Part 4 - Investigating how changes in ocean water affect oyster shells? 

Open Changes in ocean water and how it affects seashells investigation , make a copy and save in your google drive.. READ the lab instructions. Check with your teacher to find out which solution you will be testing. Gather materials,create labels with your group’s names, find the INITIAL MASS, and make initial observations. You will be recording the mass of your solutions with shells over a 2 week period, so you will need to access this document in the future

NEXT STEPS

  1. Did our investigations today help us answer the question, “What is an acid?” Provide two pieces of evidence.

  2. Are there any new questions that need to be answered about acids and what makes a substance acidic to determine what is happening to shellfish? 

  3. Complete IMT for Lesson 3.

Last update: June 5, 2023
Created: June 5, 2023