Background

Acids and bases are not only present in the chemistry lab. Acids and bases encompass our lives. Today, we will test the properties of many household substances you have encountered already.

Pre-lab Questions

Predict whether you think the following substances are an acid or a base.

• Vinegar
• Water
• HCl
• NaOH
• Aspirin
• Tums (or Pepto Bismol)
• Baking soda
• Egg whites

Objective

You will test properties of acids and bases in order to write a conclusion regarding the properties of acids and bases. How can the properties of household products determine if they are an acid or a base?

Materials

These materials will already be at each of your lab stations.

• Red and blue litmus paper strips
• Goldenrod paper
• Stirring rods
• 10 mL graduated cylinders
• Pipettes
• 150 mL beakers, labeled with solutions
• Red cabbage indicator
• Phenolphthalein
• Conductivity meters
• Well plate
• Magnesium ribbon cut into small pieces
• HCl
• Acetic acid
• Water
• Sodium bicarbonate
• NaOH (or lye)
• Borax (or other detergent)
• Milk of Magnesia (or other antacid)
• Egg whites
• Soda (coke, sprite, diet coke)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Wash your hands thoroughly before leaving the lab.
• Follow the teacher’s instructions for clean-up of materials and disposal of any chemicals.
• When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Procedure

Station 1

Activity 1, Part A: Are you acidic, neutral, or basic?

1. Take one piece of red litmus paper and a piece of blue litmus paper. Place about one inch of the paper in your mouth.
2. Wait 5 seconds, then take out the paper and see if either changed color. In your data table, record your observations. You will determine whether your saliva is acidic or basic. (red to blue means basic; blue to red means acidic, no change mean neutral)
3. Throw your used litmus papers in the trash.

Activity 1, Part B: Tasting acids & bases

1. Dip a cotton swab in cup labeled vinegar & place it on different parts of your tongue. In your data table, record where you taste the acidity.
2. Dip the other side of the cotton swab in cup labeled baking soda and place on different part of your tongue. In your data table, record where you taste the bitterness.
3. Throw your used cotton swabs in the trash.

The areas of the tongue sensitive to tastes:

Station 2

Activity 2, Part A: Secret Message

1. Dip a cotton swab into the Dixie cup of vinegar & use it to write your secret message on the paper.
2. Allow about 1-2 min for the message to dry completely. Start on part B while it is drying.
3. Read the “invisible ink” message by misting it lightly with the spray bottle filled with red cabbage juice.
4. Record your data. You may discard or keep your secret message.

Activity 2, Part B: Denaturation of milk

1. Add 2 mL of milk into each beaker labeled A & B.
2. Record initial color in your data table.
3. In beaker A, add 6 drops of water. After each drop, mix with stirring rod. Record any changes in your data table.
4. In beaker B, add 6 drops of vinegar. After each drop, mix with stirring rod. Record any changes in your data table.
5. Rinse the A & B beakers, graduated cylinder, and stirring rods.

Station 3

1. Choose 9 substances you would like to test. Obtain approximately 5 mL of each substance in the numbered graduated cylinders. Keep track of where you put each solution by recording it in the data table below.
2. In your data table, predict each substance as basic, neutral, or acidic.
3. In your data table, record the initial color of each substance to be tested.
4. In your data table, record the initial color the red cabbage indicator.
5. Using the pipette, add 5-7 drops of indicator to each beaker (up to 5 mL=80 drops) swirling the beaker after each drop. Once you have a color change that remains in solution, you may cease adding indicator. Some may NOT change, as they are neutral substances. Record your observations in the data table.
6. Organize all beakers from most acidic to most basic using as a guide for the pH scale for red cabbage juice
   1. (most acidic pink→ red → purple → blue → green → yellow most basic)
7. Estimate the pH value of each substance by using the pH scale you created with the beakers and record in the data table.
8. In your data table, classify each product as acid, base, or neutral. Rinse all glassware used.

Station 4

1. Using a pipette, place 5 drops of HCl into a clean beaker.
2. Use one piece of litmus paper to test the pH of the HCl. (Red to Blue means basic; blue to Red means acidic, no change mean neutral)
3. Record the color and determine if this is an acid or base in your data table.
4. Using a pipette, add 1 drop of phenolphthalein to the beaker. The solution will remain colorless.
5. Add sodium hydroxide, drop by drop, gently swirling the beaker gently until you see the first pale pink color that does NOT go away with shaking.
6. Using the litmus paper, record the color of the litmus paper as you did in step 3.
7. Using a different pipette, add 5 more drops of NaOH. Record the color and determine if this is an acid or base in your data table.
8. Rinse the test tube thoroughly in the sink and discard the used pH paper away in the trashcan.

Station 5

How do Acids and Bases feel?

1. Dip the tip of your finger into the beaker of water to wet it. Rub your thumb and wet finger together. It should not feel oily or slippery, as water is a neutral substance.
2. Re-wet your finger and use that finger to pick up a small amount of each powder or liquid out of its beaker and repeat the feel test. In your data table, describe each substance as either sticky or slippery. Use a paper towel to wipe your fingers.
3. Use the scoopula or pipette to take out a small portion of each substance and place each into the well plate. Ensure you know the order you placed the substances in the well plate.
4. If the material is a powder, you will need to create a solution. Using the pipette, add 2-5 drops of water to each beaker and use a stirring rod to dissolve the substances. Make sure you rinse the stirring rod in between each sample.
5. Do acids and bases conduct electricity? Using the conductivity meter, test the conductivity of each substance. DO NOT submerge the battery parts of the conductivity meters, only the end of the metal nodes needs to touch the solution in order to produce results. Do not forget to clean the nodes of the meter between use by dipping in distilled water and wiping with a paper towel.
6. How do Acids and Bases React with Metals? Carefully place ONE small piece of magnesium into each solution on your spot plate. Use a stirring rod to submerge the magnesium. Look for bubbling as a positive test, or no bubbles. Record your observations.
7. Magnesium pieces may be wrapped up in a paper towel and thrown away. Rinse the spot plate, scoopula, and dry them off. WASH YOUR HANDS THOUROUGHLY!

Observations

Activity 1

Part A

Part B

Station 2

Part A

Part B

Station 3

Station 4

Station 5

Analysis

1. Why was it important that we test water even though we know water to be neutral?
2. Many times ingested food produces excess acid in the stomach, causing pain. The pain is stopped by taking anti-acids over-the-counter medications such as Pepto-Bismol or milk-of-magnesia. How do these work?
3. List three properties of acids you discovered based on your results.
4. List three properties of bases you discovered based on your results.
5. Are most house cleaning substances acids or bases? Provide reasoning behind your conclusion.
6. Are most food product acids or bases? Provide reasoning behind your conclusion.
7. What are indicators?
8. List some of the substances you tested in activity 3 from most acidic to most basic.