Question:

EXPERIMENT 4

Acids and Bases

Materials and Equipment: Red Cabbage, Hot or Boiling Water, Pitcher, pH paper, Lemon Soda (sprite, sierra mist), White Vinegar, Apple Juice, Baking Soda, Shampoo (preferably clear), Conditioner (preferably clear), Hand Sanitizer (clear), Ammonia.

Introduction

In this experiment we will determine the pH of a variety of substances using both pH paper and a natural indicator- red cabbage juice.   Red cabbage contains a pigment called flavin.  Flavin is also found in red poppies, grapes, blue cornflowers, and apple skin.  The pigment changes color with pH so it can be used as an indicator. 

Many of the liquids in your house have acidic or basic properties.  Acids taste sour; bases taste bitter and feel slippery.  We use acids and bases everyday without much concern, however, strong acids and bases can be very dangerous.  It is important to know what chemicals in our house are dangerous and to take the proper precautions when working with them.  

The pH scale is a reflection of the concentration of hydronium ions (H3O+) present in a substance. Recall that when an acid dissolves in water, it will dissociate into its conjugate base and hydronium ions.

HA + H2O  A- + H3O+

For a strong acid, this reaction is irreversible.  When a weak acid dissolves in water, the reaction in reversible and will go both ways. 

The pH of a substance is calculated using the following equation:

pH = -log [H3O+]  (Equation 1)

The pH scale ranges from 1 to 14.  1 is considered the most acidic and 14 is the most basic.  A pH of 7 is neutral.

Red cabbage juice will be mixed with different chemicals and different colors will be produced.  The pH of the chemicals can be approximated using illustration 1:

    Illustration 1

In this experiment, we are going to first determine the pH of various household substances using two different indicators.  Once we have determined the pH, we are going to estimate the hydronium ion concentration by solving equation 1 for [H3O+].  We can rearrange equation 1 and solve for hydronium as shown:

[H3O+] = 10-pH(Equation 2)

Procedure

1. Chop cabbage into small pieces.

1. Put cabbage in a large pitcher and then cover with boiling water, let stand for 10 minutes or more (the more you wait the more potent your indicator).  This step will produce a strong odor.  You might want to set it outside before you pour the water on the cabbage.

1. Remove the cabbage from the liquid.  Discard the cabbage and save the liquid.

1. Label each cup with the name of one of the liquids.  

1. Pour about 100 ml of each individual solution into its respective glass.  For baking soda, add 3 tablespoons of baking soda into 100 ml water.

1. Add approximately 50 mL of red cabbage juice indicator to the various solutions until you see a color change.

1. Record your results in the data table.

1. Discard of the solutions in your sink.  Rinse and dry the cups.

1. Label each cup with the name of one of the liquids.  

1. Pour about 50 ml of each individual solution into its respective glass.  For baking soda, add 1.5 tablespoons of baking soda into 50 ml water.

1. Dip a small section of pH paper into each of the glasses.  Compare the color of the pH paper to the color on the pH paper container.  Record your results in the data table.

1. Add 10 teaspoons of baking soda to each of the above solutions.  Observe the color changes and record them.  Take the pH of each solution again, this time using only the pH paper.

Data

Questions

1. If the pH paper turns red, is the substance acidic, basic, or neutral? Does the red cabbage juice test agree with the pH paper or not?

1. If the pH paper turns blue, is the substance acidic, basic, or neutral? Does the red cabbage juice test agree with the pH paper or not?

1. If the pH paper does not change color or the red cabbage juice does not change color, what does this tell you about the substance that was tested?

1. Which substance was the most acidic and what was its pH reading? Which was most basic and what was its pH reading?

1. Does the addition of baking soda alter the pH of weak acids/bases?  Why or why not?

1. Does the addition of baking soda alter the pH of strong acids/bases?  Why or why not?

1. Which is more accurate- the pH paper or the red cabbage juice and why?

1. Did anything in this lab surprise you? Why?

1. How does a difference in 1 pH unit affect the H+ concentration? For example, how does the concentration of hydronium in a solution with a pH of 3 differ from one with a pH of 4? Which one is stronger or weaker? Why?

1. Look at the ingredients for each liquid you tested. Which ingredients contribute to the liquid’s pH?  (Try to categorize these by acidic and basic components.)

1. Whenever you mix an acid with a base, they neutralize each other. If this is the case, why is Alka- Seltzer used to treat stomachaches?