NAME________________________ SECTION ____________

PARTNER(S) ________________________ DATE

EXPLORING ACIDS AND BASES

Part I: Properties of Acids and Bases

As you already know, it is not advisable to taste chemicals in an effort to analyze or identify them. But you have had prior personal experience with the taste of many distinctive chemicals. Circle your response to the following.

From your general experience, you categorize the taste of lemon juice (it contains citric acid) as: sour bitter sweet

When you last got soap in your mouth (it contains traces of base), it tasted:

sour bitter sweet

An acid is a compound that produces H+ ions in water solution. Bases produce OH- ions under the same conditions. Compounds that do not form a surplus of either of these ions in aqueous solution are classified as neutral. This way of classifying compounds was developed by the Swedish chemist Arrhenius.

HCl(aq) " H⁺(aq) + Cl^-(aq) (in water) NaOH(aq) " Na⁺(aq) + OH^-(aq) (in water)

Indicators such as litmus and phenolphthalein (HPth) change color because of structural and light absorbing properties that change when they lose a hydrogen ion.

HPth(aq) Pth^-(aq) + H⁺(aq) (colorless) (pink)

HPth(aq) + OH-(aq) Pth^-(aq) + H2O(l) (colorless) (pink)

Universal indicator paper is paper that includes a number of indicators so that the paper changes pH at many pH values.

1. Access the simulation https://phet.colorado.edu/sims/html/acid-base-solutions/latest/acid-base-solutions_en.html. Choose My Solution. On the Tool menu in the lower right, choose the center tool, which is universal indicator paper. Universal indicator paper is paper that changes color depending upon the pH of the solution. For this tool, complete the following table by matching the color of the indicator paper to the pH color key in the top left. There are acid/base and strong/weak sliders that you can change in addition to choosing the concentration. After doing the colors of the pH paper,, switch the tool to the left tool, a pH probe that can measure pH exactly. HC₂H₃O₂ is acetic acid and can also have the formula CH₃COOH. NH₃ is ammonia and can also be represented as NH₄OH.

Sample	Example	Color of pH paper	Approximate pH (pH paper)	Exact pH using pH Probe	Order*
0.1 M Strong Acid	0.1 M HCl
0.001 M Strong Acid	0.001 M HCl
0.1 M Strong Base	0.1 M NaOH
0.001 M Strong Base	0.001 M NaOH
0.1 M Weak Acid	0.1 M HC2H3O2
0.001 M Weak Acid	0.001 M HC2H3O2
0.1 M Strong Base	0.1 M NH3
0.001 M Strong Base	0.001 M NH3

*compared to other solutions on pH scale (1 = lowest)

A. Now look at the picture of the molecules in the magnifying glass for 0.01 M strong acid versus 0.01 M weak acid. What main difference can you see? Note: in this simulation, you will see H₃O⁺ instead of H⁺. This is not the difference that were talking about.

B. For the same two solutions, switch the view to Graph. Record the Equilibrium Concentrations of all species except water for both solutions. What do you notice about the difference between strong and weak acids?

C. Repeat A and B for strong and weak bases. Are the results similar? Do you notice anything different?

2. Now switch to the Tool on the right, the light bulb connected to a battery and positive (red) and negative (black) terminals. Place the terminals into each of the following solutions and describe if and how the light bulb lights up.

Sample	Example	Brightness of light bulb	Order*
0.1 M Strong Acid	0.1 M HCl
0.001 M Strong Acid	0.001 M HCl
0.1 M Strong Base	0.1 M NaOH
0.001 M Strong Base	0.001 M NaOH
0.1 M Weak Acid	0.1 M HC2H3O2
0.001 M Weak Acid	0.001 M HC2H3O2
0.1 M Strong Base	0.1 M NH3
0.001 M Strong Base	0.001 M NH3

* Order of how brightly the light bulb lights (1 = brightest, some may appear the same)

A. How does brightness of the bulb relate to the amount of ions in solution?

B. How do the amounts of ions in solution relate to the (i) concentrations and (ii) strength of the acids and bases?