Chemistry 30

Properties of Acids and Bases - Lesson 1.1

Demonstrate some of the basic properties of acids and bases:

• Place small amounts of distilled water, an acid (dilute hydrochloric acid), and a base (dilute sodium hydroxide) in small beakers. Test with red and blue litmus paper. Litmus paper will turn (or stay) red in acids and blue in bases.
  
  
• Use a conductivity indicator to show that distilled water will not conduct an electric current, but both acids and bases form electrolytic solutions due to the presence of ions in solution.

  Conductivity can easily be demonstrated with a simple conductivity indicator which can be purchased from most scientific catalogues. Boreal used to carry an inexpensive version (search: "conductivity indicator"). With a little ingenuity one can easily be constructed with a battery, some wire leads, and a LED light from Radio Shack.
  

• Use a soap solution to demonstrate that bases have a slippery feel.
  
  
• Pass out some lemon slices to demonstrate that acids have a sour taste.
  
  
• CAUTION: Demonstrate that acids react with active metals such as zinc to produce hydrogen gas by dropping a small piece of zinc in a few mL of 6M HCl.

Reaction between Acids and Carbonates, or The Rubber Chicken and the Bouncing Egg

A characteristic of acids not listed in the notes is that they react with acids react with carbonates (substances containing CO₃^2-) to produce a salt, water, and carbon dioxide. Some substances that contain carbonate are egg shells, which contain calcium carbonate, and bone which also contains calcium carbonate in addition to calcium phosphate.

This demonstration is easy to do but must be set up several days in advance.

The Reaction

2 H₂C₂H₃O_{2 (aq)} + CaCO_{3 (s)} → Ca(C₂H₃O₂)_{2 (aq)} + H₂O_(l) + CO_{2 (g)}

The Bouncing Egg

• Place an uncooked egg (be sure the shell isn't cracked) in a clear container and cover with vinegar. Small bubbles will be observed as carbon dioxide is formed.
• Allow this to sit for three or more days.
• The acetic acid in the vinegar will dissolve the calcium carbonate, as shown by the reaction above. When complete the shell membrane will be exposed. The vinegar will toughen the membrane, but care will still be needed to hold the egg. The egg will feel rubbery, and will bounce if dropped from a height of a few centimeters.

The Rubber Chicken

• Place a chicken bone - a wishbone works well - in a container and cover with vinegar.
• Small bubbles will be observed as carbon dioxide is formed.
• Allow this to sit for at least three days. Test the bone periodically for flexibility - a week or more in the vinegar may be required.
• The acetic acid in the vinegar will dissolve the calcium carbonate, as shown by the reaction above. Eventually the bone will be flexible enough so that it may be tied into a knot.

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Brønsted-Lowry Bases - Lesson 1.4

A Brønsted-Lowry Base is a substance that will accept a proton from an acid. This visual demonstration shows how this exchange can occur without the involvement of OH^- or H₃O⁺ ions.

Caution

• Use caution when handling concentrated hydrochloric acid. Wear google, gloves, and a lab apron.
• Do not leave the bottles unstoppered for long (about 10 seconds)

Procedure

1. Have a stoppered reagent bottle of concentrated HCl next to a stoppered bottle of concentrated aqueous ammonia solution.
2. With the bottles a few centimeters apart, unstopper both bottles.
3. A white cloud of solid ammonium chloride, NH₄Cl will form as the ammonia accepts a hydrogen from hydrogen chloride.

The Reaction

HCl_(g) + NH_{3 (g)} → NH₄Cl_(s)