Demonstration 7: Silent Demo--Reactivity and Concentration

As the concentration of an acid decreases, reactivity with metals decreases also.

• Materials
  
  6 M Hydrochloric acid, HCl
  Three samples ( about 4 cm) of magnesium metal ribbon
  An overhead projector or a light box

• Safety
  
  6 M HCl is corrosive and must be handled eith care. Goggles should be worn.

• Directions
  
  Inform the students that this will be a silent demonstration for both the teacher and students. Information can, however, be written on the board. Place one piece of Mg ribbon into 50 ml of HCl in a 250-ml beaker. With a large-face timer or stop-watch, indicate the time needed for the Mg to react completely on the board. Show the students a second beaker with 50 ml water; then pour the acid into the water beaker. Again note the time required for a complete reaction. The teacher can continue the dilutions as long as there is an observable reaction. The discussion should contain other examples where concentration is a big factor in reaction rates and toxicity. Hopefully the students will begin to understand the importance of developing a quantitative method of expressing concentration, which will follow in the lesson.

Suggestions for Other Demonstrations

Bo, Sk, Su, and Sq refer to monographs by Borgford, Shakhashiri, Summerlin, and Sarquis listed in References. Caution: Check each reference for appropriate safety guidelines before performing these demonstrations.

Natural Indicators - Extractions of colored dyes from plants that can function as acid-base indicators. [Sk v3 50; Sq CF 27; Bo CA 92]

Properties of Acids and Bases - Demonstrates classical properties of acids and bases. [Sk v3 58; Su v2 171]

pH of Familiar Products - pH measurement of household chemicals using indicator solutions or a pH meter reveals that many substances are acids and bases. [Sk v3 65; Bo CA 133]

Disappearing Ink - A blue "ink" spot on white cloth is caused to fade and disappear. [Su v2 174; Sq CF 325]

Invisible Painting - A pattern is painted on a sheet of paper with phenolphthalein (colorless) and the pattern is developed when the paper is sprayed with sodium hydroxide solution. [Sk v3 47]

Cylinders of Color - Create color changes in solutions containing several different acid-base indicators. [Sk v3 114 (uses Dry Ice); Sq CF 127 (uses Alka Seltzer(TM)), Su v1 38]

Acid-Base Nature of Common Gases - The properties of several gases are revealed: carbon dioxide in breath [Bo CA 15], sulfur dioxide [Su v2 165]; and ammonia [Sq CF 59].

Sealed-Bag Reactions with Acids and Bases - A reaction occurs among three substances mixed in a sealed-plastic bag [Sq CF 345] or between two substances in a sealed-plastic bag [Sk v3 100].

Acid Base Titration/Conductivity - Conductiometric titrations of barium hydroxide and sulfuric acid. [Sk v3 152]

Making Hydrogen Gas from an Acid and a Base - Hydrogen gas is collected in a balloon attached to each of two flasks. One flask contains aluminum foil and hydrochloric acid; the other, aluminum foil and sodium hydroxide. [Su, v2, 33]

Acid-Base Indicators: A Voice-Activated Chemical Reaction - Allow students to speak into a flask containing bromthymol indicator. After several tries, the solution will turn yellow. [Su, v1 40]

Miscellaneous Applications - Measuring acid rain [Su CA 191], measuring soil pH [Bo CA 194], what's in baking powder [Bo CA 223], carbonates in rocks, minerals, and eggshells [Bo CA 17; Sq CF 305].

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