Demonstrations

Purpose

These demonstrations allow students to observe a wide variety of chemical reactions, most of which should not be performed by students for safety reasons. Be sure to keep students actively involved by directing them to make observations and consider questions during demonstrations. They could be asked to write down observations.

Safety

Wear eye protection and an apron throughout the demonstrations. Thoroughly wash your hands after concluding the demonstrations. Additional safety precautions are given for each demonstration.

Demonstration 1: Magnesium Burning

Materials

Safety

Caution students not to look at flame directly.

The Demonstration

Light the burner. While holding the magnesium with the crucible tongs, light the magnesium in the burner flame. Point out the evidence that a reaction has occurred.

2Mg(s) + O 2 (g) --->2MgO(s)

Demonstration 2: A Penny in Nitric Acid

Materials

Safety

Conduct the reaction in the hood-the fumes are toxic. Nitric acid is highly corrosive.

The Demonstration

This reaction must be performed in a fume hood. (The fumes of nitrogen oxides are toxic.) With the hood operating, place the penny in the beaker. Add nitric acid. Observe all color changes and products released. After the penny has reacted, add a small quantity of water to the beaker to bring about a color change. (In this reaction colorless NO is initially formed; it subsequently reacts with O 2 to form colored NO 2 , which is what students observe.) They also note the color of the solution.

Cu(s) + 4HNO 3 (aq) ---> Cu(NO 3 ) 2 (aq) + 2NO 2 (g) + 2H 2 O(l)

Demonstration 3: Formation of a Precipitate

Materials

Safety

Properly dispose of the lead compound formed.

The Demonstration

Simultaneously pour the two solutions into the beaker. The brilliant yellow precipitate lead iodide, PbI 2 , forms. Students should recognize that a chemical reaction has occurred.

Pb(NO 3 ) 2 (aq) + 2KI(aq) ---> PbI 2 (s) + 2KNO 3 (aq)

Demonstration 4: Replacement Reaction

Materials

Safety

Properly dispose of the products.

The Demonstration

Pour silver nitrate solution into the large test-tube. Support the test-tube in a rack. Drop in the copper wire. The reaction will start immediately, but will be more obvious after a few minutes as the silver begins to deposit on the surface of the copper wire. The observed formation of the new product is evidence of reaction.

Cu(s) + 2AgNO 3 (aq)---> Cu(NO 3 ) 2 (aq) + 2Ag(s)

Demonstration 5: Sodium with Water

Materials

Safety

This reaction is striking and demonstrates vividly to students that chemical reactions not only result in a change in properties, but also often release energy. However, be aware that without the precautions specified here, this demonstration can be dangerous.

The Demonstration

Fill the Petri dish about half full with cold water. Be sure that the water is cold and the piece of sodium is no larger than a grain of uncooked rice. Drop the piece of sodium into the water. Observe the reaction. Sometimes the generated hydrogen will actually burn. Small scrapings or cuttings of sodium may be disposed of by dropping them in alcohol. Some chemical companies that supply schools sell an alloy of approximately 10% sodium and 90% lead that can be used in place of pure sodium.

2Na(s) + 2H 2 O(l)---> 2NaOH(aq) + H 2 (g)

Demonstration 6: Endothermic Reaction

Materials

Safety

Properly dispose of the products.

The Demonstration

Place the barium hydroxide in the flask. [CAUTION: Ba(OH)2 is very toxic.] Add the ammonium thiocyanate. Swirl the two solids for about 20-30 sec. Give the flask to a student to hold. Evidence for the reaction is an unexpectedly cold container, indicating an endothermic process.

Ba(OH)2 (s) + 2NH 4 SCN(s)---> 2NH 3 (g) + H 2 O(l) + Ba(SCN)2 (s)

Demonstration 7: Floating Pennies

Materials

Safety

Hydrochloric acid is a corrosive substance.

The Demonstration

Pour hydrochloric acid into the cylinder so it is about 3/4 full. Use a triangular file to scrape the edge of each penny to expose the zinc core. It seems to work best to scrape twice-on opposite edges of the penny. Drop the pennies into the hydrochloric acid. Bubbles will be seen immediately, but the pennies exhibit unusual behavior after about 24 hours. They will float and sink as their density approaches that of the surrounding acid. The hollow pennies are also interesting to examine after the reaction is completed. You can prepare "day old" pennies for this purpose.

Zn(s) + 2HCl(aq) ---> H 2 (g) + ZnCl 2 (aq)

Demonstration 8: Indicators

Materials

The Demonstration

Place beakers on an overhead projector. Add just enough acid solution to cover the bottom of each beaker. Add an indicator to each. Note color. Then add base dropwise until color change occurs. Be sure to swirl contents as base is added.

Demonstration 9: Aladdin's Lamp

Materials

Large round-bottomed flask or 500-mL Erlenmeyer flask
30 % Hydrogen peroxide, H 2 O 2 , 50 mL (see Cautions below)
Potassium iodide, KI, 2 g

Safety

30% hydrogen peroxide is dangerous. It should be very carefully handled and used. Wear gloves when handling. Under no circumstances should students handle 30% hydrogen peroxide.

The Demonstration

Support the flask with a clamp on a ring stand. Pour the hydrogen peroxide into the flask. Add the KI. Within a few seconds, the KI generates so much heat as it reacts with the hydrogen peroxide that it boils the water! Using a fancy spouted kettle might be amusing.

Discussion

This decomposition takes place in two steps:

H 2 O 2 (aq) + I - (aq) ---> 2H 2 O(l) + IO - (aq)

H 2 O 2 (aq) + IO - (aq) ---> H 2 O(l) + O 2 (g) + I - (aq)

The reaction is exothermic and produces oxygen gas and steam.

Demonstration 10: Ions Need to Get Together

Materials

2 Erlenmeyer flasks, 125-mL
Erlenmeyer flask, 250-mL
Potassium iodide, KI, 2 g
Copper(II) sulfate pentahydrate, CuSO 4. 5 H 2 O, 2 g

The Demonstration

Add the blue solid, CuSO 4 . 5H 2 O, and the white solid KI to a flask. Shake them together; let students see that nothing is happening. The two salts are still as they were originally. Put the mixture of two salts in a mortar and grind vigorously with a pestle. A brown color should be visible. Point out the effort it took to get the substances to react. Then dissolve 1 to 2 g of each salt in separate 100 mL samples of water in 125-mL flasks. Pour the two solutions together. Students observe an immediate, obvious formation of a brown "gunk." This demonstrates that ions need to get together to react. In the solid state they could not do that, but in solution the ions could readily "find" each other.

4KI(aq) + 2CuSO 4 (aq) ---> 2K 2 SO 4 (aq) + 2CuI(s) + I 2 (s)
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