Dem. 44%Stability of Nitrates (Ca, Cu, AgJ

To show: The relative heat stability of nitrates of Groups I-A, I-B, and 11-A.

Materials: Ca(NOa)p, Cu(NO&, AgN03, H-3.

Procedure: Separately heat 2 g of each salt in H-3.

Observations: No reaction with Ca(N03)~. The others decompose to give off brown NOz, and form black CuO and brown AgzO. (1) 2Cu(NO& + 2Cu0 + 0% + 4N0,. (9) 4AgN03 - 2Agz0 + 4N02 + 0,. (3) Ca(NOa)? decomposes only at higher temperatures, forming Ca(NOz)z + NZ + O2 + NOt, and at still higher temperature CaO.

To show: Sequestration, the removal of multivalent cations by anions, to permit sudsformation.

Alaterials: CaClz-aq, Calgon-aq, phenolphthalein in- dicator, NaOH-aq, soap-aq, C-2.

Procedure: Project C-2 with both cells half-full of distilled water. (a) To tube 1 add 5 drops CaCh- aq + a 2 drops phenolphthalein indicator + 1 drop NaOH-aq + 10 drops soap-aq. (b) Ditto to tube 2, but also add half a dropperful of Calgon-aq. Stir both solutions vigorously.

Observations: Tube 2 fills with suds; tube 1 does not. In the former Calgon has removed Ca++.

Reactions: Ca++ + 2NaZ 2Na+ + CaZz where Z is a complex silicate.

Reference: DAUGHEHTY, T . H . , J. CHEM. EDUC., 25, 482(1948).

C. Zinc Dem. 4 5 1-Amphoteric Zn(OHJ,

To show: The amphoteric nature of zinc hydroxide. NeOH NaOH

ZnCl. a Zn(OH). F? N&ZnOs . .~ HCI HCI

d4aterials: ZnCh-aq, NaOH-aq, HC1-aq, stirrer, C-3.

Procedure: Project C-3 with each cell half-full of water. To each add 10 drops of ZnC1,-aq, followed by sufficient drops of NaOH-aq to give a permanent precipitate of Zn(OH)? iu all three cells. Stir. (a) Add HCI-aq to cell 1. (b) Add NaOH-aq to cell 3 and stir.

Observatims: The precipitate acts like a hydroxide in cell 1, giving salt + water with an acid. It acts Like an acid in cell 3, giving a salt + water with a hydroxide. In cell 1 it behaves like Zn(OH)z; in cell 3 it. behaves like H,ZnO,. This dual role is called amphoterism.

D. Cadmium Dem. 452-Zinc Sulfide Hecied in Air

Experiment developed by James A . Young.

To show: ZnS heated in air gives off SO2 which will decolorize dilute I<Aln04-aq.

Materials: ZnS, RMnOn-aq, dil. HnSOa, cell H-1.

Procedure: Put 2 g ZnS in the culture tube H-1 and lead the delivery tube into I<i\'InOa-aq, diluted to a faint pink; add 1 ml of dil. H2SOn. Project. (CARE. Be sure to loosen stopper before removing heat, to prevent liquid sucking back into generator.) Heat.

Obserualions: SO2 forms and bleaches the I<h'ln04.

Reactions: (a) ZnS + 0% (air) - ZnO + SO2

( b ) 2KMnOl + 5SOn + 2Hn0 - &SO4 + 2MnSO,+ 2 HISO, pink colorless

Dem. 453-Cadmium Ion + 5--

Experiment developed by James A . Young,

To show: Formation of cadmium sulfide.

Materials: Cd(NO&aq, (NHl)tS-aq, diagonal drop- per, hlock SD-133, GI.

Procedure: Set dropper hlock on G1 two-thirds full of water and project. Put Cd++ into one dropper and S-- into the other and insert in block. Allow the reagents to mingle.

Obsewalion: heavy yellow CdS precipitates.

E. Mercury Dem. 454--Pharaoh's Serpents

To show: Voluminous ash from burning mercuric thiocyanate.

Materials: Pharmh's serpent from "trick shops." (If not available, precipitate from Hg(NO&aq + KCNS-aq, dry, knead into a tiny cone with an equal volume of dextrin.)

Procedure: Place the cone in a coke cap in H-4; project; ignite with a match.

Obseruations: The serpent which grows upon the screen is more terrifying than the ash which forms in a macro demonstration on the lecture table.

CORRECTIONS

Dem. 381. Substitute "steady-state" for "equilibrium". Irving G. Young of Honeywell, Inc. points out that the thermodynamic criteria of equilibria do not apply here. Dem. 383. Substitute CHaCOONH4 for NHhC1. Philip G. Chen of Atlantic Union College points out that NH&I itself decolorizes pink phenolphthalein. Citrate or form- ate can also be used. If only NH,CI is available, show common ion effect by a pinch of NHGI decolorizing pink NH.OH more than it does NaOH-sq of equal pinkness.

A1054 / Journul of Chemical Education