Tested Overhead Projection Series Compiled by HUBERT N. ALYEA
Princeton University 2. PHYSICAL PROPERTIES OF WATER Aqueous Tension (Continued) Dem. 103-Deliquescence,
Using a Balance
To show: The increase in weight of MgCIz expoeed to moist air. Materials: See-saw balance (SD-20-S); MgC12, filter paper, plastic cover 5 X Z1/2 X 11/2. Procedure: Project see-saw balance SD-20-S. Fold filter paper into a "&-in. square, soak thoroughly in saturated MgCL-aq., and place on the left pan. Place a similar, dry paper in the right pan. Add water dropwise to the latter, until the balance just dips. Cover balance with plastic case. Obsemation: Within 30 min, the balance tips to the side where the MgClz has absorbed moisture from the air. (Noerdin) SD-20-A
Variefy of Balancer
The balances pictured below serve a variety of purposes. Balance SD-20-K is a Knife-edge balance useful for accurate measurements. Balance SD-20-S is a See-saw balance useful for showing which of two objects is definitely heavier. Balance SD-20-V is a Visiblescale balance enabling the student to see the actual weights on the screen. Construction of balances. Balance SD-80-K. Purchase from Edmund Scientific Co., Barrington, New Jersey Catalog No. 0000 Price $15 iucludes balance and set of weights from 10 g down to 0.1 g. Unscrew metal base and discard. Replace with a plastic box-like base, as shown in the Figure. Balance SD-204. Two nails, points upwards, are set in the plastic base, as shown in the Figure. The seesaw teeters on two small conical holes in which the nail-points rest. The left pan can located in two positions; the right pan can slide along the arm to counterbalance objects placed in the left pan. The keel is added to give stability and strengthen the seesaw. A plastic cover placed over the see-saw protects it against air-currents. (Noerdin) Balance SDdO-V. This is like SD40-S but with a clear plastic keel along which two weights can be moved, set in notches. It is less sensitive than SD20-8 but enables the student to read what weights are on the pan. Of course the object and weights must be placed as centrally on the pan as possible. 3. CHEMICAL REACTIONS OF WATER
Sa2HP04.7H20,plastic cover 5 X Z1/2 X ll/z, fdter paper. Procedure: Project balance. Place pile of P206 on a a/l-inch square of filter paper on the left pan, and the Na2HPOr.7H,0 on a similar paper on the right pan, t o extremely small overweight. Cover with plastic case. Observations: Balance trips in a few minutes, depending on how closely balanced the two pans are. The Pz06deliquesces and gains weight, the Na2HPOI.7H20 effloresces and Ioses water and weight. (Noerdin) Dem. 105-Dehydrafian
of CuS0,5H10
To show: The addition of water to CuS04,and removal of water from CuS04.5H20 Materials: CuS04.5H10, conc. HzS04,methyl alcohol, H-3. Procedure: Place equal quantities of CuSOa.5Hn0(blue vitriol) in 3 culture tubes. Add water, conc. HzS04, and a quarter-tubefull of water to one, of conc. H&O, to the middle one, and of methanol to the other. Stir. Observations: The aqueous solution is blue due to hydrated cupric ion. I n concentrated HzS04anhydrous CuSOl is formed and falls as a powder to the bottom of the tube; the supernatant liquid is colorless. With methanol the copper salt dissolves to give a clear green complex. (Noerdin) Dem. I O b H e a f of Hydration of Lime
To show: Heat evolved when unslaked lime (CaO) is treated with water (slaked). Method I. Materials: Fresh lumps of CaO, iodine flakes, H-4 Grand Canyon, medicine dropper. Pvocedure: (a) Fill coke cap with lumps of CaO, place on H-4 and project. Add a few drops of water to the CaO. (b) Repeat, but this time place a few flakes of iodine in the cap, and rest this on a pile of smaller, fresh, lumps of CaO. Add 1or 2 ml of water to the lime. Observations: (a) The heat of hydration of the CaO is sufficient to generate a cloud of steam, visible on the screen. The lumps of CaO are seen to crumble. (b) Heat sufficient to sublime the iodine (over 180°C.) is generated, and violet clouds fill the screen. (Chris Kato, Lillian Munson, Randy Kay, Lakeside Summer School, Seattle, Washingten)
Method 8. Materials: Wide mouth 2-02 glass bottle, Mo, fresh CaO. (CARE: Do not use plastic box).
To show: Change in weight of substances gaining and losing moisture from the air.
Procedure: Place gas thermometer bulb inside of the glass bottle, and half-fill with lumps of CaO. Project. Add a few ml of water. (CARE: Violent evolution of steam).
Materials: See-saw balance SD-193; dry PzOs, dry
Observatins: There is a dramatic rise in temperature.
Dem. 104-Deliquescence
and Ffflorercence
Volume 40, Number 1 1 , November 7 963
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A885
SUPPLEMENTARY
DEVICES
BALANCES
SD-20-V.
Visible-scale Balance
SD-204. See-saw Balance
I
8
SD-20-K. Knife-edge Balance A886
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Journal of Chemical Education
