Chemistry Reference
In-Depth Information
Material: Big glass bowl, plain cylinder with cover glass, test tubes, beaker,
tweezers, knife, burner, filter paper; sodium (F/C), lithium (F/C), phenol-
phthalein solution in ethanol (F), universal indicator solution (F).
Procedure: (a) Fill the glass bowl half-full with water and place it on the
overhead projector. Put a piece of sodium on the water surface and observe
the sodium ball. Repeat the experiment a couple of times.
(b) Fill the cylinder with water and open it under water with help of the
cover glass. Taking tweezers push a piece of lithium (not sodium!) into the
water under the cylinder and observe it. After the reaction invert the plain
cylinder and ignite the formed gas.
(c) Test both solutions in a test tube with each indicator solution. Let the
water from a part of the solution evaporate in the beaker.
Observation: In the bowl, the moving sodium piece, a fizzing sound, and gas
formation can be observed during the reaction. In the cylinder the colorless
gas is collected and ignited: a red flame can be observed for a short time.
The indicators change their colors: phenolphthalein from colorless to red,
universal indicator from red to blue. After evaporating the water solid white
substance remains in the beaker.
Disposal: Put remains of sodium or lithiummetal in ethanol and wait for their
dissolution. Dilute all solutions so that they can be disposed off down the drain.
E1.5.
Comparison of Petrol and a Solution of Fat in Petrol
Problem: Students think that the fat disappears when grease spots are being
removed from clothes by petrol: “the spots are gone irretrievable.” Students
have to understand that spot removing is nothing else but the dissolution of
fat in petrol and the evaporation afterwards, after the evaporation the fat
remains in the cloth.
Material: Petrol ether (F/XN/N), solution of olive oil in petrol ether, filter paper.
Procedure: Put some drops of petrol ether on one filter paper and some drops
of the fat solution on another filter paper at the same time. Observe both
papers for a minute.
Observation: The spot of the pure dissolvent shrinks and evaporates residue
free, the spot of the fat solution also shrinks, but the fat remains.
E1.6.
Burning Metals on a Balance
Problem: Due to everyday experience students believe that substances loose
mass and become lighter during combustion - like in the familiar combus-
tion of alcohol, wood, paper, or candles. Metal combustion shows that due
to the bound part of oxygen in the solid metal oxide instead of a mass
decrease there actually happens a mass increase. This can be demonstrated
in experiments on a balance. The masses of candles during combustion will
be observed later (E1.7).
Material: Beam balance, digital balance, porcelain crucible with cover,
tripod and clay triangle, crucible tongs; iron wool, magnesium ribbon (F).
Procedure: (a) Iron wool that hangs on one side of the balanced beam is to
be ignited; blow gently if necessary to accelerate the reaction and to see the
metal glowing.
