Terri Donatello [ST Edwards School, science]
Heat Transfer and Density
Next Terri had us repeat the experiment with salt water, which we made by dissolving salt in tap water. The ice cube was tap water, as above. In this case, the water near the cube (at the top) cooled much faster than the water at the bottom, the opposite of our first result. In this case the salt water, being denser than the tap water, impaired the ability of the cold (tap) water from the melting cube to fall to the bottom of the beaker, as the salt water in the beaker and this water from the cube were now about the same density as each other.
Then Terri demonstrated heat by conduction, when heat is transferred by a warmer object touching a cooler object. Here there were two thermos-like cups, one of which was filled with room temperature tap water and the other with hot water. A U-shaped metal bar was then placed with one end in the cold water and the other in the hot water. We watched the thermometers as heat was conducted through the bar, raising the temperature of the cold water by conduction, and lowering the temperature of the hot water.
Finally we felt radiant heat by holding our hands near the hotplate that we had used to heat the water.
Ron Tuinstra [Iliana Christian HS, chemistry]
Honing Observation Skills with an Exercise Using Catalysts
Ron started with two 250 mL Erlenmeyer flasks, and each contained about 50 mL of a clear liquid (it looked like water). Then Ron heated a Platinum wire red hot with a lighter, and placed it in the mouth of the first Erlenmeyer flask. We watched as the wire rapidly lost its red glow as it cooled. Nothing unusual here! He did the same with the second flask, and -- amazing to see -- the wire continued to glow! The first flask contained water, but the second contained methanol. The Platinum wire catalyzed the decomposition of the methanol vapors, releasing oxygen. The heat released from this reaction keeps the wire hot (glowing). We turned out the lights in the room to see this clearly, and it was very dramatic!
Secondly, Ron placed a 500 mL Florence flask on the table. It was stoppered and covered with Aluminum foil, so that we could not see inside. As we watched, Ron removed the stopper, and in about 30 seconds a plume of steam boiled out the top of the flask! How Come!? Ron had placed about 50 mL of 30% hydrogen peroxide in the flask. A gram of manganese dioxide had been suspended by a string above the hydrogen peroxide.. The stopper held the string in place until Ron removed it, causing the manganese dioxide to drop into the hydrogen peroxide. Manganese dioxide acts as a catalyst for the decomposition of the hydrogen peroxide. The heat given off as a result of this reaction causes a vigorous boiling, which results in a dramatic plume of steam coming out the top of the flask, typically in about 30 seconds. The manganese dioxide can be recovered from the water (which remains after the decomposition) and reused!
Pat Riley [Lincoln Park HS, chemisty] Conservation of Matter
Pat brought in some portable, top-loading digital balances. Each group had three small, stoppered bottles labeled A, B, and C. A and B each contained a few grams or so of a white powder, and bottle C was empty. First weigh each bottle with stopper. (Note: it is important to keep the cork stoppers with their respective bottles, because of the variation in mass among stoppers.) Then pour the contents of bottles A and B into bottle C, replace the stoppers, and reweigh all three bottles. Then continue by shaking bottle C . We saw the white powder change to a creamy color. Then the bottle got cold, and there was an ammonia smell that was detectable despite the stopper. A contained Ba(OH)2, and B contained NH4Cl, which reacted to make ammonia (NH3), Barium Chloride (BaCl2), and water (H20). We reweighed the bottle C (at least before a great deal of the ammonia had escaped through the cork). Its mass was more or less unchanged from when A and B were added together. The comparison of total masses demonstrates (within experimental error) that everything we put in bottle C remained there. This occurred even though the various atoms were rearranged in the chemical reaction that occurred. ... Eventually, of course, most of the ammonia would escape, decreasing the total mass of bottle C.
Notes prepared by Benjamin Stark.