Follow

Chemistry

  • I1-64: BURNING CANDLE - COMBUSTION PROCESS

    I1-63
    Demonstrate features of the burning process and to debunk myths about this supposedly well-known demonstration.

    A common pre-college experiment is to burn a candle inside of a bottle which has been turned upside down over a container of water. The water supposedly rises about one-fifth of the way up the bottle, indicating that the oxygen, about one-fifth of the air in the atmosphere, has been "used up" in the combustion process.

    The candle in our experiment is placed inside the sealed tube containing air above a colored water bath, and is then ignited by a hot wire (center photograph). The water level goes down initially, then returns to its original level just after the candle goes out. There is virtually NO CHANGE in the water level from before to after the experiment is performed (photograph at right).

    This is the CORRECT WAY to do this experiment. Two experimental errors are often made when performing this experiment: (1) if the candle is lit before the bottle is placed over it, the air is initially hot, and will pull the water up the bottle as it cools, and (2) when the bottle is placed over the candle, the hot air from the candle flame expands, and some of it might escape out of the bottom opening of the bottle. Analysis of the chemistry of this experiment shows that the final products of combustion are actually more voluminous than the initial air, but other things happen to yield no net difference in the water level.
    I1

    i1-64a

    i1-64b

  • I4-35: LOWERING THE FREEZING POINT OF WATER USING SALT

    I4-35
    Demonstrate that the freezing point of water can be reduced by putting ice in a salt water bath.
    The temperature of an ice and water mixture is measured with a digital thermometer. Adding salt to the icewater mixture reduces its temperature. The thermometer can be used to stir the mixture. By this technique the freezing point can be lowered as much as 15 degrees celcius.
    I4

    i4-35a

  • I5-41: ENDOTHERMIC REACTION - ENTROPY

    I5-41
    Aid a discussion of entropy.
    31.5 grams of barium hydroxide and 15.2 grams of ammonium thiocyanate, both solid powders initially at room temperature, are mixed. As they are stirred using the digital thermometer probe a chemical reaction occurs, producing water with barium cyanate and ammonium, both of which are soluble in the water. The solution becomes very cold, and can freeze a wettened wooden block to the bottom of the beaker.

    This experiment can be described in terms of entropy using two approaches: (1) As the mixture cools, it must become a liquid, increasing its disorder so that entropy will increase, or (2) The tendency toward disorder drives the reaction, creating the liquid reaction product.

    Note: PLEASE CALL THE LECTURE DEMONSTRATION GROUP AT 405-5995 NOT LATER THAN THE MORNING BEFORE YOUR REQUEST SO THAT WE HAVE THE TIME TO PREPARE THE NEEDED CHEMICALS.

  • I6-24: DIFFUSION VELOCITY

    I6-24
    Show that the diffusion velocity is proportional to the RMS molecular velocity.
    Cotton swabs dipped into HCl and NH4OH respectively are inserted into the ends of the tube. HCl gas (molecular weight 36.5) and NH3 or ammonia gas (molecular weight 17) begin to diffuse inward. In less than 25 minutes they meet, forming a ring of ammonium chloride (NH4Cl). The diffusion velocity is proportional inversely to the square root of the molecular mass, because according to equipartition of energy the average molecular speed is inversely proportional to the square root of the molecular mass. This means that the distances traveled by each vapor before they meet are in the proportion:

    d(NH3):d(HCl)=SQRT(36.5/17)=3:2 approximately.

    Use caution when handling chemicals. Always wear protective gloves. Please do not throw used swabs in garbage; place in included water beaker instead for proper disposal.