The disappearing liquid

Abstract

A one-liter (Pyrex) volumetric flask is modified by replacing the neck with a narrower tube (wide enough to permit insertion of a magnetic stirring bar) about 35 cm long, and adding a septum adapter to the side of the base. The procedure is as follows:1. A magnetic stirring bar and 500 mL of 95% ethanol (which can be colored with methylene blue to make it more visible) are added to the flask.2. Water which has been similarly colored is added to a bulb to which are attached a tube and hypodermic needle. Air is removed from the tubing and the needle is inserted into the magic flask via the septum. The water, being of higher density than the alcohol, will form a layer under the alcohol without mixing if care is taken.3. When the alcohol has reached the top of the neck the needle is gently removed.4. The flask is placed on a magnetic stirrer.5. When the magnetic stirrer is turned on, the water and alcohol mix; a decrease in volume occurs and the liquid level in the neck drops sharply - from 15 to 25 cm, depending on the care taken in filling the flask.It is possible to perform the experiment without modifying the volumetric flask. This can be done by adding 500 - 600 mL of 95% ethanol to the volumetric flask, in which a stirring bar has been placed. Then:1. As above.2. Slowly add water (also colored) to the bottom of the flask by means of a funnel attached by a rubber tube to a long glass tube (a 10-mL pipet would be suitable). This can take about 15 min.3. When the volumetric flask has been filled to the top, the flow is stopped with a pinch clamp and the tube is gently removed.4. The void left at the neck of the flask is completely filled with ethanol.5. As above. The decrease in height is 8-10 cm.Two questions have to be answered: (i) Where did the liquid in the neck go? (ii) Why do bubbles form on mixing?The answer to the first question lies in the partial molal volumes of alcohol and water, or the fact that the density of the mixture is greater than the average density of the two pure liquids. This can be demonstrated by using a 2-foot cylinder (4 in. i.d.) half filled with 1/4-in. styrofoam balls, on which are layered 1-in. balls to fill the cylinder to the top. Mixing the balls results in a clear decrease in the total volume of balls in the cylinder.The answer to the second question is that the solubility of air in the two liquids is significantly different, and on mixing the air is displaced owing to a change in its activity.Caution: Ethanol is extremely flammable. Care must be taken to ensure that no sparks or flames are near the demonstration.