The tops of liquids are at equilibrium between gas phase and liquid phase molecules. This means that in a closed container, there is a layer of vapor above the liquid. This causes vapor pressure. If possible, this gas will diffuse until it is evenly distributed throughout the space it is contained in.

To demonstrate this fact, we connected 9 sealed 500mL Erlenmeyer flasks together with glass and rubber tubing. The flask on the left side was filled with ~300mL of concentrated Hydrochloric Acid. The flask on the right was filled with ~300mL of concentrated NH3. The middle seven flasks were filled with ~300mL of water. Thymol Blue pH indicator was added to each of the flasks so we can see the pH changes in each of the flasks caused by the gas. At first the HCl solution was pink, the NH3 solution was blue, and the water was yellow. The flasks were all connected using glass and rubber tubing.

The vapor built up in the bottom flasks, it began to travel up the tubing into the second flasks. The gas dissolves in the water and slowly changes the pH. The pH of the water on the acid side goes down and the pH of the water on the basic side goes up. Once the pH on the acid side reaches 2.8-1.2, the color changes from yellow to pink. Once the pH on the basic side reaches 8.0-9.6, it changes from yellow to blue. Overtime, through the changing colors of the flasks, you can see how the gas is diffusing through the system.


The first time this experiment was set up, only the first flask on the acid side and basic side changed pH enough to change the indicator color. The experiment was set up again using ~500mL of each NH­3 and HCl in 500mL side arm flasks. The seven 250mL side arm flasks in the middle were filled with ~100mL of deionized water. Thymol blue was added to each of the flasks again. The flasks were connected with glass and rubber tubing with a wider inner diameter than the first attempt.

The indicator color still only changed in the first flask on the acidic side and the first 3 flasks on the basic side. Testing with pH paper showed that the HCl and NH3 were diffusing up the flasks, but the concentration wasn’t changing enough to change the indicator color.

2.5 L of HCl can be purchased for $87.40. 500mL of HCl would cost $17.48. 2.5 L of NH3 costs $87.30, so 500mL cost $17.46. It takes 0.1g of thymol blue solid to make 250mL of thymol blue solution. We used ~1mL of this solution. Thymol blue costs $30.20 per 5 g. This means we used less than $0.01 worth of indicator. 


Amount used


Hydrochloric Acid

500 mL



500 mL


Total cost of chemicals in this experiment was $34.94