Terry M. Bella
Hands-on activities are our most effective ways to help our students develop their understandings of complex topics as this allows them to apply abstract concepts to real-world activities. Grasping the concept of pH can be difficult for students because it is so abstract. Thankfully universal pH indicator allows students to see that pH change is occurring. Students can then make connections between the indicator color change and the amount of free hydrogen ions in solution.
Shells in Acid
Many iterations of this activity can be found through an online search, I am providing the general format for the activity. Students will be exposing seashells to vinegar for 24-48 hours in order to observe the corrosive effects of the acid on the shells.
You want to begin with a discussion of what shells are made of and how these organisms have pulled carbonate and calcium out of the water to form them. Discuss how this biological process can be reversed by exposing the shells to an acid that will react with the calcium carbonate, breaking down the shell, essentially reversing the reaction. You will want to acquire an assortment of seashells, either commercially or by collection, wash them and allow students to observe and handle them. Allow students to select a shell for experimentation and instruct them to take a digital photo for reference purposes. Students will then place their shells in a beaker of vinegar for 24 to 48 hours. They will note the initial reaction of the shell with the vinegar as bubbles of carbon dioxide form. Note that the ocean will not likely ever become as acidic as vinegar (pH 2) but nonetheless the effect of acid on the calcium carbonate shell is the same. The strength of the vinegar only accelerates the reaction, albeit significantly. In sufficient time (1-2 days) remove the shells, wash off, and compare the condition of the shell to that of the digital image.
Rainbow Tornado
This is a demonstration of the color change capability of universal indicator. This requires a 1000ml glass graduated cylinder, large magnetic stir bar, stirring plate, deionized water, universal pH indicator, 0.1M NaOH, and 0.1M HCl. Fill the cylinder with water and add 5ml of universal indicator. With the cylinder on the stirring plate and the magnet in place adjust the stirring until you achieve a “tornado” effect. There will be a cavitation and the typical cyclone appearance. Add either the acid or the base drop by drop. What will happen is you can momentarily observe different areas of the water column at different pH, as indicated by the indicator. Wear to gloves and goggles when handling chemicals. During the demonstration discuss with students how the color of the column is indicating the amount of free hydrogen ions. The color scale for universal pH indicator is readily available online.
This demonstration should be preceded by or in conjunction with a discussion about pH and the pH scale. The demonstration is used expose students to universal indicator in fun way and can be followed by an activity wherein students test the pH of various chemicals and determine pH via indicator color.
Bubbling Carbon Dioxide in Water
This activity will allow students to visualize and observe the effect of carbon dioxide on pH. They must first understand that dry ice is a solid form of pH and as it changes from a solid to a gas some of the gas will dissolve into the water. This activity should follow any activity that allows students to become familiar with universal pH indicator and the pH scale.
This is an activity that students can perform themselves. The supplies needed are dry ice, 250ml beakers, deionized water, and universal indicator. Students will fill the beaker with water and add 1ml of universal indicator. They then place a small piece of dry ice in the beaker and note how the pH of the solution drops as indicated by the color change. This can be done in a more controlled manner by bubbling carbon dioxide into the beaker if you have compressed gas available.