Lesson 4 - Page 1




1. about acid and base indicators
2. what acids and bases are


The word acid is derived from the Latin acidus meaning sour or tart. Properties commonly associated with aqueous solutions of inorganic acids are: sour taste; interaction with metals such as zinc and magnesium to liberate hydrogen gas; interaction with bases to produce a salt and water; ability to change the color of litmus paper from blue to red; and the reaction with carbonated water to liberate carbon dioxide. These properties are due to the hydrogen ions, H+, released by the inorganic acid in an aqueous solution.

An inorganic base is a substance that liberates hydroxide ions (OH-) in an aqueous solution. Aqueous solutions of bases are called alkaline or basic solutions. They have the following properties: a bitter taste; soapy feeling; the ability to change litmus from red to blue; and the ability to interact with acids to form a salt and water.

Students will learn about acid and base indicators in this experiment. An indicator is a colored substance usually derived from plant material, that can exist in either an acid or base form. By adding a small amount of an indicator to a solution and noting its color, it is possible to determine whether it is in the acid or base form. This in turn, can be used to determine whether the solution itself is acidic or basic. Common indicators are phenolphthalein, methyl red, bromthymol blue, and thymol blue, and litmus.


30 250 mL beakers, solutions to test, water


In the first section of the experiment the instructor is to collect 10 solutions that the students will test using pH paper. Some suggested solutions include the following: baking soda, epsom salt, table sugar, table salt, vinegar, ammonia, soft drinks, milk. Be sure that the test solutions are not very concentrated so as to not color the solution very much. Too much color in the solution will not allow the students to see the color change on the pH paper. Use water as the solvent (50-100 mL) and then add a few milliliters of solute. The students are to reference the color change with the color codes located on the pH paper containers. Following this the students are to determine whether the solution was acidic or basic based on pH.

In the second section of the experiment the students will perform a "color changing" experiment using indicators, vinegar, and ammonia. The vinegar and ammonia have been diluted to 1:3 parts water. Be sure to remind the students to be careful that the solutions do not get into their eyes. Directions are listed on the student sheets.




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