How do we determine if a substance is acidic, basic, or neutral?
California State Standard(s): SC.8.5.e. Students know how to determine whether a solution is acidic, basic, or neutral.
Student Materials (per group) Additional Teacher Materials:
o 3-4 leafs of Red Cabbage o
o 250 ml Beaker
o Hot Plate
o 10 ml Vinegar
o 5 g Baking Soda in 50 ml of water.
o 1 Pipette
o Paper Towels
Background and Misconceptions:
Acids and bases are a special class of compounds that when they react with each other produce a salt and water in a process called neutralization. Generally, an acid is able to donate a proton while a base is able to accept a proton. Indicators are used to identify acids and bases. The indicators change color in response to the pH level, which is a measure of the strength of an acid and a base. Based on a scale of 0-14, the pH scale is used as an easy way to measure the number of hydrogen ions in a solution. Acids donate hydrogen ions (protons) into a solution. Acids accept hydrogen ions from a solution. A stronger acid causes a large increase in the number of hydrogen ions in the solution while a strong base accepts large numbers of hydrogen ions from the solution. When the solution is balanced its acceptance and donation of ions is equal and the solution is said to be neutral. Water is neutral and is assigned a number of 7 on the pH scale. Bases are numbers higher than 7, with the strongest bases assigned a number of 14. (The pH scale is a logarithmic scale.) Acids have pH's that are lower than 7, with the strongest acids assigned a number of 0.
Indicators change color to indicate pH and various indicators will react differently and produce different color ranges. Red cabbage juice in this activity will remain purple if the solution is neutral, turn red if the solution is an acid, and turn green if the solution is a base. Red cabbage is not as precise as some other indicators. Some indicators measure only over a very small range of pH. For example, Methyl Yellow operates only over a pH in the range of 2.9-4. However, universal indicators such as litmus paper operate over a much wider range, albeit with much less accuracy. pH indicators are only used to approximate pH. For much more accurate measurement of pH, a meter should be used.