Red Cabbage Lab: Acids and Bases
Introduction:
Liquids all around us have either acidic or basic (alkaline) properties. For example, acids taste sour;
while, bases taste bitter and feel slippery. However, both strong acids and strong bases can be very
dangerous and burn your skin, so it is important to be very careful when using such chemicals. In
order to measure how acidic or basic a liquid is, one must use the pH (potential of hydrogen) scale as
illustrated below:
pH Scale
The pH scale ranges from 1 to 14 where 1 to 6 is classified as acidic, 7 neutral (neither a base nor an
acid) and 8 to 14 is classified as basic (or alkaline). The strength of the pH scale is determined by the
concentration of hydrogen ions (H+), where a high concentration of H+ ions indicate a low pH (i.e.
acid) and a low concentration of H+ ions indicate a high pH (i.e. alkaline or base).
Indicators:
An indicator is a chemical substance that will change colour to indicate pH. There are many different
kinds of indicator, including natural substances such as red cabbage and beetroot juice.
In this lab, you will use the juice from red cabbage as a pH indicator to test common household
liquids and determine their pH levels. You will mix cabbage juice with different household liquids
and see a colour change produced by a pigment called flavin (an anthocyanin) in red cabbage.
Through this colour change, you will be able to successfully identify the approximate pH of common
household liquids using the table below:
Materials:
Safety glasses
Test tubes
Test tube rack
Pipettes
Red cabbage indicator (AKA cabbage juice)
Other available indicators: e.g. Universal Indicator, Litmus paper, pH meter
Assortment of available acids and bases whose pH you will test
Procedure:
1) Safety glasses are to be worn at ALL times. You DON’T want to get acid in your eyes!
2) Place test tubes in rack
3) Pipette a small volume of each liquid acid and base into separate test tubes and label
appropriately below Note: i) Wash or use a new pipette for each liquid. ii) Do not mix acids
or bases – one liquid per test tube.
4) Make a prediction about the pH of each liquid below (or whether you think it is acidic,
neutral or basic).
5) Using a new pipette, add a few drops of an indicator to a test tube and record your
observations below (i.e. colour change). Note: Universal Indicator may have its own dropper.
6) Repeat for remaining test tubes.
7) Repeat previous steps using a different indicator (or share your observations with another
group that has used a different indicator).
8) Clean up when you have finished.
9) Research the actual pH of the liquids that you just tested and write below.
10) Answer questions below.
Liquid/Test
tube #
Predicted pH
(Acid, Neutral or
Base)
Cabbage: colour
change / pH
Universal
Indicator: colour
change / pH
Litmus paper:
colour change /
pH
pH meter:
colour change
/ pH
Q1. How does a difference in 1 pH unit change in terms of H+ concentration? Example: How does a
pH of 3 differ from pH of 4? Which one is stronger or weaker? Why?
Q2. Look at the ingredients for each liquid you tested. Which ingredients contribute to each of the
liquid’s pH level?
Q3. Neutralization: When you mix an acid with a base, they neutralize each other. If this is the case,
why is Alka-Seltzer used to treat stomach aches/Heartburn? (Note: excess stomach acids cause
stomach aches). We will look at this in more detail in the coming lesson/s.
Actual pH
(look it up)