LAB: RED CABBAGE INDICATOR
This unit has introduced you to some of the general properties of acids and bases. You’ve learned about the pH scale that helps tell you about the relative strength of acids (pH lower than 7) and bases (pH higher than 7). You’ve also learned about a few different compounds, called indicators, that can help you visually determine the pH of a liquid. Today, we will be making and using our own indicator to help us determine the pH of some common household liquids.
Students will follow a procedure to make their own indicator solution
Students will use their indicator to make observations of different household objects
Students will make predictions about the properties of the observed liquids
Students will explain the chemistry of a neutralization reaction
Acid: A substance that generally increases the concentration of H+ (H3O+) ions in a solution
Base: A substance that generally decreases the concentration of H+ (H3O+) ions, or increased the concentration of OH- ions in a solution
pH Scale: The scale used to quantify the strength of an acid or base relative to pure water (with a pH of 7)
Indicator: A compound used to visually determine the relative pH of a solution
Hydronium Ion: H3O+. Often created when an acid donates an H+ ion to a solution.
Hydroxide Ion: OH-. Often created when a base donates an OH- ion to a solution, or when a base removes an H+ ion from a solution.
Neutralization: A reaction involving an acid and a base where H+ ions combine with OH- ions to create neutral water (H2O)
Part 1: Making the indicator
One head of red cabbage (green cabbage will NOT work)
Large kitchen knife and cutting board
Large stock pot
Stove or other controlled heating surface
A pitcher or large bowl
A strainer or colander
Using the cutting board and kitchen knife, cut up your head of red cabbage into small pieces.
Place a few cups of your cut-up cabbage (2-4 cups will be enough) into your large stockpot
Pour water into your stock pot until the water level just covers the cabbage.
Turn the stove on to high heat and bring the water to a boil
Once the water starts boiling, turn of the stove and allow the water to cool (20-30 minutes)
Set out your storage container for your cabbage indicator solution, either a pitcher or a large cup or bowl.
Strain the cabbage mixture into your storage container, making sure to catch any large chunks of cabbage. Set aside your cabbage indicator solution for use later. You should have a few cups worth of indicator
Your cabbage indicator should have a deep purple color. Red cabbage contains the pigment anthocyanin that changes color when it is mixed with an acid or base. The pigment turns red when mixed with acids and turns blue or bluish-green when mixed with basic solutions. The amount of color change is related to the strength of the acid or base. A deeper red color would indicate a stronger acid than a light red or red-purple color.
Now we will find a few common edible acids and bases from your kitchen. Try to find any of the following. Aim for at least 4 or 5 liquids, but more is better. One of your liquids should be water.
Baking soda (dissolved in water)
Antacid (either dissolve an alka-seltzer tablet in water, or grind up a “Tums” tablet and dissolve in water)
Coffee or black Tea
Simple syrup (table sugar dissolved in water)
Pour out a small amount of your chosen liquids and let them come to room temperature. Now it’s time to apply what you’ve been learning from your lessons. Recall that acidic substances often taste “tart” or “sharp” and that bases often taste bitter and might have a slippery feel on your skin. Try tasting and feeling each liquid that you found in your kitchen. Keep track of your observations in the table below (no need to write full sentences, but try to use a few words for each observation). Keep a small portion of each liquid for use in part 3 of the project.
NOTE: The liquids used in this project are common food items and only weak acids acids or bases. You should obviously never taste or feel unknown liquids, especially those that are classified as strong acids or bases.
Part 3: Conducting the experiment
Use your background knowledge to make an informed prediction about the results of your experiment. Think about the different variables in the experiment and how they might affect each other. What variable are you changing yourself (independent variable). What variable are you measuring (dependent variable)? What things are you keeping the same throughout the experiment? Hypotheses are best in an “If/then format” that make a prediction between the variables in your experiment. For example: If I change [my independent variable], then I will see [these changes] in my [dependent variable]. For this experiment, you should try to predict which liquids will be an acid, base, or neutral.
Prepared cabbage indicator from part 1 (room temperature)
Small amounts of your acquired liquids from part 2, labeled (room temperature).
Enough clear cups or glasses for each of your liquids from part 2
A ½ or ¼ teaspoon
Tape and marker
Pour 1 cup of your cabbage indicator into a clear glass for each liquid you plan to test (if you don’t have enough cabbage indicator for each glass, you can use a smaller amount of indicator – just use the same volume for each)
Take 1 tablespoon of one of your liquids from part 2 and pour it into one of the glasses containing the cabbage indicator. Stir the mixture. Use the tape and marker to label the glass with the type of liquid you added.
Repeat step 2 with the rest of your liquids
Make observations about the color change in each glass. Try to be as specific as possible because some colors might be very similar. You are free to take pictures or use colored pencils to record your observations. You might want to create your own table to help organize your observations and make them easier to compare.
Once every mixture has had a chance to react and change color (no more than 5 minutes), identify a weak base from among your liquids (slight blue color) and your strongest acid (red color with very little purple left).
Take a very small amount of the original acidic liquid (¼ or ½ of a teaspoon) and add it to the weak base and cabbage indicator solution. Stir to mix the liquids
Make observations about any color changes that might have occured. If you didn’t notice any color changes, keep adding the acidic liquid in small amounts until the liquid returns to the neutral purple color of cabbage indicator mixed with water. Keep track of how much liquid you added until the solution turns purple
After gathering all of your data and observations, you can dispose of all your liquids and begin compiling your lab report
Which liquids did you identify as acids? Why?
Which liquids did you identify as bases? Why?
Which liquids did you identify as neutral? Why?
Could you tell the relative strengths of acids and bases in your experiment? How?
How much volume of acid did it take to neutralize the basic solution? If you added different volumes of the acid and the base to get a neutral color for the indicator, why did it take more amounts of one type of liquid to reach neutralization?
What do you think was happening to the ions during the neutralization reaction?
Write your conclusion. Don’t forget to discuss your hypothesis and use your data to support your conclusion. Talk about the concepts you learned and how they connect to the previous lessons and to real life. Evaluate the experiment itself and think about how it might be improved or expanded upon. Your conclusion should be 5 or more sentences in length. Look through the sample Lab Report Format for more guidance.
Summary of Submission Requirements:
Heading and unique title
Pre-lab, hypothesis, procedure, data, analysis, and conclusion section headings
Each section is complete and thorough
Completed lab report submitted through TurnItIn
Ignitia assignment submitted with TurnItIn Submission ID
Steps of Project Overview:
Downloadable copy of assignment
Lab Report Rubric:
Lab Report Format
Last Name Page #
A prediction of what you think the result of the project will be. Write your hypothesis before you begin the experiment. A common sentence form for a hypothesis is to use an “if…then…” statement.
List the materials that you used to complete the experiment.
Summarize the steps that you took to complete this experiment; the procedure should be written in 3rd person present tense, and it should give enough information for someone to repeat the experiment using only your procedure. Make sure to include any modifications you make.
Write a detailed description of your observations, results, and any measurements. Do not interpret or try to explain your data; that discussion belongs in your conclusion. Try to use a table to organize your data if you can.
Perform any calculations using your data and present the results here. Answer any questions from the prompt. Number your responses so that they correspond with the given questions. Do not type the questions themselves into this section; instead, restate the question within your response.
Begin by stating whether your hypothesis was true or false, and use specific data/observations/results to support your answer. Discuss how this experiment connects to some of the concepts you’ve been learning in this unit. How might these concepts apply to other areas of science or to your everyday life? Finally, discuss how the experiment went. Do you trust your results? What possible sources of errors or uncertainty were there? How might you improve the procedure to make the experiment better in the future (there is always something that can improve your data). What new questions do you have and how might you investigate them in the future (a general comment or two is fine)? A conclusion should be 5 or more sentences in length.
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