Why is the water changing its colour?丨DFRobot Science Lab EP05
Randolph 2020-05-19 12:07:09


Some liquids will show different colors at different pH values. In this lesson, we will explore what color will appear when the pH of these solutions changes, and try to make a ‘good-looking drink’.



From Molecules to Organisms: Structures and Processes - MS-LS1-5

Ecosystems: Interactions, Energy and Dynamics - MS-LS2-5 

Engineering Design - MS-ETS1-1, MS-ETS1-3 


BOSON Science Kit Materials

1、Battery Holder w/3x AAA batteries installed


2、MainBoard-3IO (m1)


3、Display Modulen(o11)


4、pH Sensor (i17)&pH probe


5、BOSON cable 

 1 X short 1 X long


 Additional Materials 

1、Clear Plastic Cups




3、Different liquids


4、Butterfly bean flower or purple cabbage juice




In addition to lemonade, the several experimental liquids in this chapter are like white vinegar, baking soda and bleach are absolutely not allowed to drink! Although the color of white vinegar and butterfly bean juice is very beautiful, shouldn't you want to drink vinegar directly?

Many drinks are sold at night markets or school celebrations, such as black fairy grass honey or orange papaya milk. But have you ever seen a drink containing two or more colors? This colorful drink is usually achieved by using the juice of the special plant such as butterfly bean flower, and then adding different pH solutions to achieve the effect of discoloration. Children can use the experiments in this chapter to find out which solutions and butterfly bean juice can produce the most beautiful color.

● Pour four different liquids into butterfly bean flower or purple cabbage juice

● Use pH sensor and display modulen to detect the pH of these liquids 

● Record the detected values in a table 

● Discuss the relationship between the pH and color of these liquids



In this activity, your group needs to mix the liquids together and use BOSON pH sensor to detect pH value when color changed. 

Activity Procedure:


1.Prepare Butterfly Bean Flower Juice.(Or purple cabbage juice, the description will not be repeated later)Immerse the butterfly pea flower in hot water for about 30 minutes, and find three containers to distribute them equally.


2.Connect the pH sensor according to the figure below, control the module and the display module, and switch the display module switch to i17 to display the pH value. 


3.Remove the front cover of the pH sensor and clean the front glass beads with pure water.


4.First use the pH sensor to measure the pH value of butterfly bean flower juice, and record it in the table below. As shown on the right, the measurement result is 7.3, which means butterfly bean flower juice is a neutral solution. In addition, the pH value of white vinegar, baking soda, lemonade, and bleach should be recorded after the initial measurement.


Tips:Before measuring different solutions, flush the sensor with pure water again, otherwise the measurement results will be inaccurate.

5.Slowly pour the white vinegar into the butterfly bean juice and carefully observe the color change. After standing for 30 seconds, measure the pH again.


6.Pour the remaining baking soda, lemon drink, and bleach into the butterfly bean juice, observe the color change, and record it in the table below. Is it colorful ?


From left to right are white vinegar, baking soda, lemonade and bleach

Data Record

Copy the Data Table below into your notebook. 



Observe and think: 

1.Observe, is there a relationship between the acidity and alkalinity of the final mixed solution and the color depth?

2.Take a walk to the market and see if the butterfly bean flower beverage has a common taste?




     ●Acid-base indicator solution

Anthocyanins are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue or black. Food plants rich in anthocyanins include the blueberry, raspberry, black rice, and black soybean, among many others that are red, blue, purple, or black. Some of the colors of autumn leaves are derived from anthocyanins.

Anthocyanins belong to a parent class of molecules called flavonoids synthesized via the phenylpropanoid pathway. They occur in all tissues of higher plants, including leaves, stems, roots, flowers, and fruits. Anthocyanins are derived from anthocyanidins by adding sugars. They are odorless and moderately astringent.


 Anthocyanins generally are degraded at higher pH. However, some anthocyanins, , are resistant to degradation at pH 8 and may be used effectively as a food colorant.

Use as environmental pH indicator 

Anthocyanins may be used as pH indicators because their color changes with pH; they are red or pink in acidic solutions (pH < 7), purple in neutral solutions (pH ≈ 7), greenish-yellow in alkaline solutions (pH > 7), and colorless in very alkaline solutions, where the pigment is completely reduced.



Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often adsorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity.

The main use of litmus is to test whether a solution is acidic or basic. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the pH range 4.5–8.3 at 25 °C (77 °F). Neutral litmus paper is purple.Litmus can also be prepared as an aqueous solution that functions similarly. Under acidic conditions, the solution is red, and under alkaline conditions, the solution is blue.

Wet litmus paper can also be used to test for water-soluble gases that affect acidity or alkalinity; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance, ammonia gas, which is alkaline, turns red litmus paper blue.

Chemical reactions other than acid–base can also cause a color change to litmus paper. For instance, chlorine gas turns blue litmus paper white – the litmus dye is bleached,[2] because of presence of hypochlorite ions. This reaction is irreversible, so the litmus is not acting as an indicator in this situation.


Consider the following questions: 

     1.Investigate the safety of anthocyanins as food additives. 

     2.Why lemonade is an alkaline solution, although it tastes sour.

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