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5 Ways to Teach Litmus Test for Carboxylic Acids 

Nabiha Khalid
Chemistry
High School
Teaching with Labster

Introduction of Litmus Test for Carboxylic Acids

When determining acidity in a solution, the litmus test litmus employs paper or litmus. Lichens are the source of the natural litmus that is used as a pH indicator. Litmus is a mixture of dyes, and the paper that is used to manufacture litmus can be dyed by soaking it in the litmus mixture. The manufacturing process determines both the hue of the litmus paper and the pH variations.

As demonstrated in figure 2, two pieces of litmus paper, one red and one blue, can be submerged in a solution to detect whether or not the solution is acidic, basic, or neutral. A test for carboxylic acids can be done with litmus paper that is blue in color. When exposed to a solution with a pH greater than 7, red litmus paper changes colour to blue. When dipped into a solution with a pH less than seven, blue litmus paper changes colour to red. When exposed to a solution with a pH value of 7, litmus paper of either colour does not alter its appearance visually.

Carboxylic acids are a type of organic acid that contain the carboxyl functional group, which is denoted by the symbol COOH. Figure 1 features a green highlighting that draws attention to this carboxyl group. The suffix -ic acid or -oic acid is added to the name of carboxylic acids. For instance, salicylic acid and benzoic acid are both examples of carboxylic acids. Because the carboxyl functional group is polar, smaller carboxylic acids are often soluble in water. This is because larger carboxylic acids are not polar. Because of their ability to donate protons (H+), carboxylic acids are classified as Bronsted-Lowry acids. In addition to this, they are often weak acids, and their boiling temperatures are typically greater than that of water.

Salicylic acid, commonly known as 2-hydroxybenzoic acid, has the following structure on a molecular level (figure 1). The highlighted functional groups are the benzene ring, the carboxylic acid, and the alcohol group. The carboxylic acid is shown in green, the benzene ring in yellow, and the alcohol group in blue.

salicylic acid

Figure 1: The molecular structure of salicylic acid.

Materials of Litmus test

Beakers, as well as test tubes and racks for test tubes

A litmus test with blue and red paper

Compound that is going to be examined

Safety of Litmus test 

Since both acidic and basic solutions are corrosive, it is important to wear gloves when working with them.

Procedure of Litmus test 

Place a drop of the liquid compound or the compound in solution on a wet piece of litmus paper that is either blue or red in color.

On litmus paper stained red: If the red hue of the litmus paper shifts to a blue color, this indicates that the chemical in question is basic.

For blue litmus paper: If the blue hue of the litmus paper changes to a red color, this indicates that the substance being tested is acidic.

This indicates that the substance has a phenolic or carboxylic group. 

The substance is considered neutral if neither the red nor the blue litmus paper changes color when exposed.

litmus test for acidity virtual lab

Figure 2: The colors of litmus paper after being dipped in basic, acidic, and neutral solutions from Labster's Litmus Test for Carboxylic Acids Virtual Lab.

Because of the carboxylic acid, the blue litmus test turns red. Because of hydroxyl group in COOH has a far more acidic character than the hydroxyl group in alcohol. When a carboxylic group is present, the litmus paper will change color from blue to red if it has been exposed to the substance being tested. The chemical reaction is given below.

R-COOH + H2O → R-COO– + H3O+

Why is the Litmus Test for Carboxylic Acids a difficult topic for students?

1. Involvement with chemical reactions

The chemical sciences are likely to strike fear into the hearts of the large percentage of students enrolled in biology classes. Analytical methods, such as litmus tests, work at the intersection of discrete chemistry concepts. As a result, biology students may become anxious simply upon hearing these ideas for the first time.

2. Confusion among terminologies

Students mistakenly classify its fundamentalists as strong, weak, or inconsequential. Then, I must specify its acidity and supply its conjugate acid. Students frequently ask the following fundamental, perplexing questions: What chemicals cause H+ ions to form in water? Which compounds can cause water to have OH-ions? What materials can flow current? What compounds turn blue litmus dye into red? What compounds can change the color of red litmus paper to blue?

3. Basics of chemistry

Students may struggle with the concept of a litmus test because it is the basics in chemistry. The majority of the time, theoretical education supported by diagrams is employed to cover the subject of litmus testing in addition to its numerous variants and the fundamental idea underlying them. It's possible that using a flow chart will assist students in visualizing what's happening in their thoughts. Students, on the other hand, have a hard time comprehending the significance of each stage that is involved in the process. The ease with which one can move from one thought to the next is the single most crucial aspect in determining how challenging the litmus test will be. Before you can have that degree of comprehension in regard to the procedure of the litmus test, you need to have sufficient knowledge in a few different topics first. This is necessary in order to prepare you for the next step. It is essential to bear in mind that the solution to this issue does not rest in figuring out the underlying principle of each subject, as was once the case while working in a theoretical setting. This is something that must be kept in mind at all times.

Five ways to teach Litmus Test for Carboxylic Acids 

1)    Building the base

Here, the basic ideas of the solution, mixture, solvent, buffer, saturation, and concentration are developed. The ability to distinguish between these ideas is a must for students. Giving real-world examples will help with this. The properties that distinguish mixtures from litmus testing are briefly explained to the pupils. During a classroom teacher-led demonstration, the students will have the opportunity to try and evaluate the physical characteristics of a few simple combinations and solutions. 

2)  Hands-on practice

Then the students are advised to perform some experimentation of litmus testing. These "hands-on" exercises are beneficial to the growth of students' practical abilities and aid in their comprehension of scientific ideas and occurrences. The development of knowledge of the hazard, risk, and safety working is one further benefit that comes as a direct result of participating in practical training, particularly in learning litmus testing.

3) Clear idea of Acid, Base, and Alkalis 

Students should have clear concepts from different angles. A specific chemical species, for instance, can be classified as an acid, a base, or an alkali based on how it reacts with water. There are three different ways to define acids. When dissolved in water, an acid increases the number of hydrogen ions present. (The Arrhenius explanation) A species that gives a proton and hydrogen ion (H+) is said to be an acid. An acid is a species that accepts an electron pair (according to the Bronsted-Lowry definition).
Similarly, the three definitions of bases are essentially the opposite of those of an acid. When dissolved in water, a base increases the number of hydroxide ions. A base is a species that will receive a proton or hydrogen ion (H+), according to the Arrhenius definition. An electron pair-donor is a base, according to the Brnsted-Lowry definition. (According to Lewis). 

A base that disintegrates in water is an alkali. For instance, sodium hydroxide and copper oxide are both bases, but only sodium hydroxide qualifies as an alkali due to its ability to dissolve in water.

4) Seeing is believing 

Students learn from things they experience firsthand. Arrange some graphical presentation of lab representation of how pH value changes in different mediums. Each of the naturally found pH indicators in Figure 3 changes color depending on pH, but each has a different method for doing so. Whether aluminum ions are present in the petals of hydrangea flowers determines their color. The aluminum ions in acidic soil can bind to the hydrangea petals and color flowers blue. On the other hand, Curcumin, anthocyanins, and litmus are chemicals whose chemical structures change when the pH changes and this alteration in chemical structure results in a change in color.

ph indicators in nature - litmus test virtual lab

Figure 3 - Naturally sourced pH indicators from Labster's Litmus Test for Carboxylic Acids Virtual Lab.

5)   Using a Virtual Lab simulation

A fantastic technique to teach litmus testing is using a virtual laboratory simulation. At Labster, we're committed to providing a fully interactive advanced diagnostic simulation laboratory that uses gamification components like narrative and score systems inside a captivating 3D setting. After touring a full-scale sewage facility, you'll work on a litmus testing of carboxylic acids. 

Litmus Test for Carboxylic Acids Virtual Lab

Check out Labster’s Litmus Test for Carboxylic Acids Virtual Lab.

References 

Ju, G., Cheng, M., & Shi, F. (2014). A pH-responsive smart surface for the continuous separation of oil/water/oil ternary mixtures. NPG Asia Materials, 6(7), e111-e111.

https://amrita.olabs.edu.in/?brch=8&cnt=1&sim=141&sub=73

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