Nature of indicators
Indicators are special coloring substances that sensitively react by changing color to the presence of an acid or a base in a solution. Often such dyes can be obtained from common fruits and berries.
As a clear natural example, we can consider blueberry juice or syrup. When dissolved in water, it forms a pink liquid. If a base, such as regular baking soda, is introduced into this system, the color instantly changes to blue. Upon subsequent addition of vinegar or citric acid, the liquid becomes pink again. Such a reversible color change proves that the added acid has completely neutralized the action of the base.
For a quick practical determination of the nature of the medium (acidic, alkaline, or neutral), special indicator strips are used in laboratories. This is paper impregnated with litmus or another sensitive substance. It is sufficient to apply a drop of the test solution to such paper and analyze the change in its color.
Titration method
Titration is a classical laboratory process of gradually adding one solution to another in order to perform precise chemical analysis. In the context of our topic, this method is used to carry out the neutralization reaction.
A special laboratory setup is assembled for the experiment:
- Burette: a long glass tube fixed in a stand. An alkali solution is poured into it (in our case, sodium hydroxide, $\text{NaOH}$).
- Conical flask: located under the burette. It contains an acid solution (diluted hydrochloric acid, $\text{HCl}$).
Before starting the process, 2–3 drops of an indicator—a violet litmus solution—are added to the flask with the acid, and the mixture is thoroughly stirred. In an acidic medium, litmus instantly turns pink. Then, the alkali solution is added dropwise from the burette into the flask. It is this process of dropwise addition of a base to an acid that is called titration. The gradual color change of the solution makes it possible to visually record the equivalence point—the moment when complete neutralization occurs.
Chemistry of the neutralization reaction
If we consider an acid from the standpoint of S. Arrhenius's theory, it is a hydrogen-containing substance of molecular structure capable of replacing or exchanging hydrogen atoms for metal atoms. The neutralization reaction itself is a classical exchange reaction between an acid and a base.
Using the reagents used as an example, the chemical equation looks as follows: $$ \text{HCl} + \text{NaOH} = \text{NaCl} + \text{H}_2\text{O} $$
As a result of the interaction, two products are formed:
- Salt (sodium chloride, $\text{NaCl}$): is an absolutely neutral product.
- Water: is formed without fail, but is visually indistinguishable in an aqueous solution.
Neither of the reaction products exhibits either basic or acidic properties.
During the addition of alkali to acid, the color of litmus changes stepwise:
- Pink color indicates the initial acidic medium (excess acid).
- Violet color signifies the moment of neutralization (strictly neutral medium).
- Blue color indicates that an excess of base has been added (the medium has become alkaline).
An important stoichiometry rule: a strictly defined volume of alkali is always required for a fixed volume of acid to achieve complete neutralization. If the reverse experiment is performed (adding acid to an alkali solution with litmus), the colors will change in reverse order: from the initial blue, through the intermediate violet, to the final pink or red.
Thermodynamics and safety rules
The most important aspect of carrying out a neutralization reaction is strict compliance with safety regulations. From a thermodynamic point of view, this process is exothermic, meaning it is accompanied by significant heat release.
It is strictly forbidden to carry out the reaction between neat (anhydrous, concentrated) substances. The interaction of concentrated reagents causes a sharp and strong heating of the mixture. This will inevitably lead to the instantaneous formation of abundant water vapor and extremely dangerous splattering of boiling liquid. To avoid injuries and accidents, before mixing acid and alkali, they must be diluted with water.
Briefly: answers to topical questions
What theories of acids and bases exist besides Arrhenius's theory?
In addition to Arrhenius's theory, the properties of acids and bases are described by the solvent-system theory, Lewis's electronic theory, and the Brønsted-Lowry (protolytic) theory.
- Solvent-system theory — acids create an acidic medium in solution, and bases form an alkaline medium.
- Lewis's electronic theory — an acid acts as an electron pair acceptor, and a base is its donor.
- Protolytic theory — an acid is a proton donor, and a base acts as its acceptor.
What safety rules must be followed when diluting concentrated acids?
When diluting concentrated acids, the order of mixing liquids must be strictly observed.
- Adding acid — concentrated acid is slowly poured into water.
- Prohibition of adding water — it is strictly forbidden to pour water into acid.
Water has a lower density and remains on the surface of the heavy acid. Due to intense heating, it instantly boils, leading to acid splashing. It is also forbidden to carry out the neutralization reaction between neat concentrated substances; they must be preliminarily diluted with water.
Litmus is easy to remember by color mixing: it is pink in acid, blue in alkali, and in a neutral medium (when pink and blue mix), it turns violet.
Class and exam questions
What are the products of a neutralization reaction?
Water and salt are formed as a result of the chemical interaction between an acid and a base. These products exhibit neither acidic nor basic properties.
Why does the solution change color stepwise during titration?
The color changes depending on the predominance of the reagent. Pink shows an excess of acid, violet shows the point of exact neutralization, and blue shows an excess of added alkali.
Why is it dangerous to mix undiluted acid and alkali?
The neutralization reaction is exothermic. Upon contact of concentrated substances, severe heating occurs, along with the formation of steam and the ejection of boiling liquid from the vessel.
What else to study on this topic
- Precise stoichiometric calculation of volumes in titration
- Svante Arrhenius's theory of acids and bases
- Design of a laboratory burette and rules for working with it
- Types of natural indicators and their chemical nature
- Mechanism of color change of organic dye indicators
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