Chapter 2 · Question 4

What is a neutralisation reaction? Explain with the help of a simple activity involving sodium hydroxide and hydrochloric acid. Give the balanced chemical equation.

Answer Revealed

Direct Answer:

A neutralisation reaction is the reaction between an acid and a base to produce a salt and water. In Activity 2.6, 2 mL of dilute NaOH is taken in a test tube and two drops of phenolphthalein indicator are added — the solution turns pink (phenolphthalein is pink in a basic medium). When dilute HCl is added drop by drop, the pink colour disappears, indicating that the base has been completely neutralised by the acid. Equation:

\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}

. The ionic basis is:

\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}

Simple Explanation

Neutralisation is when an acid and a base cancel each other out. Picture this: you have sodium hydroxide (a base) with a drop of phenolphthalein — it turns pink. Now you add hydrochloric acid drop by drop — the pink fades and finally disappears. The acid has neutralised the base. The products are just common table salt (

\text{NaCl}

) dissolved in water. Add more NaOH, and pink returns — because the base is back in excess. At the ionic level, hydrogen ions (

\text{H}^+

) from the acid and hydroxide ions (

\text{OH}^-

) from the base combine to form neutral water molecules.

Exam-Ready Structure

Neutralisation is the fundamental acid-base reaction and is demonstrated through Activity 2.6 in the NCERT textbook. 1. Definition: The reaction between an acid and a base that produces a salt and water is called a neutralisation reaction. General form: Acid + Base

\rightarrow

Salt + Water. 2. Activity 2.6 procedure: (a) Take approximately 2 mL of dilute sodium hydroxide (NaOH) solution in a test tube. (b) Add two drops of phenolphthalein indicator — the solution turns pink because phenolphthalein gives a pink colour in basic solutions. (c) Using a dropper, add dilute hydrochloric acid (HCl) drop by drop to the pink solution while gently shaking the test tube. (d) After sufficient acid is added, the pink colour disappears — this is the end point where all the base has been neutralised. (e) If a few more drops of NaOH are added back, the pink colour reappears because the solution becomes basic again. 3. Balanced chemical equation:

\text{NaOH(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}

. The salt formed is sodium chloride (common salt). 4. Ionic basis of neutralisation: Acids produce

\text{H}^+\text{(aq)}

ions in water, while bases produce

\text{OH}^-\text{(aq)}

ions. The essential neutralisation reaction is:

\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}

. The

\text{H}^+

ions from the acid and the

\text{OH}^-

ions from the base combine to form water, while the other ions remain in solution to form the salt. 5. The general neutralisation can also be written as:

\text{HX} + \text{MOH} \rightarrow \text{MX} + \text{HOH}

, where HX is an acid and MOH is a base.

Key Points

Neutralisation: Acid + Base $\rightarrow$ Salt + Water
Activity 2.6: NaOH (pink with phenolphthalein) + HCl dropwise $\rightarrow$ pink colour disappears
Equation: $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$
Ionic basis: $\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$
Phenolphthalein is pink in basic medium, colourless in acidic/neutral medium

What is a neutralisation reaction? Explain with the help of a simple activity involving sodium hydroxide and hydrochloric acid. Give the balanced chemical equation.