1 markMCQSection A

Sodium hydroxide is termed an alkali while Ferric hydroxide is not because :

Q: Sodium hydroxide is termed an alkali while Ferric hydroxide is not because :

The correct option is (b). Sodium hydroxide ($NaOH$) is termed an alkali because it is a base that dissolves in water, while ferric hydroxide ($Fe(OH)_3$) is a base but is not soluble in water.

Acids, Bases and Salts

What Happens to an Acid or a Base in a Water Solution?

Options

(a)Sodium hydroxide is a strong base, while Ferric hydroxide is a weak base.

(b)Sodium hydroxide is a base which is soluble in water while Ferric hydroxide is also a base but it is not soluble in water.

(c)Sodium hydroxide is a strong base while Ferric hydroxide is a strong acid.

(d)Sodium hydroxide and Ferric hydroxide both are strong base but the solubility of Sodium hydroxide in water is comparatively higher than that of Ferric hydroxide.

Official Answer

The correct option is (b). Sodium hydroxide ( $NaOH$ ) is termed an alkali because it is a base that dissolves in water, while ferric hydroxide ( $Fe(OH)_3$ ) is a base but is not soluble in water.

AlkaliSoluble in waterSodium hydroxideFerric hydroxideInsoluble base

Marking Scheme

11 Mark for selecting the correct option (b) with the correct definition of an alkali.

Hint

Recall the definition of an alkali from the 'Do You Know?' section: 'An alkali is a base that dissolves in water.' Check which option directly addresses water solubility.

Quick Oral Answer

An alkali is a base that dissolves in water. Sodium hydroxide is soluble in water, so it is an alkali, whereas ferric hydroxide is insoluble in water, so it is a base but not an alkali.

Analysis & Explanation

According to the NCERT text, bases generate hydroxide ( $OH^-$ ) ions in water. However, all bases do not dissolve in water. An alkali is specifically defined as a base that dissolves in water.

Why Option (b) is correct: Sodium hydroxide ( $NaOH$ ) dissolves readily in water to produce sodium ions ( $Na^+$ ) and hydroxide ions ( $OH^-$ ), making it an alkali:

$NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)$

In contrast, ferric hydroxide ( $Fe(OH)_3$ ) is a base because it can neutralize acids, but it is insoluble in water, so it cannot be classified as an alkali.

Why Option (a) is incorrect: The classification of a base as an alkali depends entirely on its solubility in water, not on whether it is strong or weak.
Why Option (c) is incorrect: Ferric hydroxide is a base, not an acid.
Why Option (d) is incorrect: Ferric hydroxide is practically insoluble in water, and strength is not the defining factor for being an alkali.

Common Mistakes

1Assuming that all bases are alkalis.
2Confusing the strength of a base (degree of ionization) with its solubility in water.

Interesting Facts

Alkalis are soapy to touch, bitter in taste, and highly corrosive. Common examples of alkalis include sodium hydroxide ( $NaOH$ ), potassium hydroxide ( $KOH$ ), and calcium hydroxide ( $Ca(OH)_2$ ).

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 1 mark in the CBSE Class 10 Science 2023 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "What Happens to an Acid or a Base in a Water Solution?" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2023 paper.