(i) Suggest a safe procedure of diluting a strong concentrated acid.

(ii) Name the salt formed when sulphuric acid is added to sodium hydroxide and write its pH.

(iii) Dry HCl gas does not change the colour of dry blue litmus paper. Why ?

Q: (i) Suggest a safe procedure of diluting a strong concentrated acid. (ii) Name the salt formed when sulphuric acid is added to sodium hydroxide and write its pH. (iii) Dry HCl gas does not change the colour of dry blue litmus paper. Why ?

**(i) Safe procedure of diluting a strong concentrated acid:** To dilute a concentrated acid, the acid must always be added slowly to water with constant stirring. Water should never be added to concentrated acid. This is because the process of dissolving an acid in water is highly exothermic. If water is added to concentrated acid, the large amount of heat generated suddenly can cause the mixture to splash out and cause severe acid burns, or even break the glass container due to excessive local heating. **(ii) Salt formed and its pH:** When sulphuric acid ($H_2SO_4$) is added to sodium hydroxide ($NaOH$), the salt formed is **sodium sulphate** ($Na_2SO_4$). Since it is a salt of a strong acid and a strong base, it is neutral in nature, and its pH is **7**. **(iii) Why dry HCl gas does not change the colour of dry blue litmus paper:** Dry $HCl$ gas does not dissociate to produce hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$) in the absence of water. Acidic behavior and the ability to change the colour of blue litmus paper to red depend entirely on the presence of these hydronium ions. Since both the $HCl$ gas and the blue litmus paper are dry, no moisture is available for ionization, and thus no colour change occurs.

Acids, Bases and Salts

Acids and Bases in Water Solution

Official Answer

(i) Safe procedure of diluting a strong concentrated acid:

To dilute a concentrated acid, the acid must always be added slowly to water with constant stirring. Water should never be added to concentrated acid. This is because the process of dissolving an acid in water is highly exothermic. If water is added to concentrated acid, the large amount of heat generated suddenly can cause the mixture to splash out and cause severe acid burns, or even break the glass container due to excessive local heating.

(ii) Salt formed and its pH:

When sulphuric acid ( $H_2SO_4$ ) is added to sodium hydroxide ( $NaOH$ ), the salt formed is sodium sulphate ( $Na_2SO_4$ ).

Since it is a salt of a strong acid and a strong base, it is neutral in nature, and its pH is 7.

(iii) Why dry HCl gas does not change the colour of dry blue litmus paper:

Dry $HCl$ gas does not dissociate to produce hydrogen ions ( $H^+$ ) or hydronium ions ( $H_3O^+$ ) in the absence of water. Acidic behavior and the ability to change the colour of blue litmus paper to red depend entirely on the presence of these hydronium ions. Since both the $HCl$ gas and the blue litmus paper are dry, no moisture is available for ionization, and thus no colour change occurs.

acid to waterexothermicsodium sulphatepH 7hydronium ionsabsence of waterionization

Marking Scheme

1For (i): State that acid must be added to water slowly with constant stirring, and explain the exothermic risk of doing the opposite (1 mark)
2For (ii): Name the salt as sodium sulphate ( $Na_2SO_4$ ) and state its pH as 7 (1 mark)
3For (iii): Explain that dry HCl does not form $H^+$ or $H_3O^+$ ions in the absence of water, which are necessary to change litmus colour (1 mark)

Hint

Remember: 'Add Acid to Water, never Water to Acid'. For the salt, combine the cation of the base with the anion of the acid. For litmus, think about what causes acidic behavior.

Quick Oral Answer

Always add acid to water slowly with stirring because the reaction is highly exothermic. The salt formed by NaOH and H2SO4 is sodium sulphate, which has a pH of 7. Dry HCl doesn't change dry litmus colour because it cannot ionize into H+ ions without water.

Analysis & Explanation

Dilution safety: Adding water to acid creates a highly concentrated zone of reaction at the surface, vaporizing the water instantly and causing splashing. Adding acid to water ensures the acid is immediately diluted in a large volume of water, which absorbs the heat safely.
Neutralization: The reaction is:

$2NaOH(aq) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + 2H_2O(l)$

Because both reactants are strong electrolytes, they neutralize each other completely, yielding a neutral salt with a pH of 7.

Role of water: Water is essential for the ionization of acids. The polar water molecules pull the $H^+$ ion away from the $Cl^-$ ion in $HCl$ , forming stable hydronium ions ( $H_3O^+$ ). Without water, $HCl$ remains in molecular form and cannot exhibit acidic properties.

Common Mistakes

1Writing 'add water to acid' by mistake.
2Stating the pH of sodium sulphate as acidic or basic instead of neutral (7).
3Forgetting to mention that water is required for the dissociation of $HCl$ into $H^+$ ions.

Interesting Facts

Concentrated sulphuric acid has a very high affinity for water, which is why the dilution process is so violently exothermic.

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Frequently Asked Questions

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

This question carries 3 marks 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 "Acids and Bases in Water Solution" from CBSE Class 10 Science.

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

This is a Short Answer question from Section C in the CBSE Class 10 Science 2023 paper.