3 marksShort AnswerSection SECTION-A

State reason for the following:

(i) Dry $\text{HCl}$ gas does not change the colour of the dry blue litmus paper.

(ii) Alcohol and glucose also contain hydrogen, but do not conduct electricity.

(iii) Concentration of $\text{H}_3\text{O}^+$ ion is affected when a solution of an acid is diluted.

Q: State reason for the following: (i) Dry $\text{HCl}$ gas does not change the colour of the dry blue litmus paper. (ii) Alcohol and glucose also contain hydrogen, but do not conduct electricity. (iii) Concentration of $\text{H}_3\text{O}^+$ ion is affected when a solution of an acid is diluted.

(i) **Dry $HCl$ gas does not change the colour of dry blue litmus paper** because the separation of $H^+$ ions from $HCl$ molecules can only occur in the presence of water. Since both the gas and the litmus paper are dry, no hydronium ions ($H_3O^+$) are formed to exhibit acidic properties. (ii) **Alcohol and glucose contain hydrogen but do not conduct electricity** because they do not dissociate into free ions (such as $H^+$ ions) when dissolved in water. Electric current in a solution is carried by free-moving ions, which are absent in glucose and alcohol solutions. (iii) **Concentration of $H_3O^+$ ions is affected on dilution** because mixing an acid with water increases the total volume of the solution. This results in a decrease in the number of hydronium ions ($H_3O^+$) per unit volume.

Acids, Bases and Salts

Properties of Acids and Bases

Official Answer

(i) Dry $HCl$ gas does not change the colour of dry blue litmus paper because the separation of $H^+$ ions from $HCl$ molecules can only occur in the presence of water. Since both the gas and the litmus paper are dry, no hydronium ions ( $H_3O^+$ ) are formed to exhibit acidic properties.

(ii) Alcohol and glucose contain hydrogen but do not conduct electricity because they do not dissociate into free ions (such as $H^+$ ions) when dissolved in water. Electric current in a solution is carried by free-moving ions, which are absent in glucose and alcohol solutions.

(iii) Concentration of $H_3O^+$ ions is affected on dilution because mixing an acid with water increases the total volume of the solution. This results in a decrease in the number of hydronium ions ( $H_3O^+$ ) per unit volume.

ionizationhydronium ionsabsence of waterdissociatefree ionsper unit volumedilution

Marking Scheme

1Explain that dry $HCl$ does not ionize into $H^+$ or $H_3O^+$ ions in the absence of water, so it cannot show acidic behavior. (1 mark)
2Explain that glucose and alcohol do not dissociate into ions in aqueous solution, hence they do not conduct electricity. (1 mark)
3Explain that dilution increases the volume of the solution, decreasing the concentration of $H_3O^+$ ions per unit volume. (1 mark)

Hint

Think about the necessity of water for ionization, the role of free ions in conducting electricity, and how adding water changes the volume of a solution.

Quick Oral Answer

Dry $HCl$ needs water to form $H^+$ ions and change litmus colour. Glucose and alcohol do not release $H^+$ ions in water, so they don't conduct electricity. Dilution decreases the concentration of $H_3O^+$ ions per unit volume.

Analysis & Explanation

Acidic behavior is strictly associated with the presence of hydrogen/hydronium ions ( $H^+$ or $H_3O^+$ ), which are only produced when an acid dissociates in water. Dry $HCl$ cannot dissociate without water. Similarly, compounds like glucose and alcohol contain hydrogen atoms covalently bonded to carbon and oxygen, meaning they do not ionize in water to release $H^+$ ions, preventing electrical conductivity. Dilution increases the solvent volume, spreading the same number of ions over a larger volume, thereby decreasing the concentration of $H_3O^+$ per unit volume.

Common Mistakes

1Stating that dry $HCl$ is not an acid (it is an acid, but it only exhibits acidic properties in water).
2Assuming all hydrogen-containing compounds are acidic.
3Thinking that dilution increases the total number of $H_3O^+$ ions instead of decreasing their concentration per unit volume.

Interesting Facts

Pure water is a very poor conductor of electricity, but even a tiny amount of acid provides enough ions to make it highly conductive.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2014?

This question carries 3 marks in the CBSE Class 10 Science 2014 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 "Properties of Acids and Bases" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2014 paper?

This is a Short Answer question from Section SECTION-A in the CBSE Class 10 Science 2014 paper.