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Acids, Bases and Salts - What Do All Acids and All Bases Have in Common

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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All acids generate hydrogen ions H+(aq)H^+(aq) in the presence of water. These H+H^+ ions cannot exist alone and combine with water molecules to form hydronium ions (H3O+H_3O^+).

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The acidic property of a substance is due to the release of H+(aq)H^+(aq) or H3O+H_3O^+ ions in solution. For example, HClHCl dissociates into H+H^+ and Cl−Cl^- ions.

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All bases generate hydroxide ions OH−(aq)OH^-(aq) in water. Bases that are soluble in water are specifically called alkalis.

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The presence of free ions (H+H^+ in acids and OH−OH^- in bases) in aqueous solutions allows them to conduct electricity.

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Compounds like glucose (C6H12O6C_6H_{12}O_6) and alcohol (C2H5OHC_2H_5OH) contain hydrogen but do not dissociate in water to produce H+H^+ ions, hence they do not show acidic character or conduct electricity.

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The process of dissolving an acid or a base in water is a highly exothermic reaction. During dilution, the concentration of ions (H3O+H_3O^+ or OH−OH^-) per unit volume decreases.

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Safety Rule: Always add acid to water slowly with constant stirring. Never add water to concentrated acid, as the heat generated may cause the mixture to splash out or break the glass container.

📐Formulae

HCl+H2O→H3O++Cl−HCl + H_2O \rightarrow H_3O^+ + Cl^-

H++H2O→H3O+H^+ + H_2O \rightarrow H_3O^+

NaOH(s)→H2ONa+(aq)+OH−(aq)NaOH(s) \xrightarrow{H_2O} Na^+(aq) + OH^-(aq)

KOH(s)→H2OK+(aq)+OH−(aq)KOH(s) \xrightarrow{H_2O} K^+(aq) + OH^-(aq)

Mg(OH)2(s)→H2OMg2+(aq)+2OH−(aq)Mg(OH)_2(s) \xrightarrow{H_2O} Mg^{2+}(aq) + 2OH^-(aq)

💡Examples

Problem 1:

Why does an aqueous solution of an acid conduct electricity, whereas a solution of glucose does not?

Solution:

Acidic solutions produce H+(aq)H^+(aq) ions, while glucose does not.

Explanation:

Electricity is conducted through a solution by the movement of ions. When HClHCl is dissolved in water, it dissociates: HCl→H++Cl−HCl \rightarrow H^+ + Cl^-. These ions act as charge carriers. However, glucose (C6H12O6C_6H_{12}O_6) does not dissociate into ions in water, so there are no free charges to conduct electricity.

Problem 2:

What happens to the concentration of hydronium ions (H3O+H_3O^+) when a solution of an acid is diluted?

Solution:

The concentration of H3O+H_3O^+ ions decreases per unit volume.

Explanation:

Dilution involves adding more solvent (water) to the solution. While the total number of H3O+H_3O^+ ions remains the same, the volume of the solution increases. Therefore, the number of ions per unit volume (molarity) decreases.

Problem 3:

Dry HClHCl gas does not change the color of dry litmus paper. Why?

Solution:

Absence of H+(aq)H^+(aq) ions in the absence of water.

Explanation:

The acidic behavior of HClHCl is only manifested when it dissociates into H+H^+ ions. This dissociation only occurs in the presence of moisture/water: HCl+H2O→H3O++Cl−HCl + H_2O \rightarrow H_3O^+ + Cl^-. Since dry HClHCl gas and dry litmus paper lack water, no ions are formed, and no color change occurs.