A student added dry HCl gas to dry NaCl and observed no reaction. However, when water was added, the solution showed acidic nature.

Question: Explain why HCl shows acidic behaviour only in aqueous solution.

Roadmap:

• Check ion formation
• Role of water

Solution:

HCl produces \([H^+]\) ions only in presence of water. Without water, ionisation does not occur, hence no acidic behaviour.

A student added solid NaOH to dry HCl gas and observed no reaction. However, when water was added, a reaction occurred.

Question: Explain why NaOH shows basic behaviour only in aqueous solution.

Roadmap:

• Check ion formation
• Role of water

Solution:

NaOH produces \([OH^-]\) ions only in aqueous solution. Without water, ionisation does not occur, so basic properties are not observed.

A student dissolved NH₄Cl in water and found the solution acidic.

Question: Explain why NH₄Cl solution is acidic.

Roadmap:

• Identify parent acid and base
• Check hydrolysis

Solution:

NH₄Cl is formed from strong acid (HCl) and weak base (NH₄OH). The ammonium ion undergoes hydrolysis, releasing \([H^+]\) ions, making the solution acidic.

A student placed copper in dilute HCl and observed no reaction, but zinc reacted vigorously.

Question: Explain the difference in behaviour.

Roadmap:

• Check reactivity series
• Compare positions of Cu and Zn

Solution:

Zinc is above hydrogen in the reactivity series and displaces hydrogen from acid. Copper is below hydrogen and cannot displace it, so no reaction occurs.

A student mixed equal volumes of HCl and NH₄OH and observed the solution was slightly acidic.

Question: Explain why the solution is not neutral.

Roadmap:

• Identify strength of acid and base
• Check salt formed

Solution:

HCl is a strong acid and NH₄OH is a weak base. The salt NH₄Cl undergoes hydrolysis producing \([H^+]\), making the solution acidic.

A student added HCl to black CuO and observed a blue solution.

Question: Explain the observation.

Roadmap:

• Identify nature of CuO
• Write reaction

Solution:

CuO is a basic oxide. It reacts with HCl to form CuCl₂ (blue solution) and water. The colour change confirms salt formation.

Dry HCl gas does not change the colour of dry litmus paper, but aqueous HCl does.

Question: Explain the reason.

Solution:

In absence of water, HCl does not ionise to produce \([H_3O^+]\) ions. In aqueous solution, ionisation occurs, showing acidic behaviour.

Two solutions have pH 3 and pH 5. Which is more acidic and by how much?

Solution:

pH 3 solution is more acidic. It is 100 times more acidic than pH 5 (since each unit = 10×).

Why is baking powder preferred over baking soda alone?

Answer: Baking powder contains tartaric acid which neutralises Na₂CO₃ formed, preventing bitter taste.

A blue crystalline solid turns white on heating but regains blue colour on adding water.

Identify the compound and explain.

Solution:

The compound is hydrated copper sulphate \([CuSO_4 \cdot 5H_2O]\). Heating removes water forming white anhydrous CuSO₄. Adding water restores hydrated form and blue colour.