Physical and Chemical Properties of Acids

In acidic solutions, litmus paper turns red and phenolphthalein remains colourless. In alkaline solutions, litmus turns blue and phenolphthalein turns pink/magenta.

Both observations match the behaviour of indicators in an acidic solution: litmus → red, phenolphthalein → colourless.

The solution is acidic. Both indicators confirm this — neither result is consistent with neutral or alkaline conditions.

We need to use the indicator results (litmus = red, phenolphthalein = colourless) to classify the solution as acidic, alkaline, or neutral.

In an alkaline solution, litmus would turn blue and phenolphthalein would turn pink/magenta. Neither of these occurred, so alkaline is eliminated.

In a neutral solution, litmus stays purple/unchanged and phenolphthalein remains colourless. However, the litmus turned red here, which rules out neutral.

The solution clearly reacted with both indicators, demonstrating that it does contain ions (specifically H⁺ ions). Pure non-ionic solutions would not affect indicators this way.

The only remaining option is 'The solution is acidic', which is confirmed by both indicator results.

A single arrow (→) in HCl's equation means the reaction goes in one direction only — complete dissociation. A double arrow (⇌) in acetic acid's equation means the reaction goes in both directions — partial dissociation.

Strong acids like HCl completely dissociate (all molecules break into ions). Weak acids like acetic acid only partially dissociate (an equilibrium exists between dissociated and undissociated molecules).

The key difference shown by the equations is that HCl completely dissociates while acetic acid only partially dissociates — this is the definition of strong vs. weak acids.

The question asks about the key difference shown by the two equations. We need to look at what structural feature of the equations distinguishes them.

While it is true that different anions are produced (Cl⁻ vs CH₃COO⁻), this is not the key difference highlighted by the equations. Both produce H⁺ ions regardless.

Concentration and strength are different concepts. The equations show dissociation behaviour, not how much acid is dissolved. No concentration information is given in these equations.

Both equations show aqueous (aq) solutions, so both are dissolved in water. The state symbols in the equations do not support this claim.

The single arrow (→) vs double arrow (⇌) is the key visual difference — confirming that HCl completely dissociates while acetic acid only partially dissociates.