Which of the following gases will bleach moist red litmus paper? I. Cl2, II. SO2, III. SO3, IV. HCl

A weak acid is an acid that only partially dissociates or ionizes in an aqueous solution. This means that only a small fraction of its molecules break apart into hydrogen ions ($\text{H}^+$) and conjugate base ions.

• (A) is not corrosive: Corrosiveness is related to the reactivity of an acid, not its strength (degree of ionization). Some weak acids can still be corrosive.
• (B) is slightly ionized in water: This statement directly aligns with the definition of a weak acid. They undergo incomplete ionization in water.
• (C) does not produce salts with alkalis: All acids, whether strong or weak, react with alkalis (bases) to form salt and water through neutralization.
• (D) does not conduct electric current in aqueous solution: Since weak acids do ionize to some extent, they produce ions that can conduct electricity, though less effectively than strong acids.

The most accurate description of a weak acid among the choices is that it is slightly ionized in water.

The final answer is $\boxed{\text{B}}$