A weak acid is one which

Step 1: Analyze the question The question asks for the definition of a weak acid. The key idea is how far the acid ionizes (dissociates into ions) in water.

Step 2: Explain the concept

• Strong acids (e.g., HCl, H₂SO₄, HNO₃) ionize completely in water. Almost every molecule splits into H⁺ ions and anions.
• Weak acids (e.g., ethanoic acid CH₃COOH, carbonic acid H₂CO₃, citric acid) ionize only partially or slightly in water. This sets up an equilibrium in which most of the acid stays as un-ionized molecules:

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

Strength is about the degree of ionization. It is not the same as concentration or corrosiveness.

Step 3: Evaluate each option

• A. is not corrosive: Wrong. Corrosiveness depends on concentration and the nature of the acid, not on whether it is weak. Concentrated weak acids can still be corrosive.
• B. completely ionizes in water: Wrong. This describes a strong acid.
• C. does not produce salt with alkali: Wrong. All acids, weak ones included, react with alkalis to form salt and water (neutralization). For example, CH₃COOH + NaOH → CH₃COONa + H₂O.
• D. slightly ionizes in water: Correct. A weak acid only partially ionizes, giving a low concentration of H⁺ ions.

Step 4: Answer

Exam tip: Remember "Strong = Complete, Weak = Partial." In the exam, watch out for options that confuse strength with concentration (dilute/concentrated) or with corrosiveness. Those are common distractors.