In basic chemistry, which of the following chemical elements is common to all acids and is responsible for their characteristic acidic behaviour in aqueous solution?

Introduction / Context:
Acids are an important class of compounds in chemistry, studied from school level onwards. A very common examination question asks what feature all acids share in common, regardless of whether they are strong or weak, mineral or organic. This question checks the understanding that the essential feature of acids in aqueous solution is the presence of a particular element.

Given Data / Assumptions:

• The question asks which element is common to all acids.

• Options include hydrogen, carbon, sulphur, oxygen and phosphorus.

• We assume acids are considered in their usual definition in water based solutions.

• The learner is expected to have basic knowledge of the structure of acids.

Concept / Approach:
According to the Arrhenius and Brønsted–Lowry concepts, an acid in aqueous solution is a substance that can donate hydrogen ions (H⁺) or protons. For example, hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and acetic acid (CH₃COOH) all contain hydrogen that can be released as H⁺ in water. This means that hydrogen is the element common to all acids in the usual school level treatment.

Step-by-Step Solution:
Step 1: Think of a few common acids such as HCl, H₂SO₄, HNO₃ and CH₃COOH. Step 2: Observe that all of these formulae start with or contain hydrogen atoms. Step 3: Recall that in water, these compounds release hydrogen ions H⁺, which are responsible for acidic properties such as sour taste and ability to turn blue litmus red. Step 4: Check the options and look for hydrogen among them. Step 5: Conclude that hydrogen is the element common to all acids and select it as the answer.

Verification / Alternative check:
Any basic chemistry textbook defines acids either as substances that produce hydrogen ions in aqueous solution (Arrhenius definition) or as proton donors (Brønsted–Lowry definition), both of which focus on hydrogen. Even when advanced theories like Lewis acids are studied later, school level multiple choice questions about ordinary acids almost always refer to hydrogen based definitions. This makes hydrogen the correct and standard answer.

Why Other Options Are Wrong:
Carbon is present in organic acids such as acetic acid but is not present in all mineral acids like hydrochloric acid. Sulphur appears in sulphuric acid but not in every acid. Oxygen is found in many oxo acids like H₂SO₄ and HNO₃, yet there are important acids such as HCl and HBr that do not contain oxygen. Phosphorus appears only in specific acids like phosphoric acid. Hence, none of these elements is common to every acid, whereas hydrogen is.

Common Pitfalls:
Some learners mistakenly choose oxygen because many well known acids have O in their formula. This happens if they think only of sulphuric, nitric and carbonic acids and forget about simple acids like HCl and HBr. Others may be confused by more advanced definitions of acidity that involve electron pair acceptance. For basic general science and entry level exams, the safe rule is to remember that all typical acids contain hydrogen that can be released as H⁺ in water.

Final Answer:
The element that is common to all acids is hydrogen.