In acid–base chemistry, which element is common to all acids and is responsible for their ability to produce H+ ions in aqueous solution?

Introduction / Context:
This question focuses on a key idea in acid–base chemistry: what makes a substance an acid. According to the Arrhenius definition, an acid is a substance that increases the concentration of hydrogen ions (H+) in aqueous solution. Even in broader definitions such as Brønsted–Lowry, acids are proton donors, and a proton is simply a hydrogen ion. Therefore, hydrogen is the element that must be present in all acids, because it is the source of the H+ ions that give acids their characteristic properties, such as sour taste, reactivity with metals, and ability to neutralise bases.

Given Data / Assumptions:
- The question asks for the element common to all acids.
- Options include hydrogen, oxygen, carbon, and sulphur.
- It is assumed that you are familiar with formulae of common acids like HCl, H2SO4, HNO3, CH3COOH, and so on.
- The focus is on the element that defines acid behaviour in water by providing H+ ions.

Concept / Approach:
To classify a substance as an acid, it must have at least one hydrogen atom that can be released as a proton (H+). In water, strong acids such as HCl, HNO3, and H2SO4 fully or almost fully ionise to release H+ ions, while weak acids like CH3COOH partially ionise. Oxygen is present in many acids (like H2SO4 and HNO3) but not in all, because hydrochloric acid (HCl) contains no oxygen. Carbon appears in many organic acids, such as acetic acid, but not in mineral acids like HCl. Sulphur appears in some acids (like H2SO4) but is clearly not universal. Hydrogen alone is present in every acid and is the element that yields the characteristic H+ ions in aqueous solution.

Step-by-Step Solution:
Step 1: Recall the Arrhenius definition of an acid as a substance that increases H+ ion concentration in water. Step 2: Examine formulas of common acids: HCl, H2SO4, HNO3, H3PO4, CH3COOH. All of them contain hydrogen atoms. Step 3: Recognise that oxygen is present in many acids (for example H2SO4, HNO3) but not in all (HCl does not contain oxygen). Step 4: Notice that carbon is present only in organic acids such as acetic acid and not in strong mineral acids like HCl or H2SO4. Step 5: Observe that sulphur appears only in certain acids like sulfuric acid and is missing from many others. Step 6: Conclude that hydrogen is the only element common to all acids, making option A the correct answer.

Verification / Alternative check:
Look at a variety of acids studied in school chemistry: hydrochloric acid (HCl), hydrobromic acid (HBr), nitric acid (HNO3), sulfuric acid (H2SO4), phosphoric acid (H3PO4), acetic acid (CH3COOH), and citric acid (C6H8O7). Every one of these formulas includes hydrogen, and the acidic behaviour is linked to the ability of at least one of these hydrogen atoms to be donated as H+ in solution. In contrast, bases such as NaOH or Ca(OH)2 contain hydroxide ions (OH-) rather than acidic hydrogen. This clear pattern across many examples confirms that hydrogen is the element common to all acids.

Why Other Options Are Wrong:
Oxygen is not present in acids like HCl and HBr, so it cannot be common to all acids. Carbon is absent in many mineral acids, such as HCl and HNO3, so it also cannot be universal. Sulphur is present only in sulfur containing acids like H2SO4 and is not found in acids like HCl or HNO3. Therefore, options B, C, and D are not correct. Only hydrogen appears in the formula of every acid and is central to the definitions of acids in both Arrhenius and Brønsted–Lowry theories.

Common Pitfalls:
Some students confuse the fact that many common acids contain oxygen and may incorrectly assume oxygen is always present. Others associate carbon strongly with organic chemistry and may think that carbon must be in all acids, forgetting about mineral acids that contain no carbon. To avoid these mistakes, remember that the defining behaviour of an acid is its ability to donate H+ ions, and the element that supplies these H+ ions is hydrogen. Focusing on this functional role makes it clear why hydrogen is the correct answer.

Final Answer:
The element common to all acids, which allows them to produce H+ ions in solution, is hydrogen.