In basic chemistry, is pure water considered a pure substance or a mixture?

Introduction / Context:
Questions about whether water is a pure substance or a mixture are classic conceptual checks in introductory chemistry. They test a learner's understanding of the definitions of pure substance, element and compound, and the difference between an idealised, perfectly pure sample and everyday samples that may contain impurities. Pure water, represented by the formula H2O, is a compound that can be classified as a pure substance because it has a fixed composition and uniform properties throughout.

Given Data / Assumptions:
- The focus is on pure water, meaning only H2O with no significant impurities.
- A pure substance has a fixed composition and constant properties at given conditions.
- Compounds are pure substances formed from elements combined in fixed ratios.
- Everyday natural water may contain dissolved salts and gases, but the question emphasises pure water in the chemical sense.

Concept / Approach:
A pure substance can be either an element or a compound. Elements contain only one kind of atom. Compounds contain atoms of different elements chemically bonded in a fixed ratio. Water is a compound of hydrogen and oxygen in a 2:1 ratio. A key point is that a compound with fixed composition and uniform properties is also considered a pure substance. Mixtures, in contrast, consist of two or more substances physically combined, where the ratio can vary and components can often be separated by physical means.

Step-by-Step Solution:
Step 1: Recall that water has the formula H2O, meaning each molecule contains two hydrogen atoms and one oxygen atom. Step 2: Recognise that this composition is fixed; pure water always contains hydrogen and oxygen in the same 2:1 ratio. Step 3: Apply the definition of a compound: a substance formed when two or more elements chemically combine in a fixed proportion, which fits water. Step 4: Apply the definition of a pure substance: a material with uniform composition and properties that cannot be separated into simpler substances by physical methods, which also fits water. Step 5: Conclude that pure water is a pure substance, specifically a compound, and not a mixture.

Verification / Alternative check:
One verification is to compare water with a clear example of a mixture, such as salt water. In salt water, the proportions of salt and water can vary and salt can be recovered by simple physical processes like evaporation. In pure water, there is nothing to separate except by chemical decomposition into hydrogen and oxygen gases. Another check is to note that pure water has a sharp boiling point at standard pressure, which is a characteristic of pure substances, while mixtures often have a boiling range. These checks support the conclusion that pure water is a pure substance.

Why Other Options Are Wrong:
Option B claims water is always a mixture because it contains dissolved substances, but that applies only to natural or impure samples, not to chemically pure water. Option C states that water is pure only when it is part of a solution, which is backwards, since solutions are mixtures by definition. Option D suggests that compounds are never pure substances, but in chemistry compounds such as water, carbon dioxide and sodium chloride are standard examples of pure substances. Therefore these alternatives do not match accepted definitions.

Common Pitfalls:
Students sometimes confuse real world tap water, which contains minerals and gases, with the idealised pure water used in chemical definitions. This leads them to think that water must always be a mixture. Another error is to think that only elements can be pure substances, ignoring that compounds with fixed composition are also classified as pure. Clarity about the distinction between pure water and natural water samples helps avoid these misunderstandings.

Final Answer:
In chemistry, pure water is classified as a pure substance, so the correct statement is Yes, pure water is a pure substance made only of H2O molecules.