Which of the following everyday materials is not a pure chemical substance?

Introduction / Context:
Distinguishing between pure substances and mixtures is a key skill in basic chemistry and in understanding materials around us. A pure substance has a fixed composition and uniform properties, while a mixture consists of two or more substances physically combined with variable composition. This question connects that conceptual knowledge to familiar examples such as oxygen, carbon dioxide, water and milk, encouraging learners to think about what is really present in each case at the particle level.

Given Data / Assumptions:
- Option A is oxygen, represented by O2 in its common molecular form.
- Option B is carbon dioxide, represented by CO2.
- Option C is water, represented by H2O in its pure form.
- Option D is milk, a common food material.
- We consider ideal pure samples of gases and water, and typical everyday milk which contains many components.

Concept / Approach:
Pure substances include elements and compounds with definite composition and properties. Oxygen is an element and can be considered a pure substance when it consists only of O2 molecules. Water and carbon dioxide are compounds with fixed ratios of elements, so chemically pure water and pure carbon dioxide are also pure substances. Milk, in contrast, is a complex mixture of water, fats, proteins, lactose, minerals and other components, forming an emulsion and suspension. Therefore milk is not a pure substance but a heterogeneous mixture.

Step-by-Step Solution:
Step 1: Evaluate oxygen. Oxygen gas is composed of identical O2 molecules and is an element, so it can be a pure substance. Step 2: Evaluate carbon dioxide. CO2 has a fixed ratio of one carbon atom to two oxygen atoms and is a compound, so pure CO2 is a pure substance. Step 3: Evaluate water. Pure H2O has a constant composition and sharp boiling point, so it is also a pure substance in the chemical sense. Step 4: Evaluate milk. Milk contains water, fat droplets, dissolved sugars, proteins and salts, all physically combined but not chemically bonded in fixed ratios, so it is a mixture rather than a pure substance. Step 5: Conclude that out of the given options, milk is the one that is not a pure chemical substance.

Verification / Alternative check:
A quick verification is to consider whether the material can be represented by a single chemical formula. Oxygen can be written as O2, carbon dioxide as CO2, and pure water as H2O. Milk cannot be represented by a single simple formula because it is made up of many different molecules and particles. Another check is to ask whether the material can be separated into simpler components by physical methods such as filtration or centrifugation. Milk can be separated into cream, skimmed milk and other fractions, which confirms that it is a mixture. These checks support the classification of milk as not being a pure substance.

Why Other Options Are Wrong:
Option A, oxygen, is an element that meets the definition of a pure substance. Option B, carbon dioxide, is a compound with fixed composition and also qualifies as a pure substance when isolated. Option C, water, is a compound that is similarly a pure substance when free of impurities. These three options therefore do not answer the question asking for the one that is not a pure substance. Milk is the only correct choice.

Common Pitfalls:
Learners sometimes assume that anything they drink or see in daily life must be pure or that anything that looks uniform must be a pure substance. Milk often appears uniform to the naked eye but is actually a colloidal mixture. Another mistake is to think that only elements can be pure substances and that compounds cannot, which is incorrect. Remembering that both elements and compounds can be pure substances, while food materials like milk are usually mixtures, helps avoid these errors.

Final Answer:
The material that is not a pure chemical substance is Milk.