Which of the following substances is not a chemical compound but instead a mixture or alloy?

Introduction / Context:
In chemistry, it is important to distinguish between pure compounds and mixtures. Compounds have a fixed chemical formula and definite proportions of elements that are chemically bonded, whereas mixtures and alloys contain components that are physically combined and not in a fixed ratio. This question asks you to identify which of the listed substances is not a chemical compound but rather a mixture, specifically an alloy.

Given Data / Assumptions:

• Brass is a commonly used metallic material in industry and household items.
• Quick lime, marble and soda ash all have well defined chemical formulas.
• We assume standard definitions of compound, mixture and alloy.
• The options mix both chemical compounds and a metallic mixture.

Concept / Approach:
A chemical compound has a fixed composition and can be represented by a chemical formula. Examples include calcium oxide, calcium carbonate and sodium carbonate. An alloy is a homogeneous mixture of metals or of a metal with other elements, and its exact composition can vary within certain limits. Brass is a classic example of an alloy, typically made from copper and zinc in variable proportions. It does not have a single fixed chemical formula, so it is considered a mixture rather than a compound. Therefore, among the options, brass is the correct choice as the substance that is not a chemical compound.

Step-by-Step Solution:
Step 1: Identify quick lime as calcium oxide, which has the fixed chemical formula CaO. Step 2: Identify marble as primarily calcium carbonate, with the formula CaCO3. Step 3: Recognise soda ash as sodium carbonate, with the formula Na2CO3. Step 4: Recall that brass is an alloy of copper and zinc with variable composition and no single fixed formula. Step 5: Conclude that brass is not a pure compound but a mixture and therefore the correct answer.

Verification / Alternative check:
When you look up the composition of brass, you will find ranges such as 60 percent copper and 40 percent zinc or other similar proportions. The exact ratio can change depending on the type of brass and its intended use. In contrast, CaO, CaCO3 and Na2CO3 have fixed stoichiometric ratios of their elements. This difference in compositional flexibility clearly shows that brass is a mixture, while the others are compounds. The additional option calcium chloride, with formula CaCl2, is also a compound, which reinforces that brass is the only mixture in the list.

Why Other Options Are Wrong:
Quick lime is calcium oxide, a compound with a definite formula and fixed ratio of calcium and oxygen.
Marble, although a natural material, is mainly calcium carbonate and is considered a compound in introductory chemistry contexts.
Soda ash refers to sodium carbonate, which has the fixed chemical formula Na2CO3 and is a pure compound.
Calcium chloride is another well defined compound with formula CaCl2, not a mixture or alloy.

Common Pitfalls:
Students may mistakenly think that natural materials like marble are mixtures, but in basic textbook treatment they are often considered primarily as examples of specific compounds. The more important clue here is the term alloy. If you remember that an alloy is a mixture of metals and that brass is a common alloy, the question becomes straightforward. Always look for whether a substance has a fixed formula when deciding if it is a compound or a mixture.

Final Answer:
The substance that is not a chemical compound but is instead a mixture or alloy is brass.