Which one of the following combinations forms a homogeneous mixture: starch and sugar, methanol and water, graphite and charcoal, or calcium carbonate and calcium bicarbonate?

Introduction / Context:
This question tests your understanding of the difference between homogeneous and heterogeneous mixtures. Identifying whether a given combination of substances forms a uniform mixture or not is a key skill in basic chemistry and in laboratory practice.

Given Data / Assumptions:

• Several pairs of substances are listed: starch and sugar, methanol and water, graphite and charcoal, calcium carbonate and calcium bicarbonate and sand with iron filings.
• We are asked which pair forms a homogeneous mixture.
• Homogeneous mixture means uniform composition throughout and no visible boundaries between components.
• We assume room temperature and typical conditions unless stated otherwise.

Concept / Approach:
Homogeneous mixtures, such as true solutions, have components that are evenly distributed at the molecular or ionic level. Heterogeneous mixtures show visible phases or non uniform distribution. Liquids that are completely miscible, like methanol and water, form homogeneous mixtures. Solid solid combinations usually remain heterogeneous unless one solid dissolves into the other or forms an alloy, which is not the case here for the listed pairs.

Step-by-Step Solution:
1) Starch and sugar are two distinct solids. When mixed as powders, they remain physically separate particles, so the mixture is heterogeneous.2) Methanol is an alcohol that is completely miscible with water in all proportions, forming a clear, uniform solution.3) Therefore, methanol and water together form a homogeneous mixture because no phase boundary is visible and composition is uniform.4) Graphite and charcoal are both forms of carbon, but mixing them simply gives a physical mixture of particles, which is heterogeneous.5) Calcium carbonate and calcium bicarbonate remain separate solids in a simple mixture, and sand with iron filings clearly shows two distinct solid phases, so these are heterogeneous as well.

Verification / Alternative check:
We can think in terms of phases. A homogeneous mixture should look like a single phase to the naked eye, such as a clear liquid or uniform solid solution. A methanol water mixture appears as a single, transparent liquid, confirming homogeneity. In contrast, mixed powders or granules almost always show visible specks of each component, indicating multiple phases. Thus, all the solid solid pairs in the options are heterogeneous, which further confirms methanol and water as the only correct choice.

Why Other Options Are Wrong:
Starch and sugar: Mixing these powders yields distinct particles, not a uniform single phase; this is a heterogeneous mixture.
Graphite and charcoal: Even though both are forms of carbon, they do not dissolve into each other and remain separate solid particles.
Calcium carbonate and calcium bicarbonate: These are different salts, and a simple physical mixture of their powders is heterogeneous.
Sand and iron filings: Both components are clearly visible and can be separated with a magnet, showing a heterogeneous mixture.

Common Pitfalls:
Students sometimes think that any pair of substances made of similar elements, such as graphite and charcoal, must form a homogeneous mixture. However, similarity in composition does not guarantee homogeneity. Another mistake is to forget that miscible liquids like alcohol and water always form homogeneous solutions. A good habit is to ask whether the mixture shows a single uniform phase or multiple distinct phases to classify it correctly.

Final Answer:
The homogeneous mixture among the given pairs is formed by methanol and water.