Which of the following is an example of a heterogeneous mixture?

Introduction / Context:
Matter around us often exists as mixtures of different substances rather than pure compounds or elements. Mixtures can be classified as homogeneous, where composition is uniform throughout, or heterogeneous, where composition is not uniform and different phases may be visible. This question asks you to identify which given substance is best regarded as a heterogeneous mixture in basic school-level chemistry.

Given Data / Assumptions:
- Options are air, table salt, soda and brass.
- We use standard textbook classifications for introductory chemistry.
- Soda refers to carbonated soft drink or soda water, a mixture of water, dissolved carbon dioxide and sometimes flavouring.

Concept / Approach:
A homogeneous mixture has the same composition throughout and appears as a single phase to the naked eye. Air is usually considered a homogeneous mixture of gases. Brass is a homogeneous solid solution (alloy) of copper and zinc. Table salt, NaCl, is not a mixture at all; it is a pure compound. Soda, especially when carbon dioxide is actively coming out as bubbles, is more appropriately treated as a heterogeneous mixture of liquid and gas phases, with separate bubbles visible in the liquid. Therefore, among the given options, soda is the best example of a heterogeneous mixture at an everyday level.

Step-by-Step Solution:
Step 1: Analyse table salt. It is chemically sodium chloride, a single compound, not a mixture. Step 2: Consider air. It is a mixture of gases (mainly nitrogen and oxygen) that is uniform at ordinary scales and appears as a single gas phase, so it is treated as homogeneous. Step 3: Consider brass. It is a solid alloy of copper and zinc where the metals form a uniform solid solution, so it is homogeneous as well. Step 4: Look at soda. It contains liquid water, dissolved carbon dioxide and visible gas bubbles that are a separate phase, giving non-uniform composition when observed closely. Step 5: Conclude that soda is the heterogeneous mixture among the given options.

Verification / Alternative check:
When you pour soda into a glass, you can clearly see gas bubbles rising through the liquid. These bubbles are regions rich in carbon dioxide gas, distinct from the surrounding liquid water and solutes. This two-phase system (gas and liquid) is a defining characteristic of a heterogeneous mixture. In contrast, air at standard conditions appears as a single-phase gas and brass appears as a uniform solid, supporting their classification as homogeneous mixtures or solutions rather than heterogeneous mixtures.

Why Other Options Are Wrong:
Air: Generally treated as a homogeneous mixture of gases because its composition is uniform enough at the scales considered in basic chemistry and physics.
Table salt: It is a pure compound, sodium chloride, not a mixture of different substances in variable proportions.
Brass: This is a homogeneous solid alloy, a uniform mixture of copper and zinc atoms in one solid phase, so it is not heterogeneous.

Common Pitfalls:
Learners sometimes think that any mixture of gases or metals must be heterogeneous because its components are different. However, uniform gaseous mixtures and solid alloys are considered homogeneous. Another issue is ignoring the presence of visible bubbles or separate phases in soda and classifying it as homogeneous. Remember that heterogeneity relates to visible or detectable non-uniformity in composition or phase. Seeing more than one phase, such as gas bubbles in a liquid, is a clear sign of a heterogeneous mixture.

Final Answer:
Among the options given, soda is best regarded as a heterogeneous mixture.