Which of the following statements correctly describes a characteristic feature of a mixture in basic chemistry?

Introduction / Context:
In introductory chemistry, students learn to distinguish between elements, compounds and mixtures. Mixtures are particularly important because many everyday materials, such as air, seawater and soil, are mixtures rather than pure substances. Understanding what makes a mixture different from a compound helps in topics like separation techniques and solution chemistry. This question asks which statement correctly describes a key characteristic of mixtures, focusing on composition and the way components are combined.

Given Data / Assumptions:

• Mixtures consist of two or more substances present together.
• Components of a mixture are not chemically combined but physically combined.
• Mixtures can be homogeneous (uniform) or heterogeneous (non uniform).
• Compounds have fixed formulas and are chemically combined, unlike mixtures.

Concept / Approach:
A mixture is defined as a physical combination of two or more substances where each substance retains its own chemical identity. The composition of a mixture can vary, meaning the components are not present in a fixed ratio. Mixtures can be homogeneous, such as salt dissolved in water, or heterogeneous, such as sand in water. In contrast, a compound is a pure substance with a fixed formula and definite ratio of atoms chemically bonded together. The approach is to identify the option that recognises variability of composition, physical combination and the possibility of both homogeneous and heterogeneous forms.

Step-by-Step Solution:
Step 1: Recall that mixtures are formed when two or more substances are physically combined without forming new chemical bonds between them. Step 2: Understand that mixtures can have variable composition; the proportions of components can change without altering the nature of the mixture. Step 3: Recognise that mixtures can be homogeneous, like air or sugar solution, where composition is uniform, or heterogeneous, like oil and water, where different regions have different compositions. Step 4: Compounds, unlike mixtures, have fixed formulas and the atoms are chemically combined in definite ratios. Step 5: Therefore, the statement that a mixture can be homogeneous or heterogeneous and that the components are physically combined is the correct description.

Verification / Alternative check:
Examples from daily life support this definition. Air is a homogeneous mixture of gases, with nitrogen and oxygen present in varying proportions depending on location and conditions. Seawater is a mixture of water and dissolved salts with variable salinity. Granite is a heterogeneous mixture of minerals that can be seen as different coloured grains. In all these cases, the components can be separated by physical methods, and their relative amounts are not fixed by a simple chemical formula. Textbooks consistently define mixtures in this way, confirming the correctness of the chosen statement.

Why Other Options Are Wrong:
Option A describes a pure substance with a fixed chemical formula, which is characteristic of a compound, not a mixture. Option B states that components are chemically combined in a fixed ratio, again describing a compound like water or sodium chloride. Option C claims that components are always in one specific ratio only, which contradicts the defining feature that mixture composition can vary. Option E suggests that a mixture always consists of only one type of atom, which is actually the definition of an element, not a mixture. Only option D correctly states that a mixture can be homogeneous or heterogeneous and that its components are physically combined rather than chemically bonded in a fixed ratio.

Common Pitfalls:
A frequent mistake is to confuse mixtures with compounds because both can look uniform at first glance. For example, a sugar solution looks uniform, but it is still a mixture since sugar and water can be separated by evaporation. Another pitfall is assuming that any uniform sample must be a pure substance; in reality, many homogeneous samples are mixtures. To avoid confusion, ask whether the sample could be separated by physical means and whether its composition can vary. If the answer is yes, it is likely a mixture rather than a pure compound.

Final Answer:
A correct characteristic of a mixture is that It can be homogeneous or heterogeneous and its components are physically combined without a fixed chemical formula.