Which of the following statements about chemical compounds is correct according to basic chemistry?

Introduction / Context:
In basic chemistry, substances are often classified as elements, compounds or mixtures. Understanding what makes compounds distinct from elements and mixtures is essential for interpreting chemical formulas and reactions. This question asks which statement correctly describes chemical compounds and their key properties. The correct statement should reflect that compounds are pure substances with fixed composition that can be broken down only by chemical changes, not by simple physical methods.

Given Data / Assumptions:
- A compound is formed when two or more elements combine chemically in a fixed ratio.
- Compounds have properties that are usually very different from those of the elements that form them.
- Physical methods such as filtration, decantation and simple distillation are usually used to separate mixtures, not to decompose compounds into elements.
- The options present different possible descriptions, and we need to select the one that matches standard textbook definitions.

Concept / Approach:
A chemical compound is a pure substance made of two or more elements that are chemically combined in a fixed proportion by mass. Compounds have a definite composition and specific properties that differ from the properties of the constituent elements. To break a compound into its elements or into simpler compounds, a chemical reaction is required, such as electrolysis or thermal decomposition. In contrast, mixtures can often be separated into their components by physical methods, and their compositions can vary. Therefore, the correct statement should emphasise that compounds are pure substances and that separation into elements requires chemical change.

Step-by-Step Solution:
Step 1: Consider option A, which states that compounds are pure substances that can be decomposed into simpler substances only by chemical means. Step 2: Recognise that this matches the standard definition of a compound, highlighting both purity and the need for chemical reactions to separate components. Step 3: Examine option B, which claims that compounds have properties similar to their component elements. This is incorrect because compounds usually have very different properties. Step 4: Examine option C, which suggests that compounds can be separated into their elements by simple physical methods. This describes mixtures, not compounds. Step 5: Examine options D and E, which also describe variable composition or mixture like behaviour, both of which contradict the idea of a compound as a substance with fixed composition.

Verification / Alternative check:
Common examples support this understanding. Water is a compound made from hydrogen and oxygen, but its properties differ dramatically from those of hydrogen gas and oxygen gas. Water always has the same composition, with two hydrogen atoms for each oxygen atom. To separate water into hydrogen and oxygen, electrolysis, a chemical process, is required. Physical processes like filtration cannot achieve this. Similarly, sodium chloride is a compound that can be broken down into sodium and chlorine only by chemical reactions, and its properties are very different from those of metallic sodium and chlorine gas. These examples confirm the description in option A.

Why Other Options Are Wrong:
Option B is wrong because compounds rarely have properties similar to those of their constituent elements. For example, sodium chloride is not a soft reactive metal or a toxic green gas; it is a white crystalline solid. Option C is incorrect because physical methods such as filtration or simple distillation can separate mixtures but not decompose compounds into elements. Option D suggests that compound composition can vary, which is characteristic of mixtures like salt solutions, not of pure compounds. Option E confuses compounds with mixtures and ignores the fact that compounds have a single chemical formula and fixed composition.

Common Pitfalls:
Students may confuse compounds with mixtures, especially when both are found together in everyday materials. Another pitfall is to think that any combination of substances is a compound, even when the combination is not chemically bonded. To avoid these mistakes, learners should remember that compounds have fixed composition, distinct properties and require chemical reactions to break them down, whereas mixtures can vary in composition and are often separated by physical means. Keeping these distinctions clear helps in correctly identifying compounds and understanding their behaviour.

Final Answer:
The true statement about compounds is that They are pure substances that can be decomposed into simpler substances only by chemical means.